Best Practices for CMS SignedData with Regards to Signed Attributes
draft-ietf-lamps-cms-euf-cma-signeddata-00
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| Authors | Daniel Van Geest , Falko Strenzke | ||
| Last updated | 2025-11-20 | ||
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draft-ietf-lamps-cms-euf-cma-signeddata-00
Limited Additional Mechanisms for PKIX and SMIME D. Van Geest
Internet-Draft CryptoNext Security
Intended status: Best Current Practice F. Strenzke
Expires: 24 May 2026 MTG AG
20 November 2025
Best Practices for CMS SignedData with Regards to Signed Attributes
draft-ietf-lamps-cms-euf-cma-signeddata-00
Abstract
The Cryptographic Message Syntax (CMS) has different signature
verification behaviour based on whether signed attributes are present
or not. This results in a potential existential forgery
vulnerability in CMS and protocols which use CMS. This document
describes the vulnerability and lists best practices and mitigations
for such a vulnerability.
About This Document
This note is to be removed before publishing as an RFC.
The latest revision of this draft can be found at https://lamps-
wg.github.io/cms-euf-cma-signeddata/draft-ietf-lamps-cms-euf-cma-
signeddata.html. Status information for this document may be found
at https://datatracker.ietf.org/doc/draft-ietf-lamps-cms-euf-cma-
signeddata/.
Discussion of this document takes place on the Limited Additional
Mechanisms for PKIX and SMIME Working Group mailing list
(mailto:spasm@ietf.org), which is archived at
https://mailarchive.ietf.org/arch/browse/spasm/. Subscribe at
https://www.ietf.org/mailman/listinfo/spasm/.
Source for this draft and an issue tracker can be found at
https://github.com/lamps-wg/cms-euf-cma-signeddata.
Status of This Memo
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
Task Force (IETF). Note that other groups may also distribute
working documents as Internet-Drafts. The list of current Internet-
Drafts is at https://datatracker.ietf.org/drafts/current/.
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This Internet-Draft will expire on 24 May 2026.
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document authors. All rights reserved.
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Please review these documents carefully, as they describe your rights
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . 3
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 4
3. Best Practices . . . . . . . . . . . . . . . . . . . . . . . 4
3.1. Don't use id-data . . . . . . . . . . . . . . . . . . . . 4
3.2. Recipient Verification . . . . . . . . . . . . . . . . . 5
4. Mitigations . . . . . . . . . . . . . . . . . . . . . . . . . 5
4.1. Recipient Detection . . . . . . . . . . . . . . . . . . . 5
4.2. Sender Detection . . . . . . . . . . . . . . . . . . . . 6
5. Security Considerations . . . . . . . . . . . . . . . . . . . 6
6. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 7
7. References . . . . . . . . . . . . . . . . . . . . . . . . . 7
7.1. Normative References . . . . . . . . . . . . . . . . . . 7
7.2. Informative References . . . . . . . . . . . . . . . . . 8
Appendix A. RFCs Using the id-data EncapsulatedContentInfo Content
Type . . . . . . . . . . . . . . . . . . . . . . . . . . 9
A.1. RFC 8894 Simple Certificate Enrolment Protocol . . . . . 10
A.2. RFC 8572 Secure Zero Touch Provisioning (SZTP) . . . . . 11
A.3. S/MIME RFCs . . . . . . . . . . . . . . . . . . . . . . . 11
A.4. RFC 6257 Bundle Security Protocol Specification . . . . . 11
A.5. RFC 5655 IP Flow Information Export (IPFIX) . . . . . . . 11
A.6. RFC 5636 Traceable Anonymous Certificate . . . . . . . . 12
A.7. RFC 5126 CMS Advanced Electronic Signatures (CAdES) . . . 12
A.8. RFC 5024 ODETTE File Transfer Protocol 2 . . . . . . . . 12
A.9. RFC 3126 Electronic Signature Formats for long term
electronic signatures . . . . . . . . . . . . . . . . . . 12
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Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 12
Authors' Addresses . . . . . . . . . . . . . . . . . . . . . . . 12
1. Introduction
The Cryptographic Message Syntax (CMS) [RFC5652] signed-data content
type allows any number of signers in parallel to sign any type of
content.
