Public Notary Transparency S. Kent
Internet Draft BBN Technologies
Expires: January 2016 July 2015
Intended Status: Informational
Attack Model for Certificate Transparency
draft-ietf-trans-threat-analysis-02
Abstract
This document describes an attack model for the Web PKI context in
which security mechanisms to detect mis-issuance of web site
certificates will be developed. The model provides an analysis of
detection and remediation mechanisms for both syntactic and semantic
mis-issuance. The model introduces an outline of attacks to organize
the discussion. The model also describes the roles played by the
elements of the Certificate Transparency (CT) system, to establish a
context for the model.
Status of this Memo
This Internet-Draft is submitted to IETF in full conformance with
the provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF), its areas, and its working groups. Note that other
groups may also distribute working documents as Internet-Drafts.
Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
time. It is inappropriate to use Internet-Drafts as reference
material or to cite them other than as "work in progress."
The list of current Internet-Drafts can be accessed at
http://www.ietf.org/1id-abstracts.html
The list of Internet-Draft Shadow Directories can be accessed at
http://www.ietf.org/shadow.html
This Internet-Draft will expire on January 31, 2016.
Kent Expires January 2016 [Page 1]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
Table of Contents
1. Introduction....................................................... 3
2. Semantic mis-issuance.............................................. 7
2.1. Non-malicious Web PKI CA context ............................... 7
2.1.1. Certificate logged ......................................... 8
2.1.1.1. Benign log.............................................. 8
2.1.1.1.1. Self-monitoring Subject ............................. 8
2.1.1.1.2. Benign third party Monitor .......................... 8
2.1.1.2. Mis-behaving log........................................ 8
2.1.1.2.1. Self-monitoring Subject ............................. 9
2.1.1.2.2. Benign third party Monitor .......................... 9
2.1.1.3. Mis-behaving third party Monitor....................... 10
2.1.2. Certificate not logged .................................... 10
2.1.2.1. Self-monitoring Subject................................ 10
2.1.2.2. Careful browser........................................ 10
2.2. Malicious Web PKI CA context .................................. 11
2.2.1. Certificate logged ........................................ 11
2.2.1.1. Benign log............................................. 11
2.2.1.1.1. Self-monitoring Subject ............................ 11
2.2.1.1.2. Benign third party Monitor ......................... 11
2.2.1.2. Mis-behaving log....................................... 12
2.2.1.2.1. Monitors - third party and self .................... 12
2.2.1.3. Mis-behaving third party Monitor....................... 12
2.2.2. Certificate not logged .................................... 12
2.2.2.1. Careful browser........................................ 13
3. Syntactic mis-issuance............................................ 13
3.1. Non-malicious Web PKI CA context .............................. 13
3.1.1. Certificate logged ........................................ 13
3.1.1.1. Benign log............................................. 13
3.1.1.2. Mis-behaving log or third party Monitor................ 14
3.1.1.3. Self-monitoring Subject and Benign third-party Monitor. 15
3.1.1.4. Careful browser........................................ 15
3.1.2. Certificate not logged .................................... 15
3.2. Malicious Web PKI CA context .................................. 15
3.2.1. Certificate logged ........................................ 15
3.2.1.1. Benign log............................................. 15
3.2.1.2. Mis-behaving log or third party Monitor................ 16
3.2.1.3. Self-monitoring Subject and Benign third party Monitor. 16
3.2.1.4. Careful browser........................................ 16
3.2.2. Certificate is not logged ................................. 17
4. Issues Applicable to Sections 2 and 3............................. 17
4.1. How does a Subject know which Monitor(s) to use? .............. 17
4.2. How does a Monitor discover new logs? ......................... 17
4.3. CA response to report of a bogus or erroneous certificate ..... 17
Kent Expires January 2016 [Page 2]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
4.4. Browser behavior .............................................. 18
4.5. Remediation for a malicious CA ................................ 18
4.6. Auditing - detecting mis-behaving logs ........................ 19
5. Security Considerations........................................... 20
6. IANA Considerations............................................... 20
7. Acknowledgments................................................... 20
8. References........................................................ 21
8.1. Normative References .......................................... 21
8.2. Informative References ........................................ 21
Author's Addresses................................................... 21
Copyright Statement.................................................. 21
1. Introduction
Certificate transparency (CT) is a set of mechanisms designed to
detect, deter, and facilitate remediation of certificate mis-
issuance. The term certificate mis-issuance is defined here to
encompass violations of either semantic or syntactic constraints.
The fundamental semantic constraint for a certificate is that it was
issued to an entity that is authorized to represent the Subject (or
Subject Alternative) named in the certificate. (It is also assumed
that the entity requested the certificate from the CA that issued
it.) Throughout the remainder of this document we refer to a
semantically mis-issued certificate as "bogus.")
