]> Intra-handshake Attestation Considered Harmful (CVE-2026-33697 of CVSS 7.5 and several other CVEs of up to expected CVSS 9.8 upcoming) TU Dresden, Germany
muhammad_usama.sardar@tu-dresden.de
Shanghai Guan An Information Technology Co., Ltd., China
bluedognull@gmail.com
Independent
aurestarnull@gmail.com
Shanghai Guan An Information Technology Co., Ltd., China
havan12050544@gmail.com
SEAT AI agents Intra-handshake attestation CVE-2026-33697 The draft aims to provide technical details of CVE-2026-33697 and EUVD-2026-16488, which is substantial technical evidence of how intra-handshake attestation fails in practice, even without physical access. Moreover, since continuous attestation is generally required, intra-handshake attestation adds unnecessary complexity. The results are backed by the research and the artifacts in state-of-the-art tool, ProVerif, under Apache-2.0 license for reproducibility, and have been acknowledged by the relevant stakeholders. About This Document The latest revision of this draft can be found at . Status information for this document may be found at . Source for this draft and an issue tracker can be found at .
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: 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 responsibly disclosed the vulnerability in intra-handshake attestation -- as noted in security advisory issued -- to the vendors, which resulted in of CVSS 7.5.
Credits CVEs and finders
CVE Finders
CVE-2026-33697 Muhammad Usama Sardar, Viacheslav Dubeyko, and Jean-Marie Jacquet
Threat Model The threat model is explained in Sec. 6.1 of .
Detailed Vulnerability Disclosure Timeline and Public Acknowledgements by Affected Vendors 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 published [Severity = HIGH (CVSS 7.5)] 26 March, 2026
ERISA published EUVD [Severity = HIGH (CVSS 7.5)] 26 March, 2026
Acknowledgment by Privasys for rustls [Severity = HIGH (CVSS 7.5)] 9 July, 2026
Acknowledgment by Privasys for go [Severity = HIGH (CVSS 7.5)] 10 July, 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
EU ERISA European Union's ERISA has independently published with CVSS 7.5 to acknowledge this vulnerability.
Comparison with Other Vulnerabilities in Confidential Computing Literature Severity is based on NIST metrics. 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 7.5 High
The comparison of the above with CVSS 7.5 for 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.
More CVEs Further formal analysis has led to the following potential CVEs for intra-handshake attestation (currently under disclosure):
  • 1 potential CVE of expected CVSS 9.8
  • 2 potential CVEs of expected CVSS 9.1
  • 1 potential CVE of expected CVSS 8.7
  • 2 potential CVEs of expected CVSS 7.5
  • 2 potential CVEs of expected CVSS 7.4
  • 2 potential CVEs of expected CVSS 6.3
These are preliminary estimates of scores, not final assigned score. They are still under review.
Vulnerable Implementations At least the following implementations are vulnerable:
  • Meta's AI: and [Severity = HIGH (CVSS 7.5)]
  • Cocos AI: security advisory [Severity = HIGH (CVSS 7.8)], and [Severity = HIGH (CVSS 7.5)]
  • Edgeless Systems Contrast: security advisory [Severity = HIGH (CVSS 7.4)]
  • CCC Attestation SIG's adopted project intra-handshake attestation: declared vulnerable to relay attacks and archived
  • Privasys rustls: Acknowledgment of applicability of [Severity = HIGH (CVSS 7.5)]
  • Pirvasys go: Acknowledgment of applicability of [Severity = HIGH (CVSS 7.5)]
If you are aware of any other intra-handshake attestation implementation, please let us know so that we can check and responsibly disclose the vulnerabilities to them.
Vulnerable Protocol Specifications At least the following protocol specifications with intra-handshake attestation path are vulnerable to and :
  • draft-fossati-tls-attestation: symbolic proof of insecurity; draft withdrawn after the CVE
  • draft-fossati-seat-early-attestation: symbolic and (paper-and-pen-based) computational proof of insecurity (originally done for -04 and applies also to -06)
    • As a SEAT WG participant pointed out, please note that both and contain a link to security advisory that contains a link to our SEAT email that contains the G3 property (cf. ) that this draft does not satisfy.
    • Some WG participants successfully reproduced the vulnerability by substituting the right value of rdata in the shared formal model that led to the CVE.