CMS gives a signer two options when generating a signature on some
content:
* Generate a signature on the whole content; or
* Compute a hash over the content, place this hash in the message-
digest attribute in the SignedAttributes type, and generate a
signature on the SignedAttributes. The SignedAttributes type is
placed in the signedAttrs field of the SignedData type.
The resulting signature does not commit to the presence of the
SignedAttributes type, allowing an attacker to influence verification
behaviour. An attacker can perform two different types of attacks:
1. Take an arbitrary CMS signed message M which was originally
signed with SignedAttributes present and rearrange the structure
such that the SignedAttributes field is absent and the original
DER-encoded SignedAttributes appears as an encapsulated or
detached content of type id-data, thereby crafting a new
structure M' that was never explicitly signed by the signer. M'
has the DER-encoded SignedAttributes of the original message as
its content and verifies correctly against the original signature
of M.
2. Let the signer sign a message of the attacker's choice without
SignedAttributes. The attacker chooses this message to be a
valid DER-encoding of a SignedAttributes object. The attacker
can then add this encoded SignedAttributes object to the signed
message and change the signed message to the one that was used to
create the messageDigest attribute within the SignedAttributes.
The signature created by the signer is valid for this arbitrary
attacker-chosen message.
This vulnerability was presented by Falko Strenzke to the LAMPS
working group at IETF 121 [LAMPS121] and is detailed in [Str23].
Section 5.3 of [RFC5652] states:
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signedAttrs is a collection of attributes that are signed. The
field is optional, but it MUST be present if the content type of
the EncapsulatedContentInfo value being signed is not id-data.
Thus, if a verifier accepts a content type of id-data in the
EncapsulatedContentInfo type when used in SignedData, then a
SignerInfo within the SignedData may or may not contain a signedAttrs
field and will be vulnerable to this attack. On the other hand, if
the verifier doesn't accept a content type of id-data, the sender
always adds the signedAttrs field, and the recipient verifies that
signedAttrs is present, the attack will not succeed.
Due to the limited flexibility of either the signed or the forged
message in either attack variant, the fraction of vulnerable systems
can be assumed to be small. But due to the wide deployment of the
affected protocols, such instances cannot be excluded.
2. Conventions and Definitions
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "NOT RECOMMENDED", "MAY", and
"OPTIONAL" in this document are to be interpreted as described in
BCP 14 [RFC2119] [RFC8174] when, and only when, they appear in all
capitals, as shown here.
3. Best Practices
This section describes the best practices to avoid the vulnerability
at the time of writing.
3.1. Don't use id-data
New uses of the CMS SignedData type MUST disallow the id-data
EncapsulatedContentInfo content type. [WG: MUST NOT or SHOULD NOT?]
When a protocol which already uses the id-data
EncapsulatedContentInfo content type within SignedData is updated, it
SHOULD deprecate the use of id-data and use a different (new or
existing) identifier. A partial list of such identifiers is found in
the "CMS Inner Content Types" IANA subregistry within the "Media Type
Sub-Parameter Registries". [WG: overreach or not?]
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When a protocol which already uses the id-data
EncapsulatedContentInfo content type within SignedData is updated and
cannot deprecate the use of id-data, the protocol SHOULD be updated
to either always require or always forbid the inclusion of the
signedAttrs field. If the protocol is modified to make such a
requirement, a recipient MUST check whether the signedAttrs field is
present or absent as specified by the protocol, and fail processing
if the appropriate condition is not met.
3.2. Recipient Verification
When a recipient receives a CMS SignedData, it SHOULD check that the
EncapsulatedContentInfo content type value is one expected by the
protocol and fail processing if it isn't. [WG: MUST or SHOULD?]
When a recipient receives a CMS SignedData where the
EncapsulatedContentInfo content type is not id-data, it SHOULD verify
both that the correct content type was received and that the
SignedData contains the signedAttrs field and fail processing if
either of these conditions is not met. [WG: MUST or SHOULD?]
4. Mitigations
If the id-data EncapsulatedContentInfo content type continues to be
used, the following mitigations MAY be applied to protect against the
vulnerability described in Section 1.
4.1. Recipient Detection
This mitigation is performed by a recipient when processing
SignedData.