Kent Expires January 2016 [Page 3]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
A certificate is characterized as syntactically mis-issued if it
violates syntax constraints associated with the class of certificate
that it purports to represent. Syntax constraints for certificates
are established by certificate profiles, and typically are
application-specific. For example, certificates used in the Web PKI
environment might be characterized as domain validation (DV) or
extended validation (EV) certificates. Certificates used with
applications such as IPsec or S/MIME have different syntactic
constraints from those in the Web PKI context.
There are two classes of beneficiaries of CT: certificate Subjects
and relying parties (RPs). In the initial focus context of CT, the
Web PKI, Subjects are web sites and RPs are browsers employing HTTPS
to access these web sites.
A certificate Subject benefits from CT because CT helps detect
certificates that have been mis-issued in the name of that Subject.
A Subject learns of a bogus certificate (issued in its name), via
the Monitor function of CT. The Monitor function may be provided by
the Subject itself, i.e., self-monitoring, or by a third party
trusted by the Subject. When a Subject is informed of certificate
mis-issuance by a Monitor, the Subject is expected to request/demand
revocation of the bogus certificate. Revocation of a bogus
certificate is the primary means of remedying mis-issuance. (If a
browser vendor distributes a "blacklist" of bogus certificates or
CAs that mis-issued certificates and modifies the browser to reject
any certificates on the list or for which the issuing CA is on the
list, this too remedies mis-issuance. Throughout the remainder of
this document references to certificate revocation as a remedy
encompass this and analogous forms of browser behavior, if
available. Note: there are no IETF standards defining a browser
blacklist capability.)
Note that a Subject can benefit from the Monitor function of CT even
if the Subject's certificate has not been logged. Monitoring of logs
for certificates issued in the Subject's name suffices to detect
mis-issuance targeting the Subject, if the bogus certificate is
logged.
A relying party (e.g., browser) benefits from CT if it rejects a
bogus certificate, i.e., treats it as invalid. An RP is protected
from accepting a bogus certificate if that certificate is revoked,
and if the RP checks the revocation status of the certificate. (An
RP is also protected if a browser vendor "blacklists" a certificate
or a CA as noted above.) An RP also may benefit from CT if the RP
validates an SCT associated with a certificate, and rejects the
certificate if the SCT is invalid. If an RP verified that a
Kent Expires January 2016 [Page 4]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
certificate that claims to have been logged has a valid log entry,
the RP would have a higher degree of confidence that the certificate
is genuine. However, checking logs in this fashion imposes a burden
on RPs and on logs. Moreover, the existence of a log entry does not
ensure that the certificate is not mis-issued. Unless the
certificate Subject is monitoring the log(s) in question, a bogus
certificate will not be detected by CT mechanisms. Finally, if an RP
were to check logs for individual certificates that would disclose
to logs the identity of web sites being visited by the RP, a privacy
violation. Thus this attack model assumes that RPs will not check
log entries.
Note that all RPs may benefit from CT even if they do nothing with
SCTs. If Monitors inform Subjects of mis-issuance, and if a CA
revokes a certificate in response to a request from the
certificate's legitimate Subject, then an RP benefits without having
to implement any CT-specific mechanisms.
Logging [TRANS] is the central element of CT. Logging enables a
Monitor to detect a bogus certificate based on reference information
provided by the certificate Subject. Logging of certificates is
intended to deter mis-issuance, by creating a publicly-accessible
record that associates a CA with any certificates that it mis-
issues. Logging does not remedy mis-issuance; but it does facilitate
remediation by providing the information needed to enable revocation
of bogus certificates in some circumstances.
Auditing is a function employed by CT to detect mis-behavior by
logs. Auditing is intended to deter mis-issuance that is abetted by
misbehaving logs. Auditing detects log mis-behavior by noting
inconsistencies between SCTs issued by a log and the log entries
published by the log. (A failure to make available log entries in a
timely fashion, consistent with the MMD for the log is also a form
of misbehavior that can be detected by the Audit function.) Such
inconsistencies indicate log misbehavior and suggest that Monitors
ought not trust such logs. Thus the Audit function does not detect
mis-issuance per se. There is no agreed-upon Audit function design
for CT at the time of this writing. As a result, the model merely
notes where Auditing is needed to detect certain classes of attacks.
Figure 1 (below) illustrates the data exchanges among the major
elements of the CT system, based on the log specification [TRANS]
and on the assumed behavior of other CT system elements as described
above. This Figure does not include the Audit function, because
there is not yet agreement on how that function will work in a
distributed, privacy-preserving fashion.