    • An informal reasoning is that binder is not directly derived from any shared secret in this draft.
  • draft-ritz-seat-facts: symbolic proof of insecurity
    • violates G3 property in our analysis
    • unnecessary complexity is itself a security concern
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
Please see Sec. 6.2 of for details.
Security Properties (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)
Please see Sec. 6.3 of for details.
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.
  • It may not be possible to achieve level 3 in intra-handshake attestation alone without additional assumptions.
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 ❌ ❌ ❌
Please see Sec. 7.1 and Figure 5 of for details of attacks.
Expected Results 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
Implications of Findings
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.
  • As demonstrated by our symbolic analysis using ProVerif, the protocol specifications draft-fossati-seat-early-attestation and draft-ritz-seat-facts remain vulnerable to CVE-2026-33697. We have also proved that draft-fossati-seat-early-attestation-04 and draft-fossati-seat-early-attestation-06 violate the security theorems in the computational model.
Implications of Findings for IETF LAKE WG
  • Similar problems occur for protocol specification lake-ra.
Implications of Findings for IETF TLS WG
  • draft-fossati-tls-attestation-09 is vulnerable to CVE-2026-33697. Thankfully, the authors have withdrawn draft-fossati-tls-attestation-10.
  • Remote attestation within the handshake is very dangerous, since to our knowledge, it is one of the highest scored published vulnerabilities in confidential computing literature (see ).
Implications of Findings for Agent2Agent From a security perspective, intra-handshake attestation does more damage than protection for AI agents.
Technical Details
Tool We use state-of-the-art symbolic security analysis tool ProVerif for the specification of the protocols.
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 () -- 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. Note: Similar to the fixed version of diversion attacks in intra-handshake attestation from our previous work, we model non-PSK-based handshake. From ID-Crisis paper:
  • For modeling TLS 1.3, we consider handshakes based on Diffie-Hellman over either finite fields or elliptic curves, represented as (EC)DHE. This is because we are unaware of any publicly available specification or implementation of attested TLS with PSK-based handshakes.
While it would be nice to model PSK-based handshake, the rationale is that the correlation properties studied in this work do not necessarily require it. Note: The artifacts consider the case of server authentication only, as client authentication is optional in TLS 1.3. No claims are made about other configurations.
Technical Report Technical report is available at . It is accepted for publication at ESORICS 2026.
Vulnerabilities Sec. 7.1 of presents the technical details with abstract attack traces of the vulnerabilities.
Mitigation Sec. 7.2 of presents the technical details of the proposed mitigation.
Artifacts Artifacts are available at under Apache-2.0 License.
Media Coverage Several media enthusiasts and bloggers have covered the vulnerabilities to protect the community from the harm of intra-handshake attestation.
  • The Register
  • (Japanese) BlackHatNewsTokyo
  • (Several languages) Hackernoon
  • Apple podcast
  • Information Security News
  • TheNextGenTechInsider
  • DailySecurityReview
  • SC World
  • 01 Quantum
  • (Russian) Security Lab
  • (German) blogspan
  • (Chinese) Sina
  • data4biz
  • (Russian) ITSec
  • (Chinese) smzdm
  • (Chinese) donews
  • (Chinese) ifeng
  • dugganusa
  • dugganusa repo
  • spoitus
  • lavx news
  • sohu
  • (Persian) news.ditty
  • (Russian) LiMP VPN
  • daily.dev
  • warden
  • GCVE.eu
  • coderlegion
  • Anjuna Security
  • freenode
  • (Chinese) csdn
  • osintsights
  • (Turkish) hardwaremania
  • akber
  • ad-hoc news
  • AIMultiple
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"
Security Researchers Several credible security researchers, such as the following, have publicly attested to it.
  • Michael Pak
  • Rodrigo Branco
  • Bart Preneel
  • Thorsten Strufe
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 refers to Confidential Computing, and attested TLS is the core trust mechanism of CC.
Reviews
Conference Reviews has been peer-reviewed and accepted for publication at ESORICS 2026.