If the EncapsulatedContentInfo content type is id-data and
signedAttrs is not present, check if the encapsulated or detached
content is a valid DER-encoded SignedAttributes structure and fail if
it is. The mandatory contentType and messageDigest attributes, with
their respective OIDs, should give a low probability of a legitimate
message which just happens to look like a DER-encoded
SignedAttributes structure being flagged.
| However, a malicious party could intentionally present messages
| for signing that are detected by the countermeasure and thus
| introduce errors into the application processing that might be
| hard to trace for a non-expert.
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4.2. Sender Detection
This mitigation is performed by a sender who signs data received from
a 3rd party (potentially an attacker).
If the sender will be using the id-data content type and will not be
setting the signedAttrs field, check that the 3rd party content is
not a DER-encoded SignedAttributes structure, and fail if it is.
Note that also in this case, a malicious party could intentionally
present messages that trigger this countermeasure and thereby trigger
hard-to-trace errors during the signing process.
5. Security Considerations
The vulnerability is not present in systems where the use of
signedAttrs is mandatory, for example: SCEP, Certificate
Transparency, RFC 4018 firmware update, German Smart Metering CMS
data format. Any protocol that uses an EncapsulatedContentInfo
content type other than id-data is required to use signed attributes.
However, this security relies on a correct implementation of the
verification routine that ensures the correct content type and
presence of signedAttrs.
The vulnerability is not present when the message is signed and then
encrypted, as the attacker cannot learn the signature.
Conceivably vulnerable systems:
* Unencrypted firmware update denial of service
- Secure firmware updates often use signatures without
encryption. If the forged message can bring a device, due to
lack of robustness in the parser implementation, into an error
state, this may lead to a denial of service vulnerability. The
possibility of creating a targeted exploit can be excluded with
greatest certainty in this case due to the lack of control the
attacker has over the forged message.
* Dense message space
- If a protocol has a dense message space, i.e. a high
probability that the forged message represents a valid command
or the beginning of a valid command, then, especially if the
parser is permissive with respect to trailing data, there is a
risk that the message is accepted as valid. This requires a
protocol where messages are signed but not encrypted.
* Signing unstructured data
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- Protocols that sign unencrypted unstructured messages, e.g.
tokens, might be affected in that the signature of one token
might result in the corresponding forged message being another
valid token.
* External signatures over unstructured data
- The probably strongest affected class of systems would be one
that uses external signatures, i.e. CMS or PKCS#7 signatures
with absent content (that may be transmitted encrypted
separately) over unstructured data, e.g. a token of variable
length. In that case the attacker could create a signed data
object for a known secret.
* Systems with permissive parsers
- In addition to potential issues where the protocol parser is
permissive (e.g. with respect to trailing space), if the CMS
parser is permissive (e.g. allows non-protocol content types,
or allows missing signedAttrs with content types either than
id-data) then this could result in accepting invalid messages.
It is generally not good security behaviour to sign data received
from a 3rd party without first verifying that data. Section 4.2
describes just one verification step that can be performed, specific
to the vulnerability described in Section 1.
6. IANA Considerations
This document has no IANA actions.
7. References
7.1. Normative References
[RFC2119] Bradner, S., "Key words for use in RFCs to Indicate
Requirement Levels", BCP 14, RFC 2119,
DOI 10.17487/RFC2119, March 1997,
<https://www.rfc-editor.org/rfc/rfc2119>.
[RFC5652] Housley, R., "Cryptographic Message Syntax (CMS)", STD 70,
RFC 5652, DOI 10.17487/RFC5652, September 2009,
<https://www.rfc-editor.org/rfc/rfc5652>.
[RFC8174] Leiba, B., "Ambiguity of Uppercase vs Lowercase in RFC
2119 Key Words", BCP 14, RFC 8174, DOI 10.17487/RFC8174,
May 2017, <https://www.rfc-editor.org/rfc/rfc8174>.
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7.2. Informative References
[LAMPS121] Strenzke, F., "EUF-CMA for CMS SignedData", 6 November
2024, <https://datatracker.ietf.org/meeting/121/materials/
slides-121-lamps-cms-euf-cma-00>.
