Internet-Draft TODO - Abbreviation July 2026
Sardar Expires 1 February 2027 [Page]
Workgroup:
SEAT
Internet-Draft:
draft-intra-handshake-fail-01
Published:
Intended Status:
Informational
Expires:
Author:
M. U. Sardar
TU Dresden, Germany

Intra-handshake Attestation Considered Harmful (CVE-2026-33697 of CVSS 7.5)

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. Moreover, since continuous attestation is required, intra-handshake attestation adds unnecessary complexity. The results are backed by the research [Intra-handshake.fail] and the ProVerif artifacts [Intra-handshake.fail-repo] under Apache-2.0 license for reproducibility.

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

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/.

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."

This Internet-Draft will expire on 1 February 2027.

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Table of Contents

1. Introduction

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:

Table 1: Binding mechanisms, implementations and ProVerif artifacts
No. Binding mechanism Used in Artifacts
1. Client’s TLS nonce Meta's AI binder1
2. Client’s attestation nonce - binder2
3. Early exporter - binder3
4. Server’s public key - binder4
5. Combination of #2 and #3 - binder5
6. Combination of #2 and #4 Edgeless Systems Contrast; Cocos AI; CCC Attestation SIG's adopted project intra-handshake attestation binder6
7. Combination of #2, #3, and #4 draft-fossati-tls-attestation-06 binder7 
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.

We responsibly disclosed the vulnerability in intra-handshake attestation -- as noted in security advisory issued -- to the vendors, which resulted in [CVE-2026-33697] of CVSS 7.5.

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. Credits

We discovered the vulnerability jointly with Viacheslav Dubeyko and Jean-Marie Jacquet.

4. Detailed Vulnerability Disclosure Timeline and Acknowledgements by Affected Vendors

Table 2: Detailed vulnerability disclosure timeline and acknowledgements
Event Date
Our initial responsible disclosure to vendor 07 Oct, 2025
Acknowledgement by vendor 14 Dec, 2025
Information to the IETF 11 Jan, 2026
Public announcement by vendor 27 Feb, 2026
Cocos AI published security advisory [Severity = HIGH (CVSS 7.8)] 23 March, 2026
CVE (CVE-2026-33697) published [Severity = HIGH (CVSS 7.5)] 26 March, 2026
CCC implementation declared vulnerable to relay attacks 17 July, 2026
Vulnerable CCC implementation repo archived 22 July, 2026
Vulnerable draft draft-fossati-tls-attestation withdrawn by authors 23 July, 2026
Edgeless Systems published security advisory [Severity = HIGH (CVSS 7.4)] 29 July, 2026

5. Comparison with Other Vulnerabilities in Confidential Computing Literature

Severity is based on NIST metrics.

Table 3: Comparison with other vulnerabilities in confidential computing literature
Vulnerability CVE CVSS Severity
wiretap.fail No CVE (Intel and AMD announcements) - None
TEE.fail No CVE - None
TDXdown Intel 2.5 Low
Staleus CVE-2025-54509 4.0 Medium
BreakFAST CVE-2025-61972 4.2 Medium
BadRAM AMD 5.3 Medium
BreakFAST CVE-2025-61971 5.9 Medium
Fabricked CVE-2025-54510 5.9 Medium
Intra-handshake.fail CVE-2026-33697 7.5 High

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.

6. Affected Implementations

At least the following implementations are affected:

7. Binding Levels

  1. DH shared secret (gxy) used as shared secret between client and server

  2. Handshake traffic key (htsc) used for encryption of handshake messages

  3. Application traffic key (astc) used for encryption of application data

8. 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)

9. Main Results

Table 4: Main results
Property Mechanism #1,2,4,6 Mechanism #3,5,7 Proposed mechanism
G1 : Correlation of Evidence to gxy ❌ ✅ ✅
G2 : Correlation of Evidence to kch ❌ ❌ ✅
G3 : Correlation of Evidence to kc ❌ ❌ ❌

9.1. Expected Results

Table 5: Expected results
No. Binding mechanism Artifacts Expected results
1. Client’s TLS nonce binder1 binder1
2. Client’s attestation nonce binder2 binder2
3. Early exporter binder3 binder3
4. Server’s public key binder4 binder4
5. Combination of #2 and #3 binder5 binder5
6. Combination of #2 and #4 binder6 binder6
7. Combination of #2, #3, and #4 binder7 binder7
8. Proposed proposal proposal

10. Implications of Findings

10.1. Implications of Findings for IETF SEAT WG

  • We believe post-handshake attestation alone, such as 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 and draft-ritz-seat-facts 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. We are not aware of any security property that hybrid proposals can achieve that post-handshake attestation alone cannot achieve.

10.3. Implications of Findings 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 Section 5).

Given the high-severity vulnerabilities, we recommend that the
developers and maintainers of intra-handshake attestation MUST
urgently move to post-handshake attestation.

10.4. Implications of Findings for Agent2Agent

Intra-handshake attestation does more damage than protection for AI agents.

11. Technical Details

11.1. Tool

We use state-of-the-art symbolic security analysis tool ProVerif for the specification of the protocols.

11.2. Modeling

The formal model uses the fixed version of diversion attacks in intra-handshake attestation 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 as the baseline, rather than showing the same diversion attacks from ID-Crisis paper, 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.

11.3. Technical Report

Technical report is available at [Intra-handshake.fail]. It is accepted for publication at ESORICS 2026.

11.4. Artifacts

Artifacts are available at [Intra-handshake.fail-repo] under Apache-2.0 License.

12. Media Coverage

Several media enthusiasts have covered the vulnerabilities to protect the community from the harm of intra-handshake attestation.

If you have written an article on this and would like to be added here, please send us a PR or an email with the subject "media coverage of intra-handshake.fail"

12.1. Security Researchers

Credible security researchers, such as the following, have attested to it.

12.2. Germany's BSI

Germany's Federal Office for Information Security (Bundesamt für Sicherheit in der Informationstechnik) has attested to it. Carina Hilt, deputy press spokesperson at BSI, told The Register:

CC alone cannot satisfy the requirements for digital sovereignty.

dependencies on other services, such as identity and key
management etc., are also not mitigated by CC.

13. Reviews

13.1. Conference Reviews

[Intra-handshake.fail] has been peer-reviewed and accepted for publication at ESORICS 2026.

13.3. Researchers outside of IETF/IRTF

Some researchers have approached us confirming the proof-of-concept of the vulnerabilities in intra-handshake attestation. More information will be added once their pre-prints/papers are public.

14. Security Considerations

All of this document is about the insecurity of intra-handshake attestation.

15. IANA Considerations

This document has no IANA actions.

16. References

16.1. Normative References

[RFC2119]
Bradner, S., "Key words for use in RFCs to Indicate Requirement Levels", BCP 14, RFC 2119, DOI 10.17487/RFC2119, , <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, , <https://www.rfc-editor.org/rfc/rfc8174>.

16.2. Informative References

[CVE-2026-33697]
CVE, "CoCoS attested TLS is vulnerable to relay attacks via extracted ephemeral TLS keys", , <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", , <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", , <https://github.com/muhammad-usama-sardar/intra-handshake.fail>.

Acknowledgments

We would like to thank our co-authors of paper for their valuable contributions:

We gratefully acknowledge the following for insightful discussions on this work:

We also gratefully acknowledge the following who gave feedback on previous state-of-the-art that we utilize as the basis:

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.

Author's Address

Muhammad Usama Sardar
TU Dresden, Germany