IETF/IRTF Several participants of the IETF/IRTF have attested to the results by independently reproducing the results and reviewing the code. Some of the participants have independently reproduced the results by developing their own formal models and a proof-of-concept implementation of the vulnerabilities. Some of the messages are mentioned below:
  • https://mailarchive.ietf.org/arch/msg/seat/B7F1Dj_rjs8I0Kg3yCp3Rap0XeE/
  • https://mailarchive.ietf.org/arch/msg/seat/aEV9dUFotAQzHndk23qBcwBT3as/
  • https://mailarchive.ietf.org/arch/msg/seat/3Hv0E1sfXsvyBtl6AgY8j-SHHiw/
  • https://mailarchive.ietf.org/arch/msg/seat/5LJ6i9svomnhpyPHWPejM7fMmXQ/
  • https://mailarchive.ietf.org/arch/msg/seat/V_YqGUY3fEwaFwwyfA9DshpHet0/
  • https://mailarchive.ietf.org/arch/msg/seat/JF_cwmHHEbrJ_W5V6yEetWWnii4/
  • https://mailarchive.ietf.org/arch/msg/seat/P_CYTycg0KG7cbKauFA-kVgX2jo/
  • https://mailarchive.ietf.org/arch/msg/seat/ZJjJXpYwZ5nCVmz_W4FK6XiFEY4/
  • https://mailarchive.ietf.org/arch/msg/seat/4so3LxHOOXHS1wnvhuoeWWgeCHk/
  • https://mailarchive.ietf.org/arch/msg/seat/Q6Jmc58v0c1lDV3ujIY0AX_ofGA/
  • https://mailarchive.ietf.org/arch/msg/seat/n4Me5QPCvwhxcEJndWePyishcoo/
  • https://mailarchive.ietf.org/arch/msg/seat/beRzNNvwMifkRfJPfxecGoHpTDs/
  • https://mailarchive.ietf.org/arch/msg/cfrg/U5YHd91lYjiqCTt9BZyVDNFeUpM/
  • https://mailarchive.ietf.org/arch/msg/seat/aFCo4BMRDSUynvN9AQJatPjnXag/
  • https://mailarchive.ietf.org/arch/msg/seat/wb_Ys9MZd9u9oM2Bk-8tv7fvXGg/
  • https://mailarchive.ietf.org/arch/msg/seat/ov8f-7cZKK5RZ-Mmjc6IhVSB-Fk/
  • https://mailarchive.ietf.org/arch/msg/seat/2uUuaD1DygjNDM4GT_-rYbwTiJk/
  • https://mailarchive.ietf.org/arch/msg/seat/pB39abN1QrH4_ATM_E78vxPTuxk/
  • https://mailarchive.ietf.org/arch/msg/seat/PxKCxMHe-SAiR9uhOOllrK4mUA4/
  • https://mailarchive.ietf.org/arch/msg/seat/T1xupUBwqYEBSHCTXgSHXZtdqz8/
  • https://mailarchive.ietf.org/arch/msg/seat/hRw46FwgmVdi9fqZm2fjKbln_IA/
  • https://mailarchive.ietf.org/arch/msg/seat/UG7yE_klmRSxNy2HX6fzuFonDjM/
  • https://mailarchive.ietf.org/arch/msg/seat/2hpeIldeFfE6o9q6L9Vkt00ACKA/
  • https://mailarchive.ietf.org/arch/msg/seat/gc2ij0vboehS_-v10-SNslxaZC0/
  • https://mailarchive.ietf.org/arch/msg/seat/oO4mAfq5HJZptDDNrSnd7zDdX18/
  • https://mailarchive.ietf.org/arch/msg/seat/XuJc_yEJPCMIuYcv2OM7XDogRCU/
  • https://mailarchive.ietf.org/arch/msg/seat/nVHlnbFIEh-cPQeMuDVOqx5YvWQ/
  • https://mailarchive.ietf.org/arch/msg/seat/xVU3C7qUOngcip7B4ZO5MJUT9Xg/
  • https://mailarchive.ietf.org/arch/msg/seat/1gCcPw-7NopDRzzBzA3dFIgo3Rs/
  • https://mailarchive.ietf.org/arch/msg/seat/t8aobzB374lWiLzrVrORY7kGYyQ/
  • https://mailarchive.ietf.org/arch/msg/seat/m3UyB6XLQzxaucejE_o8Pn41uSI/
  • https://mailarchive.ietf.org/arch/msg/seat/gqHqcbbKva_oGE-jEDZu243gf-4/
  • https://mailarchive.ietf.org/arch/msg/seat/QD8QB1WVL-toNovGQ2Tk6DmmeEM/
  • https://mailarchive.ietf.org/arch/msg/seat/vXN2pifZ5GXcC1xLwSfLCnUcFUE/
  • https://mailarchive.ietf.org/arch/msg/seat/MGFXinb85XSaLkqjBEhmBZC7PcI/
  • https://mailarchive.ietf.org/arch/msg/seat/js9VI4PB8yYmhg2ObaZB1a22fL4/
  • https://mailarchive.ietf.org/arch/msg/seat/0RzORzX_VdlY5UQ_MWMmxZlnrjs/
  • https://mailarchive.ietf.org/arch/msg/seat/W3MH1BDSUbm1WUPxGQihaIc1zTk/
  • https://mailarchive.ietf.org/arch/msg/seat/2_aGylmFHoLmqN7BBcYVoH-rNJk/
  • https://mailarchive.ietf.org/arch/msg/seat/VyifG8zP5aworb_S1NR9FEUEXW0/
  • https://mailarchive.ietf.org/arch/msg/seat/CYwvM75z6rTId2A3ZZvZJmHxOig/
  • https://mailarchive.ietf.org/arch/msg/ufmrg/29xFZX5C4oSGkpZAvXT_7YLW2Vc/