[RFC2633] Ramsdell, B., Ed., "S/MIME Version 3 Message
Specification", RFC 2633, DOI 10.17487/RFC2633, June 1999,
<https://www.rfc-editor.org/rfc/rfc2633>.
[RFC3126] Pinkas, D., Ross, J., and N. Pope, "Electronic Signature
Formats for long term electronic signatures", RFC 3126,
DOI 10.17487/RFC3126, September 2001,
<https://www.rfc-editor.org/rfc/rfc3126>.
[RFC3851] Ramsdell, B., Ed., "Secure/Multipurpose Internet Mail
Extensions (S/MIME) Version 3.1 Message Specification",
RFC 3851, DOI 10.17487/RFC3851, July 2004,
<https://www.rfc-editor.org/rfc/rfc3851>.
[RFC5024] Friend, I., "ODETTE File Transfer Protocol 2.0", RFC 5024,
DOI 10.17487/RFC5024, November 2007,
<https://www.rfc-editor.org/rfc/rfc5024>.
[RFC5126] Pinkas, D., Pope, N., and J. Ross, "CMS Advanced
Electronic Signatures (CAdES)", RFC 5126,
DOI 10.17487/RFC5126, March 2008,
<https://www.rfc-editor.org/rfc/rfc5126>.
[RFC5636] Park, S., Park, H., Won, Y., Lee, J., and S. Kent,
"Traceable Anonymous Certificate", RFC 5636,
DOI 10.17487/RFC5636, August 2009,
<https://www.rfc-editor.org/rfc/rfc5636>.
[RFC5655] Trammell, B., Boschi, E., Mark, L., Zseby, T., and A.
Wagner, "Specification of the IP Flow Information Export
(IPFIX) File Format", RFC 5655, DOI 10.17487/RFC5655,
October 2009, <https://www.rfc-editor.org/rfc/rfc5655>.
[RFC5751] Ramsdell, B. and S. Turner, "Secure/Multipurpose Internet
Mail Extensions (S/MIME) Version 3.2 Message
Specification", RFC 5751, DOI 10.17487/RFC5751, January
2010, <https://www.rfc-editor.org/rfc/rfc5751>.
[RFC6257] Symington, S., Farrell, S., Weiss, H., and P. Lovell,
"Bundle Security Protocol Specification", RFC 6257,
DOI 10.17487/RFC6257, May 2011,
<https://www.rfc-editor.org/rfc/rfc6257>.
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[RFC8551] Schaad, J., Ramsdell, B., and S. Turner, "Secure/
Multipurpose Internet Mail Extensions (S/MIME) Version 4.0
Message Specification", RFC 8551, DOI 10.17487/RFC8551,
April 2019, <https://www.rfc-editor.org/rfc/rfc8551>.
[RFC8572] Watsen, K., Farrer, I., and M. Abrahamsson, "Secure Zero
Touch Provisioning (SZTP)", RFC 8572,
DOI 10.17487/RFC8572, April 2019,
<https://www.rfc-editor.org/rfc/rfc8572>.
[RFC8894] Gutmann, P., "Simple Certificate Enrolment Protocol",
RFC 8894, DOI 10.17487/RFC8894, September 2020,
<https://www.rfc-editor.org/rfc/rfc8894>.
[Str23] Strenzke, F., "ForgedAttributes: An Existential Forgery
Vulnerability of CMS and PKCS#7 Signatures", 22 November
2023, <https://ia.cr/2023/1801>.
Appendix A. RFCs Using the id-data EncapsulatedContentInfo Content Type
This appendix lists RFCs which use the id-data content type in
EncapsulatedContentInfo. It is a best-effort list by the authors at
time of authorship. The list can be used as a starting point to
determine if any of BCPs in this document can be applied.
The following table summarizes the RFCs' usages of signed attributes.
