Intra-handshake.fail (CVE-2026-33697 of CVSS 7.5)
draft-intra-handshake-fail-00
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| Document | Type | Active Internet-Draft (individual) | |
|---|---|---|---|
| Author | Muhammad Usama Sardar | ||
| Last updated | 2026-07-06 | ||
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draft-intra-handshake-fail-00
agent2agent M. U. Sardar
Internet-Draft TU Dresden, Germany
Intended status: Informational 6 July 2026
Expires: 7 January 2027
Intra-handshake.fail (CVE-2026-33697 of CVSS 7.5)
draft-intra-handshake-fail-00
Abstract
The draft aims to provide technical details of CVE-2026-33697, which
is substantial technical evidence of how intra-handshake attestation
fails in practice.
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://muhammad-
usama-sardar.github.io/intra-handshake-fail/draft-intra-handshake-
fail.html. Status information for this document may be found at
https://datatracker.ietf.org/doc/draft-intra-handshake-fail/.
Source for this draft and an issue tracker can be found at
https://github.com/muhammad-usama-sardar/intra-handshake-fail.
Status of This Memo
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Copyright Notice
Copyright (c) 2026 IETF Trust and the persons identified as the
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 . . . . . . . . . . . . . . . . . . . . . . . . 2
1.1. Detailed vulnerability disclosure timeline . . . . . . . 3
1.1.1. Comparison with other vulnerabilities in confidential
computing literature . . . . . . . . . . . . . . . . 3
1.2. Overview . . . . . . . . . . . . . . . . . . . . . . . . 4
1.3. Affected Implementations . . . . . . . . . . . . . . . . 5
1.4. Binding Levels . . . . . . . . . . . . . . . . . . . . . 5
1.5. Correlation Goals . . . . . . . . . . . . . . . . . . . . 5
1.6. Main results . . . . . . . . . . . . . . . . . . . . . . 5
1.6.1. Implications of Research for IETF SEAT WG . . . . . . 6
1.6.2. Implications of Research for IETF TLS WG . . . . . . 6
1.6.3. Implications of Research for Agent2Agent . . . . . . 7
2. Conventions and Definitions . . . . . . . . . . . . . . . . . 7
3. Technical Details . . . . . . . . . . . . . . . . . . . . . . 7
3.1. Tool . . . . . . . . . . . . . . . . . . . . . . . . . . 7
3.2. Modeling . . . . . . . . . . . . . . . . . . . . . . . . 7
3.3. Technical Report . . . . . . . . . . . . . . . . . . . . 7
3.4. Artifacts . . . . . . . . . . . . . . . . . . . . . . . . 7
4. Security Considerations . . . . . . . . . . . . . . . . . . . 8
5. IANA Considerations . . . . . . . . . . . . . . . . . . . . . 8
6. References . . . . . . . . . . . . . . . . . . . . . . . . . 8
6.1. Normative References . . . . . . . . . . . . . . . . . . 8
6.2. Informative References . . . . . . . . . . . . . . . . . 8
Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . 8
Author's Address . . . . . . . . . . . . . . . . . . . . . . . . 10
1. Introduction
We _responsibly_ disclosed the vulnerability in intra-handshake
attestation -- as noted in security advisory
(https://github.com/ultravioletrs/cocos/security/advisories/GHSA-
vfgg-mvxx-mgg7) issued -- to the vendors, which resulted in CVE
([CVE-2026-33697]) of CVSS 7.5.
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1.1. Detailed vulnerability disclosure timeline
* our initial responsible disclosure to vendor: 07 Oct, 2025
* acknowledgement by vendor: 14 Dec, 2025
* information to IETF (https://mailarchive.ietf.org/arch/msg/
rats/6gbqx0XY8WYrH3Mx4vO8n2-uKgY/): 11 Jan, 2026
* public announcement (https://web.archive.org/web/20260227160554/
https://www.ultraviolet.rs/blog/tee-tls-privacy/) by vendor: 27
Feb, 2026
* security advisory issued
(https://github.com/ultravioletrs/cocos/security/advisories/GHSA-
vfgg-mvxx-mgg7): 23 March, 2026 {*Severity = HIGH (CVSS 7.8)*}
* CVE (CVE-2026-33697 (https://www.cve.org/CVERecord?id=CVE-
2026-33697)) published: 26 March, 2026 {*Severity = HIGH (CVSS
7.5)*}
1.1.1. Comparison with other vulnerabilities in confidential computing
literature
* wiretap.fail (https://wiretap.fail/files/wiretap.pdf): *No* CVE
(Intel (https://www.intel.com/content/www/us/en/security-
center/announcement/intel-security-announcement-
2025-10-28-001.html) and AMD
(https://www.intel.com/content/www/us/en/security-
center/announcement/intel-security-announcement-
2025-10-28-001.html) announcements)
* TEE.fail (https://tee.fail/files/paper.pdf): *No* CVE
* TDXdown (https://dl.acm.org/doi/10.1145/3658644.3690230): CVSS
*2.5* by Intel (https://www.intel.com/content/www/us/en/security-
center/announcement/intel-security-announcement-
2024-10-08-001.html)
* Staleus (https://xca-attacks.github.io/staleus/
staleus_usenix26.pdf): CVE-2025-54509
(https://nvd.nist.gov/vuln/detail/CVE-2025-54509): CVSS *4.0*
* BreakFAST (https://xca-attacks.github.io/breakfast/
breakfast_oakland26.pdf): CVE-2025-61972
(https://nvd.nist.gov/vuln/detail/CVE-2025-6197): CVSS *4.2* and
CVE-2025-61971 (https://nvd.nist.gov/vuln/detail/CVE-2025-61971):
CVSS *5.9*
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* BadRAM (https://badram.eu/badram.pdf): CVSS *5.3* by AMD
(https://www.amd.com/en/resources/product-security/bulletin/amd-
sb-3015.html)
* Fabricked (https://xca-attacks.github.io/fabricked/
fabricked_usenix26.pdf): CVE-2025-54510
(https://nvd.nist.gov/vuln/detail/cve-2025-54510): CVSS *5.9*
The comparison of the above with CVSS *7.5* for
[Intra-handshake.fail] indicates that attested TLS is not mature yet
compared to the rest of the confidential computing stack, and is
currently one of the weakest links in the ecosystem.