Kent Expires January 2016 [Page 5]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
+----+ +---------+ +---------+
| CA |---[ 1]-->| Log |<---[8]---| Monitor |
| | | | | |
| |<--[ 2]---| |----[9]-->| |
| | | | | |
| |---[ 3]-->| |<--[10]---| |
| | | | | |--------+
| |<--[ 4]---| |---[11]-->| | |
| | | | +---------+ |
| | | | |
| | | | +---------+ |
| | | |<--[8]----| Self- | |
| | | | | Monitor | |
| | | |---[9]--->|(Subject)| |
| | | | | | |
| | | |<--[10]---| | [12]
| | | | | | |
| | | |---[11]-->| | |
| | +---------+ +---------+ |
| | |
| | +---------+ +---------+ |
| |---[ 5]-->| Website |---[7]--->| Browser | |
| | |(Subject)| +---------+ |
| |<--[ 6]-->| |<----------------------------+
+----+ +---------+
[ 1] Retrieve accepted root certs
[ 2] accepted root certs
[ 3] Add chain to log/add PreCertChain to log
[ 4] SCT (embedded in pre-cert, if applicable)
[ 5] send cert + SCTs (or cert with embedded SCTs)
[ 6] Revocation request/response (in response to detected
mis-issuance)
[ 7] cert + SCTs (or cert with embedded SCTs)
[ 8] Retrieve entries from Log
[ 9] returned entries from log
[10] Retrieve latest STH
[11] returned STH
[12] bogus/erroneous cert notification
Figure 1. Data Exchanges Between Major Elements of the CT System
Kent Expires January 2016 [Page 6]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
Certificate mis-issuance may arise in one of several ways. The ways
by which CT enables a Subject (or others) to detect and redress mis-
issuance depends on the context and the entities involved in the
mis-issuance. This attack model applies to use of CT in the Web PKI
context. If CT is extended to apply to other contexts, each context
will require its own attack model, although most elements of the
model described here are likely to be applicable.
Because certificates are issued by CAs, the top level
differentiation in this analysis is whether the CA that mis-issued a
certificate did so maliciously or not. Next, for each scenario, the
model considers whether or not the certificate was logged. Scenarios
are further differentiated based on whether the logs and monitors
are benign or malicious and whether a certificate's Subject is self-
monitoring or is using a third party Monitoring service. Finally,
the analysis considers whether a browser is performing checking
relevant to CT. The scenarios are organized as illustrated by the
following outline:
Web PKI CA - malicious vs non-malicious
Certificate - logged vs not logged
Log - benign vs malicious
Third party Monitor - benign vs malicious
Certificate's Subject - self-monitoring (or not)
Browser - careful (or not)
The following sections examine each of these cases. As noted above,
the focus here is on the Web PKI context, although most of the
analysis is applicable to other PKI contexts.
Conventions used in this document
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
2. Semantic mis-issuance
2.1. Non-malicious Web PKI CA context
In this section, we address the case where the CA has no intent to
issue a bogus certificate.
A CA may have mis-issued a certificate as a result of an error or, in
the case of a bogus certificate, because it was the victim of a
Kent Expires January 2016 [Page 7]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
social engineering attack or an attack such as the one that affected
DigiNotar
[https://www.vasco.com/company/about_vasco/press_room/news_archive/20
11/news_diginotar_reports_security_incident.aspx]. In the case of an
error, the CA should have a record of the erroneous certificate and
be prepared to revoke this certificate once it has discovered and
confirmed the error. In the event of an attack, a CA may have no
record of a bogus certificate.
2.1.1. Certificate logged
2.1.1.1. Benign log
The log (or logs) is benign and thus is presumed to provide
consistent, accurate responses to requests from all clients.
If a bogus (pre-)certificate has been submitted to one or more logs
prior to issuance to acquire an embedded SCT, or post-issuance to
acquire a standalone SCT, detection of mis-issuance is the
responsibility of a Monitor.
2.1.1.1.1. Self-monitoring Subject
If a Subject is tracking the log(s) to which a certificate was
submitted, and is performing self-monitoring, then it will be able to
detect a bogus (pre-)certificate and request revocation. In this
case, the CA will make use of the log entry (supplied by the Subject)
to determine the serial number of the bogus certificate, and
investigate/revoke it. (See Sections 4.1, 4.2 and 4.3.)
2.1.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect a bogus certificate and will alert the Subject. The
Subject, in turn, will ask the CA to revoke the bogus certificate. In
this case, the CA will make use of the log entry (supplied by the
Subject) to determine the serial number of the bogus certificate, and
revoke it (after investigation). (See Sections 4.1, 4.2 and 4.3.)
2.1.1.2. Mis-behaving log
In this case, the bogus (pre-)certificate has been submitted to one
or more logs, each of which generate an SCT for the submission. In
this scenario, a log probably will suppress a bogus certificate log
entry, or it may create an entry for the certificate but report it
selectively. (A mis-behaving log also could create and report
Kent Expires January 2016 [Page 8]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
entries for bogus certificates that have not been issued by the
indicated CA (hereafter called "fake"). Unless a Monitor validates
the associated certificate chains up to roots that it trusts, these
fake bogus certificates could cause the Monitors to report non-
existent semantic problems to the Subject who would in turn report
them to the purported issuing CA. This might cause the CA to do
needless investigative work or perhaps incorrectly revoke and re-
issue the Subject's real certificate. Note that for every certificate
submitted to a log, the log MUST verify a complete certificate chain
up to one of the roots it accepts. So creating a log entry for a fake
bogus certificate marks the log as mis-behaving.)