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+===========+========================================+
| RFC | Signed Attributes Usage |
+===========+========================================+
| [RFC8894] | Requires the used of signed attributes |
+-----------+----------------------------------------+
| [RFC8572] | Says nothing about signed attributes |
+-----------+----------------------------------------+
| [RFC8551] | RECOMMENDS signed attributes |
+-----------+----------------------------------------+
| [RFC6257] | Forbids signed attributes |
+-----------+----------------------------------------+
| [RFC5751] | RECOMMENDS signed attributes |
+-----------+----------------------------------------+
| [RFC5655] | Says nothing about signed attributes |
+-----------+----------------------------------------+
| [RFC5636] | Forbids signed attributes |
+-----------+----------------------------------------+
| [RFC5126] | Requires signed attributes |
+-----------+----------------------------------------+
| [RFC5024] | Says nothing about signed attributes |
+-----------+----------------------------------------+
| [RFC3851] | RECOMMENDS signed attributes |
+-----------+----------------------------------------+
| [RFC3126] | Requires signed attributes |
+-----------+----------------------------------------+
| [RFC2633] | RECOMMENDS signed attributes |
+-----------+----------------------------------------+
Table 1: RFCs using id-data
An RFC requiring or forbidding signed attributes does not necessarily
mean that a recipient will enforce this requirement when verifying,
their CMS implementation may simply process the message whether or
not signed attributes are present. If one of the signed attributes
is necessary for the recipient to successfully verify the signature
or to successfully process the CMS data then the vulnerability will
not apply; at least not when assuming the signer is well-behaved and
always signs with signed attributes present in accordance with the
applicable specification.
A.1. RFC 8894 Simple Certificate Enrolment Protocol
Figure 6 in Section 3 of [RFC8894] specifies id-data as the
EncapsulatedContentInfo content type, and shows the use of
signedAttrs. The document itself never refers to signed attributes,
but instead to authenticated attributes and an
authenticatedAttributes type. Errata ID 8247 clarifies that it
should be "signed attributes" and "signedAttrs".
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Since SCEP requires the use of signedAttrs with the id-data
EncapsulatedContentInfo content type, and the recipient must process
at least some of the signed attributes, it is not affected by the
vulnerability.
A.2. RFC 8572 Secure Zero Touch Provisioning (SZTP)
Section 3.1 of [RFC8572] allows the use of the id-data content type,
although it also defines more specific content types. It does not
say anything about signed attributes.
A.3. S/MIME RFCs
[RFC8551], [RFC5751], [RFC3851], and [RFC2633] require the use of the
id-data EncapsulatedContentInfo content type.
Section 2.5 of [RFC8551] says:
Receiving agents MUST be able to handle zero or one instance of
each of the signed attributes listed here. Sending agents SHOULD
generate one instance of each of the following signed attributes
in each S/MIME message:
and
Sending agents SHOULD generate one instance of the
signingCertificate or signingCertificateV2 signed attribute in
each SignerInfo structure.
So the use of signed attributes is not an absolute requirement.
A.4. RFC 6257 Bundle Security Protocol Specification
Section 4 of [RFC6257] says:
In all cases where we use CMS, implementations SHOULD NOT include
additional attributes whether signed or unsigned, authenticated or
unauthenticated.
It does not specify what the behaviour should be if signed attributes
are found by the receiver.
A.5. RFC 5655 IP Flow Information Export (IPFIX)
[RFC5655] is a file format that uses CMS for detached signatures. It
says nothing about the use of signed attributes.
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A.6. RFC 5636 Traceable Anonymous Certificate
Appendix C.1.2 of [RFC5636] says:
The signedAttr element MUST be omitted.
It does not specify what the behaviour should be if signed attributes
are found by the receiver.
A.7. RFC 5126 CMS Advanced Electronic Signatures (CAdES)
Section 4.3.1 of [RFC5126] specifies mandatory signed attributes.
One of the signed attributes is used to determine which certificate
is used to verify the signature, so CaDES is not affected by the
vulnerability.
A.8. RFC 5024 ODETTE File Transfer Protocol 2
[RFC5024] uses the id-data EncapsulatedContentInfo content type and
says nothing about signed attributes.
A.9. RFC 3126 Electronic Signature Formats for long term electronic
signatures
Section 6.1 of [RFC3126] requires the MessageDigest attribute, which
is a signed attribute.
Acknowledgments
TODO acknowledge.
Authors' Addresses
Daniel Van Geest
CryptoNext Security
Email: daniel.vangeest@cryptonext-security.com
Falko Strenzke
MTG AG
Email: falko.strenzke@mtg.de
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