Further formal analysis of *production* implementation of intra-
handshake attestation has led to discovery of another class of
attacks and will potentially lead to three CVEs (currently under
_responsible_ disclosure) each with an expected *CVSS 9.1*.
1.2. Overview
This draft presents the formal specification and analysis of the
candidate binding mechanisms for binding in intra-handshake
attestation for standardization for attested TLS protocols:
1. Client’s TLS nonce: used in Meta's AI (https://ai.meta.com/
static-resource/private-processing-technical-whitepaper)
2. Client’s attestation nonce
3. Early exporter
4. Server’s public key
5. Combination of #2 and #3
6. Combination of #2 and #4: used in: Edgeless Systems Contrast
(https://github.com/CCC-Attestation/meetings/blob/main/materials/
MarkusRudy.contrast-atls-ccc-attestation.pdf), Cocos AI
(https://www.ultraviolet.rs/products/cocos-ai/), and CCC
Attestation SIG's adopted project attested TLS proof of concept
(https://github.com/ccc-attestation/attested-tls-poc)
7. Combination of #2, #3, and #4: proposed in draft-fossati-tls-
attestation-06 (https://www.ietf.org/archive/id/draft-fossati-
tls-attestation-06.html)
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We provide a formal proof of insecurity of all the above candidate
binding mechanisms of intra-handshake attestation using the
state-of-the-art tool ProVerif and propose a mitigation for the
discovered security vulnerabilities. Our study reveals that it may
not be possible to achieve strong application-traffic (level 3)
binding using intra-handshake attestation alone.
1.3. Affected Implementations
* Meta's AI (https://ai.meta.com/static-resource/private-processing-
technical-whitepaper)
* Cocos AI (https://github.com/ultravioletrs/cocos)
* Edgeless Systems Contrast (https://github.com/edgelesssys/
contrast)
* CCC Attestation SIG's adopted project attested TLS proof of
concept (https://github.com/ccc-attestation/attested-tls-poc)
1.4. Binding Levels
1. DH shared secret
2. Handshake traffic key
3. Application traffic key
1.5. Correlation Goals
We consider TLS Server as RATS Attester, which is typical in
confidential computing.
1. Correlation of Evidence to a DH Shared Secret (G1)
2. Correlation of Evidence to Client’s Handshake Traffic Key (G2)
3. Correlation of Evidence to Client’s Application Traffic Key (G3)
1.6. Main results
* All analyzed binding mechanisms and the corresponding
implementations of intra-handshake attestation are vulnerable to
relay attacks.
* Early exporter helps achieve level 1 binding.
* Our proposed mechanism helps achieve level 2 binding.
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* It may not be possible to achieve level 3 in intra-handshake
attestation alone without additional assumptions.
+====================+====================+===========+===========+
| Property | Mechanism #1,2,4,6 | Mechanism | Proposed |
| | | #3,5,7 | mechanism |
+====================+====================+===========+===========+
| G1 : Correlation | ❌ | ✅ | ✅ |
| of Evidence to gxy | | | |
+--------------------+--------------------+-----------+-----------+
| G2 : Correlation | ❌ | ❌ | ✅ |
| of Evidence to kch | | | |
+--------------------+--------------------+-----------+-----------+
| G3 : Correlation | ❌ | ❌ | ❌ |
| of Evidence to kc | | | |
+--------------------+--------------------+-----------+-----------+
Table 1
1.6.1. Implications of Research for IETF SEAT WG
* We believe post-handshake attestation alone, such as draft-
fossati-seat-expat (https://datatracker.ietf.org/doc/draft-
fossati-seat-expat/), can achieve level 3 binding.