2.1.1.2.1. Self-monitoring Subject
If a mis-behaving log suppresses a bogus certificate log entry, a
Subject performing self-monitoring will not detect the bogus
certificate. CT relies on an Audit mechanism to detect log
misbehavior, as a deterrent. (It is anticipated that logs that are
identified as persistently mis-behaving will cease to be trusted by
Monitors and by non-malicious CAs.) It is not clear if such a
mechanism will be viable if there are a very large number of self-
monitoring Subjects.
2.1.1.2.2. Benign third party Monitor
Because a mis-behaving log will suppress a bogus certificate log
entry (or report such entries inconsistently) a benign third party
Monitor that is protecting the targeted Subject also will not detect
a bogus certificate. In this scenario, CT relies on a distributed
Auditing mechanism [gossip] to detect log misbehavior, as a
deterrent. (See Section 4.6 below.) However, a Monitor (third-party
or self) must participate in the Audit mechanism in order to become
aware of log misbehavior.
If the mis-behaving log has logged the bogus certificate when it
issuing the associated SCT, it will try to hide this from the
Subject (if self-monitoring) or from the Monitor protecting the
Subject. It does so by presenting them with a view of its log
entries and STH that does not contain the bogus certificate. To
other entities, the log presents log entries and an STH that include
the bogus certificate. This discrepancy can be detected if there is
an exchange of information about the log entries and STH between the
entities receiving the view that excludes the bogus certificate and
entities that receive a view that includes it, i.e., a distributed
Audit mechanism.
Kent Expires January 2016 [Page 9]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
If a malicious log does not create an entry for a bogus certificate
(for which an SCT has been issued), then any Monitor/Auditor that
sees the bogus certificate will detect this when it checks with the
log for log entries and STH (see Section 2.1.2.)
2.1.1.3. Mis-behaving third party Monitor
A third party Monitor that mis-behaves will not notify the targeted
Subject of a bogus certificate. This is true irrespective of whether
the Monitor checks the logs or whether the logs are benign or
malicious/conspiring.
Note that independent of any mis-issuance on the part of the CA, a
mis-behaving Monitor could issue false warnings to a Subject that it
protects. These could cause the Subject to report non-existent
semantic problems to the issuing CA and cause the CA to do needless
investigative work or perhaps incorrectly revoke and re-issue the
Subject's certificate.
2.1.2. Certificate not logged
If the CA does not submit a pre-certificate to a log, whether a log
is benign or mis-behaving does not matter. The same is true if a
Subject is issued a certificate without an SCT and does not log the
certificate itself, to acquire an SCT. Also, since there is no log
entry in this scenario, there is no difference in outcome between a
benign and a mis-behaving third party Monitor. In both cases, no
Monitor (self or third-party) will detect a bogus certificate based
on Monitor functions and there will be no consequent reporting of the
problem to the Subject or by the Subject to the CA based on
examination of log entries.
2.1.2.1. Self-monitoring Subject
A Subject performing self-monitoring will be able to detect the lack
of an embedded SCT in the certificate it received from the CA. The
Subject SHOULD notify the CA if the Subject believes that its
certificate was supposed to be logged. If the certificate was
supposed to be logged, but was not, the CA can use the certificate
supplied by the Subject to investigate and remedy the problem. In the
context of a benign CA, a failure to log the certificate might be the
result of an operations error, or evidence of an attack on the CA.
2.1.2.2. Careful browser
If a browser rejects certificates without SCTs (see Section 4.4.),
CAs may be "encouraged" to log the certificates they issue. This, in
Kent Expires January 2016 [Page 10]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
turn, would make it easier for Monitors to detect bogus
certificates. However, it is not clear how such behavior by browsers
can be deployed incrementally throughout the Internet. As a result,
this model does not assume that browsers will reject a certificate
that is not accompanied by an SCT. In the absence of logging,
browsers generally do not benefit from CT, since certificates have
to be logged to enable detection of mis-issuance by Monitors, and to
trigger subsequent revocation.
2.2. Malicious Web PKI CA context
In this section, we address the scenario in which the mis-issuance
is intentional, not due to error. The CA is not the victim but the
attacker.
2.2.1. Certificate logged
2.2.1.1. Benign log
A bogus (pre-)certificate may be submitted to one or more benign logs
prior to issuance, to acquire an embedded SCT, or post-issuance to
acquire a standalone SCT. The log (or logs) replies correctly to
requests from clients.
2.2.1.1.1. Self-monitoring Subject
If a Subject is checking the logs to which a certificate was
submitted and is performing self-monitoring, it will be able to
detect the bogus certificate and will request revocation. The CA may
refuse to revoke, or may substantially delay revoking, the bogus
certificate. The CA could make excuses about inadequate proof that
the certificate is bogus, or argue that it cannot quickly revoke the
certificate because of legal concerns, etc. In this case, the CT
mechanisms will have detected mis-issuance, but the information
logged by CT does not help remedy the problem. (See Sections 3 and
5.)
2.2.1.1.2. Benign third party Monitor
If a benign third party monitor is checking the logs to which a
certificate was submitted and is protecting the targeted Subject, it
will detect the bogus certificate and will alert the Subject. The
Subject will then ask the CA to revoke the bogus certificate. As in
2.2.1.1.1, the CA may or may not revoke the certificate.
Kent Expires January 2016 [Page 11]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
2.2.1.2. Mis-behaving log
A bogus (pre-)certificate may have been submitted to one or more
logs that are mis-behaving, e.g., conspiring with an attacker. These
logs may or may not issue SCTs, but will hide the log entries from
some or all Monitors.
2.2.1.2.1. Monitors - third party and self
If log entries are hidden from a Monitor (third party or self), the
Monitor will not be able to detect issuance of a bogus certificate.
The Audit function of CT is intended to detect logs that conspire to
delay or suppress log entries (potentially selectively), based on
consistency checking of logs. (See 2.1.1.2.2.) If a Monitor learns
of misbehaving log operation, it alerts the Subjects that it is
protecting, so that they no longer acquire SCTs from that log. The
Monitor also avoids relying upon such a log in the future. However,
unless a distributed Audit mechanism proves effective in detecting
such misbehavior, CT cannot be relied upon to detect this form of
mis-issuance. (See Section 4.6 below.)
2.2.1.3. Mis-behaving third party Monitor
If the third party Monitor that is "protecting" the targeted Subject
is mis-behaving, then it will not notify the targeted Subject of any
mis-issuance or of any malfeasant log behavior that it detects
irrespective of whether the logs it checks are benign or
malicious/conspiring.
2.2.2. Certificate not logged
Because the CA is presumed malicious, it may choose to not submit a
(pre-)certificate to a log. This means there is no SCT for the
certificate.
When a CA does not submit a certificate to a log, whether a log is
benign or mis-behaving does not matter. Also, since there is no log
entry, there is no difference in behavior between a benign and a mis-
behaving third-party Monitor. Neither will report a problem to the
Subject.
A bogus certificate would not be delivered to the (legitimate)
Subject. So the Subject, acting as a self-Monitor, cannot detect the
issuance of a bogus certificate in this case.
Kent Expires January 2016 [Page 12]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
2.2.2.1. Careful browser
If careful browsers reject certificates without SCTs, CAs may be
"encouraged" to log certificates (see section 4.4.) However, it is
not clear how such behavior by browsers can be deployed
incrementally throughout the Internet. As a result, this model does
not assume that browsers will reject a certificate that is not
accompanied by an SCT. In the absence of logging, browsers generally
do not benefit from CT, since certificates have to be logged to
enable detection of mis-issuance by Monitors, and to trigger
subsequent revocation.
3. Syntactic mis-issuance
3.1. Non-malicious Web PKI CA context
This section analyzes the scenario in which the CA has no intent to
issue a syntactically incorrect certificate. Throughout the
remainder of this document we refer to a syntactically incorrect
certificate as "erroneous".
3.1.1. Certificate logged
3.1.1.1. Benign log
If a (pre-)certificate is submitted to a benign log, syntactic mis-
issuance can (optionally) be detected, and noted. This will happen
only if the log performs syntactic checks in general, and if the log
is capable of performing the checks applicable to the submitted
(pre-)certificate. (A (pre-)certificate SHOULD be logged even if it
fails syntactic validation; logging takes precedence over detection
of syntactic mis-issuance.) If syntactic validation fails, this can
be noted in an SCT extension returned to the submitter.
. If the (pre-)certificate is submitted by the non-malicious issuing
CA, and if the CA has a record of the certificate content, then
the CA SHOULD remedy the syntactic problem and re-submit the
(pre-)certificate to a log or logs. If this is a pre-certificate
submitted prior to issuance, syntactic checking by a log helps
avoid issuance of an erroneous certificate. If the CA does not
have a record of the certificate contents, then presumably it
was a bogus certificate and the CA SHOULD revoke it.
. If a certificate is submitted by its Subject, and is deemed
erroneous, then the Subject SHOULD contact the issuing CA and
request a new certificate. If the Subject is a legitimate
subscriber of the CA, then the CA will either have a record of
Kent Expires January 2016 [Page 13]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
the certificate content or can obtain a copy of the certificate
from the Subject. The CA will remedy the syntactic problem and
either re-submit a corrected (pre-)certificate to a log and send
it to the Subject or the Subject will re-submit it to a log.
Here too syntactic checking by a log enables a Subject to be
informed that its certificate is erroneous and thus may hasten
issuance of a replacement certificate.
. If a certificate is submitted by a third party, that party might
contact the Subject or the issuing CA, but because the party is
not the Subject of the certificate it is not clear how the CA
will respond.