* The research suggests that recent hybrid proposals (combination of
intra-handshake attestation and post-handshake attestation) draft-
fossati-seat-early-attestation (https://datatracker.ietf.org/doc/
draft-fossati-seat-early-attestation/04/) and draft-ritz-seat-
facts (https://datatracker.ietf.org/doc/draft-ritz-seat-facts/00/)
may add unnecessary complexity of intra-handshake attestation
without adding any security benefit compared to post-handshake
attestation alone, such as draft-fossati-seat-expat
(https://datatracker.ietf.org/doc/draft-fossati-seat-expat/).
1.6.2. Implications of Research for IETF TLS WG
* Remote attestation _within_ the handshake is very dangerous, since
to our knowledge, it is one of the highest scored vulnerabilities
in confidential computing literature (see this
(https://github.com/CCC-Attestation/formal-spec-KBS#comparison-
with-other-vulnerabilities-in-confidential-computing-literature)).
Given the high-severity vulnerabilities, the developers and maintainers
of intra-handshake attestation MUST urgently move to post-handshake
attestation.
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1.6.3. Implications of Research for Agent2Agent
Intra-handshake attestation does more damage than protection for AI
agents.
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. Technical Details
3.1. Tool
We use state-of-the-art symbolic security analysis tool ProVerif
(https://ieeexplore.ieee.org/document/9833653) for the specification
of the protocols.
3.2. Modeling
The formal model uses the fixed version of diversion attacks in
intra-handshake attestation (https://github.com/CCC-Attestation/
formal-spec-id-crisis/tree/main/TLS-a/fix) from our previous work as
the starting point to focus on relay attacks in intra-handshake
attestation in this work. The rationale is that we consider it more
useful to show the added value of this contribution to the community
by using the fixed version of diversion attacks in intra-handshake
attestation (https://github.com/CCC-Attestation/formal-spec-id-
crisis/tree/main/TLS-a/fix) as the baseline, rather than showing the
same diversion attacks from ID-Crisis paper (https://dl.acm.org/
doi/10.1145/3779208.3785387), and the discovered CVE
([CVE-2026-33697]) -- which the previous analysis could not find --
practically demonstrates the added value. This modeling choice makes
it clear that even with the diversion attacks fixed, high-severity
relay attacks would still remain in intra-handshake attestation.
3.3. Technical Report
Technical report is available at [Intra-handshake.fail].
3.4. Artifacts
Artifacts are available at [Intra-handshake.fail-repo] under Apache-
2.0 License.
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4. Security Considerations
All of this document is about the insecurity of intra-handshake
attestation.
5. IANA Considerations
This document has no IANA actions.
6. References
6.1. 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>.
[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>.
6.2. Informative References
[CVE-2026-33697]
CVE, "CoCoS attested TLS is vulnerable to relay attacks
via extracted ephemeral TLS keys", March 2026,
<https://www.cve.org/CVERecord?id=CVE-2026-33697>.
[Intra-handshake.fail]
Sardar, M. U., Dubeyko, V., and J.-M. Jacquet, "Intra-
handshake.fail (CVE-2026-33697): High-severity CVE in
Attested TLS", September 2026,
<https://www.researchgate.net/publication/408219182_Intra-
handshakefail_CVE-2026-33697_High-
severity_CVE_in_Attested_TLS>.
[Intra-handshake.fail-repo]
Sardar, M. U., Dubeyko, V., and J.-M. Jacquet, "Intra-
handshake.fail (CVE-2026-33697): High-severity CVE in
Attested TLS", June 2026,
<https://github.com/CCC-Attestation/formal-spec-KBS>.
Acknowledgments
We would like to thank our co-authors of paper for their valuable
contributions:
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* Viacheslav Dubeyko
* Jean-Marie Jacquet
We gratefully acknowledge the following for insightful discussions on
this work:
* Eric Rescorla
* Juho Forsén
* Markus Rudy
* Mariam Moustafa
* Bruno Blanchet
* Steve Kremer
* Tjaden Hess
* Martin Thomson
* Yuning Jiang
* Pavel Nikonorov
* Casey Wilson
* Danko Miladinovic
* Songbo Bu
* Nathanael Ritz
We also gratefully acknowledge the following who gave feedback on
previous state-of-the-art (https://github.com/CCC-Attestation/formal-
spec-id-crisis) that we utilize as the basis:
* Tuomas Aura
* Ionut Mihalcea
* Thomas Fossati
* Hannes Tschofenig
* Yaron Sheffer
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* Laurence Lundblade
* Giridhar Mandyam
* Christopher Patton
* Jonathan Hoyland
* Richard Barnes
Several others at the IETF, IRTF, and CCC have contributed by
providing feedback.
We sincerely thank Karthikeyan Bhargavan, Bruno Blanchet, and Nadim
Kobeissi for the foundational formal model of draft 20 of TLS 1.3 in
their work (https://ieeexplore.ieee.org/document/7958594).
Author's Address
Muhammad Usama Sardar
TU Dresden, Germany
Email: muhammad_usama.sardar@tu-dresden.de
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