Bottom line: Syntactic mis-issuance of a certificate can be avoided
by a CA if it makes use of logs that are capable of performing these
checks for the types of certificates that are submitted, and if the
CA acts on the feedback it receives. If a CA uses a log that does not
perform such checks, or if the CA requests checking relative to
criteria not supported by the log, then syntactic mis-issuance will
not be detected or avoided by this mechanism. Similarly, syntactic
mis-issuance can be remedied if a Subject submits a certificate to a
log that performs syntactic checks, and if the Subject asks the
issuing CA to fix problems detected by the log. (The issuer is
presumed to be willing to re-issue the certificate, correcting any
problems, because the issuing CA is not malicious.)
3.1.1.2. Mis-behaving log or third party Monitor
A log or Monitor that is conspiring with the attacker or is
independently malicious, will either not perform syntactic checks,
even though it claims to do so, or simply not report errors. The log
entry and the SCT for an erroneous certificate will assert that the
certificate syntax was verified.
As with detection of semantic mis-issuance, a distributed Audit
mechanism could, in principle, detect mis-behavior by logs or
Monitors with respect to syntactic checking. For example, if for a
given certificate, some logs (or Monitors) are reporting syntactic
errors and some which claim to do syntactic checking, are not
reporting these errors, this is indicative of misbehavior by these
logs and/or Monitors.
Note that a malicious log (or Monitor) could report syntactic errors
for a syntactically valid certificate. This could result in
reporting of non-existent syntactic problems to the issuing CA, which
might cause the CA to do needless investigative work or perhaps
incorrectly revoke and re-issue the Subject's certificate.
Kent Expires January 2016 [Page 14]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
3.1.1.3. Self-monitoring Subject and Benign third-party Monitor
If a Subject or benign Monitor performs syntactic checks, it will
detect the erroneous certificate and the issuing CA will be notified
(by the Subject). If the Subject is a legitimate subscriber of the
CA, then the CA will either have a record of the certificate content
or can obtain a copy of the certificate from the Subject. The CA
SHOULD revoke the erroneous certificate (after investigation) and
remedy the syntactic problem. The CA SHOULD either re-submit the
corrected (pre-)certificate to one or more logs and then send the
result to the Subject, or send the corrected certificate to the
Subject, who will re-submit it to one or more logs.
3.1.1.4. Careful browser
If a browser rejects an erroneous certificate and notifies the
Subject and/or the issuing CA, then syntactic mis-issuance will be
detected (see Section 4.) Unfortunately, experience suggests that
many browsers do not perform thorough syntactic checks on
certificates, and so it seems unlikely that browsers will be a
reliable way to detect erroneous certificates. Moreover, a protocol
used by a browser to notify a Subject and/or CA of an erroneous
certificate represents a DoS potential, and thus may not be
appropriate. This argues for syntactic checking to be performed by
other elements of the CT system, e.g., logs and/or Monitors.
3.1.2. Certificate not logged
If a CA does not submit a certificate to a log, there can be no
syntactic checking by the log. Detection of syntactic errors will
depend on a Subject performing the requisite checks when it receives
its certificate from a CA.
3.2. Malicious Web PKI CA context
This section analyzes the scenario in which the CA's issuance of a
syntactically incorrect certificate is intentional, not due to error.
The CA is not the victim but the attacker.
3.2.1. Certificate logged
3.2.1.1. Benign log
Because the CA is presumed to be malicious, the CA may cause the log
to not perform checks, in one of several ways. (See [DOMVAL] and
[EXTVAL] for more details on validation checks and CCIDs).
Kent Expires January 2016 [Page 15]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
1. The CA may assert that the certificate is being issued w/o
regard to any guidelines (the "no guidelines" reserved CCID).
2. The CA may assert a CCID that has not been registered, and thus
no log will be able to perform a check.
3. The CA may check to see which CCIDs a log declares it can
check, and chose a registered CCID that is not checked by the log
in question. In this fashion the CA can prevent the log from
performing checks, and the SCT and log entry will not contain an
indication of a failed check.
4. The CA may submit a (pre-) certificate to a log that is known
to not perform any syntactic checks, and thus avoid syntactic
checking.
3.2.1.2. Mis-behaving log or third party Monitor
A mis-behaving log or third party Monitor will either not perform
syntactic checks or not report any problems that it discovers. (See
3.1.1.2 for further problems).
3.2.1.3. Self-monitoring Subject and Benign third party Monitor
Irrespective of whether syntactic checks are performed by a log, a
malicious CA will acquire an embedded SCT, or post-issuance will
acquire a standalone SCT. If Subjects or Monitors perform syntactic
checks that detect the syntactic mis-issuance and report the problem
to the CA, a malicious CA may do nothing or may delay action to
remedy the problem.
3.2.1.4. Careful browser
As noted above (3.1.1.4) many browsers fail to perform thorough
syntax checks on certificates. Such browsers would benefit from
having such checks performed by a log and reported in the SCT.
(Remember, in this scenario, the log is benign.) However, if a
browser does not discriminate against certificates that do not
contain SCTs (or that are not accompanied by an SCT in the TLS
handshake), only minimal benefits might accrue to them from syntax
checks perform by logs.
If a browser accepts certificates that do not appear to have been
syntactically checked by a log (as indicated by the SCT), a
malicious CA need not worry about failing a log-based check.
Similarly, if there is no requirement for a browser to reject a
certificate that was logged by an operator that does not perform
Kent Expires January 2016 [Page 16]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
syntactic checks, the fourth approach noted in 3.2.1.1 will succeed
as well. If a browser were configured to know which versions of
certificate types are applicable to its use of a certificate, the
second and third strategies noted above could be thwarted.
3.2.2. Certificate is not logged
Since certificates are not logged in this scenario, a Monitor (third-
party or self) cannot detect the issuance of an erroneous
certificate. Thus there is no difference between a benign or a
malicious/conspiring log or a benign or conspiring/malicious Monitor.
(A Subject MAY detect a syntax error by examining the certificate
returned to it by the Issuer.) However, even if errors are detected
and reported to the CA, a malicious/conspiring CA may do nothing to
fix the problem or may delay action.
4. Issues Applicable to Sections 2 and 3
4.1. How does a Subject know which Monitor(s) to use?
If a CA submits a bogus certificate to one or more logs, but these
logs are not tracked by a Monitor that is protecting the targeted
Subject, CT will not remedy this type of mis-issuance attack. It is
not clear whether every Monitor MUST offer to track every Subject
that requests protection. Absent such a guarantee, how do Subjects
know which set of Monitors will provide "sufficient" coverage? If a
Subject acts as its own Monitor, this problem is solved for that
Subject.
4.2. How does a Monitor discover new logs?
It is not clear how a (self-)Monitor becomes aware of all (relevant?)
logs, including newly created logs. The means by which Monitors
become aware of new logs MUST accommodate self-monitoring by a
potentially very large number of web site operators. If there are
many logs, it may not be feasible for a (self-) Monitor to track all
of them.
4.3. CA response to report of a bogus or erroneous certificate
A CA being presented with evidence of a bogus or erroneous
certificate, in the form of a log entry and/or SCT, will need to
examine its records to determine if it has knowledge of the
certificate in question. It also will likely require the targeted
Subject to provide assurances that it is the authorized entity
representing the Subject name (subjectAltname) in question. Thus a
Subject should not expect immediate revocation of a contested
Kent Expires January 2016 [Page 17]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
certificate. The time frame in which a CA will respond to a
revocation request usually is described in the CPS for the CA. Other
certificate fields and extensions may be of interest for forensic
purposes, but are not required to effect revocation nor to verify
that the certificate to be revoked is bogus or erroneous, based on
applicable criteria. The SCT and log entry, because each contains a
timestamp from a third party, is probably valuable for forensic
purposes (assuming a non-conspiring log operator).
4.4. Browser behavior
If a browser is to reject a certificate that lacks an embedded SCT,
or is not accompanied by an SCT transported via the TLS handshake,
this behavior needs to be defined in a way that is compatible with
incremental deployment. Issuing a warning to a (human) user is
probably insufficient, based on experience with warnings displayed
for expired certificates, lack of certificate revocation status
information, and similar errors that violate RFC 5280 path validation
rules. Unless a mechanism is defined that accommodates incremental
deployment of this capability, attackers probably will avoid
submitting bogus certificates to (benign) logs as a means of evading
detection.
4.5. Remediation for a malicious CA
A targeted Subject might ask the parent of a malicious CA to revoke
the certificate of the non-cooperative CA. However, a request of this
sort may be rejected, e.g., because of the potential for significant
collateral damage. A browser might be configured to reject all
certificates issued by the malicious CA, e.g., using a CA hot list
distributed by a browser vendor. However, if the malicious CA has a
sufficient number of legitimate clients, treating all of their
certificates as bogus or erroneous still represents serious
collateral damage. If this specification were to require that a
browser can be configured to reject a specific, bogus or erroneous
certificate identified by a Monitor, then the bogus or erroneous
certificate could be rejected in that fashion. This remediation
strategy calls for communication between Monitors and browsers, or
between Monitors and browser vendors. Such communication has not been
specified, i.e., there are no standard ways to configure a browser to
reject individual bogus or erroneous certificates based on
information provided by an external entity such as a Monitor.
Moreover, the same or another malicious CA could issue new bogus or
erroneous certificates for the targeted Subject, which would have to
be detected and rejected in this (as yet unspecified) fashion. Thus,
for now, CT does not seem to provide a way to remedy this form of
attack, even though it provides a basis for detecting such attacks.
Kent Expires January 2016 [Page 18]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
4.6. Auditing - detecting mis-behaving logs
The combination of a malicious CA and one or more conspiring logs
motivates the definition of an audit function, to detect conspiring
logs. If a Monitor protecting a Subject does not see bogus
certificates, it cannot alert the Subject. If one or more SCTs are
present in a certificate, or passed via the TLS handshake, a browser
has no way to know that the logged certificate is not visible to
Monitors. Only if Monitors and browsers reject certificates that
contain SCTs from conspiring logs (based on information from an
auditor) will CT be able to detect and deter use of such logs. Thus
the means by which a Monitor performing an audit function detects
such logs, and informs browsers must be specified for CT to be
effective in the context of mis-behaving logs.
Absent a well-defined mechanism that enables Monitors to verify that
data from logs are reported in a consistent fashion, CT cannot claim
to provide protection against logs that are malicious or may
conspire with, or are victims of, attackers effecting certificate
mis-issuance. The mechanism needs to protect the privacy of users
with respect to which web sites they visit. It needs to scale to
accommodate a potentially large number of self-monitoring Subjects
and a vast number of browsers, if browsers are part of the
mechanism. Even when an Audit mechanism is defined, it will be
necessary to describe how the CT system will deal with a mis-
behaving or compromised log. For example, will there be a mechanism
to alert all browsers to reject SCTs issued by such a log? Absent a
description of a remediation strategy to deal with mis-behaving or
compromised logs, CT cannot ensure detection of mis-issuance in a
wide range of scenarios.
Monitors play a critical role in detecting semantic certificate mis-
issuance, for Subjects that have requested monitoring of their
certificates. A monitor (including a Subject performing self-
monitoring) examines logs for certificates associated with one or
more Subjects that are being "protected". A third-party Monitor must
obtain a list of valid certificates for the Subject being monitored,
in a secure manner, to use as a reference. It also must be able to
identify and track a potentially large number of logs on behalf of
its Subjects. This may be a daunting task for Subjects that elect to
perform self-monitoring.
Note: A Monitor must not rely on a CA or RA database for its
reference information or use certificate discovery protocols; this
information must be acquired by the Monitor based on reference
certificates provided by a Subject. If a Monitor were to rely on a
CA or RA database (for the CA that issued a targeted certificate),
Kent Expires January 2016 [Page 19]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
the Monitor would not detect mis-issuance due to malfeasance on the
part of that CA or the RA, or due to compromise of the CA or the
RA. If a CA or RA database is used, it would support detection of
mis-issuance by an unauthorized CA. A Monitor must not rely on
certificate discovery mechanisms to build the list of valid
certificates since such mechanisms might result in bogus or
erroneous certificates being added to the list.
As noted above, Monitors represent another target for adversaries
who wish to effect certificate mis-issuance. If a Monitor is
compromised by, or conspires with, an attacker, it will fail to
alert a Subject to a bogus or erroneous certificate targeting that
Subject, as noted above. It is suggested that a Subject request
certificate monitoring from multiple sources to guard against such
failures. Operation of a Monitor by a Subject, on its own behalf,
avoids dependence on third party Monitors. However, the burden of
Monitor operation may be viewed as too great for many web sites, and
thus this mode of operation ought not be assumed to be universal
when evaluating protection against Monitor compromise.
5. Security Considerations
A threat model is, by definition, a security-centric document. Unlike
a protocol description, a threat model does not create security
problems nor does it purport to address security problems. This model
postulates a set of threats (i.e., motivated, capable adversaries)
and examines classes of attacks that these threats are capable of
effecting, based on the motivations ascribed to the threats.
6. IANA Considerations
None.
7. Acknowledgments
The author would like to thank David Mandelberg and Karen Seo for
help with the editing and formatting, and other members of the TRANS
working group for reviewing this document.
Kent Expires January 2016 [Page 20]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
8. References
8.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels," BCP 14, RFC 2119, March 1997.
8.2. Informative References
[TRANS] Laurie, B., Langley, A., Kasper, E., Messeri, E.,
Stradling, R., "Certificate Transparency," draft-ietf-
trans-rfc6962-bis-07 (March 9, 2015), work in progress.
[DOMVAL] Kent, S., "Syntactic and Semantic Checks for Domain
Validation Certificates," draft-kent-trans-domain-
validation-cert-checks-01, (December 2014), work in
progress.
[EXTVAL] Kent, S., "Syntactic and Semantic Checks for Extended
Validation Certificates," draft-kent-trans-extended-
validation-cert-checks-01 (December 2014), work in
progress.
[gossip] Nordberg, L, Gillmore, D, "Gossiping in CT," draft-linus-
trans-gossip-ct-01, (March 9, 2015), work in progress
Author's Addresses
Stephen Kent
BBN Technologies
10 Moulton Street
Cambridge MA 02138
USA
Phone: +1 (617) 873-3988
Email: skent@bbn.com
Copyright Statement
Copyright (c) 2015 IETF Trust and the persons identified as the
document authors. All rights reserved.
Kent Expires January 2016 [Page 21]
Internet-Draft Attack Model for Certificate Mis-issuance July 2015
This document is subject to BCP 78 and the IETF Trust's Legal
Provisions Relating to IETF Documents
(http://trustee.ietf.org/license-info) in effect on the date of
publication of this document. Please review these documents
carefully, as they describe your rights and restrictions with respect
to this document. Code Components extracted from this document must
include Simplified BSD License text as described in Section 4.e of
the Trust Legal Provisions and are provided without warranty as
described in the Simplified BSD License.
Kent Expires January 2016 [Page 22]