  • https://mailarchive.ietf.org/arch/msg/seat/u1HxYW9cJfVpi3Cf9q06ehwpYGE/
  • https://mailarchive.ietf.org/arch/msg/seat/SG_A0016a-KMnXAkGtMUxokZmjc/
  • https://mailarchive.ietf.org/arch/msg/seat/3w7-OW2CAVr0-QBz97eAMxB_nMI/
Main Questions In short, four main questions have been raised by WG participants in support of our work:
  • What security property hybrid (intra- + post-handshake attestation) provides that post-handshake attestation alone cannot provide?
  • Since continuous attestation is required in most use cases, how is additional complexity of intra-handshake attestation justified? Use cases with one-time attestation can be covered by doing attestation round immediately after Connection Establishment Time: see reference.
  • What is the benefit of doing signatures of remote attestation within the handshake (as this latency can be exploited)? We add that verification of signatures is also time consuming, which can be exploited too. See reference.
  • How evidence is bound to the secure channel without involving any shared secret?
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.
Security Considerations All of this document is about the insecurity of intra-handshake attestation. By no means should the vendors mentioned in this draft be considered less secure than any other vendors implementing intra-handshake attestation solutions. In particular, those who have closed-source implementations are most likely more vulnerable than the open-source ones, since the former cannot easily be reviewed by the security community. Even extensive security reviews -- of closed-source implementations -- by cybersecurity firms often do not perform formal analysis, and thus such reviews may miss corner cases and subtle vulnerabilities.
Ethical Considerations We (i.e., the super set of all authors involved in this research) are ethical researchers aiming to protect the community from the potential harm caused by the exploitability of the vulnerabilities in intra-handshake attestation. We have responsibly disclosed the vulnerabilities to the respective developers and maintainers following their respective disclosure processes and provided them our proposed mitigations and requested them to take rapid action. We have released only the formal analysis for published CVE. To minimize exploit in the wild, we have not publicly released the proof-of-concept exploit code. We have not retrieved any real data from any real system. We have not released any key to any public forum or to any person. To the best of our abilities, knowledge, and understanding, we have tried to explain the vulnerabilities to the authors of vulnerable drafts draft-fossati-tls-attestation, draft-fossati-seat-early-attestation, and draft-ritz-seat-facts for at least half a year at several forums, including but not limited to CCC Attestation SIG and IETF/IRTF. Please see the (non-exhaustive list of) recordings and the archives. We sincerely thank the authors of draft-fossati-tls-attestation for withdrawing their draft to protect further exploits.
IANA Considerations This document has no IANA actions.
Normative References Intra-handshake.fail (CVE-2026-33697): High-severity CVE in Attested TLS Intra-handshake.fail (CVE-2026-33697): High-severity CVE in Attested TLS CoCoS attested TLS is vulnerable to relay attacks via extracted ephemeral TLS keys CVE CoCoS attested TLS is vulnerable to relay attacks via extracted ephemeral TLS keys ENISA
Acknowledgments We would like to thank our co-authors of paper for their valuable contributions:
  • Viacheslav Dubeyko
  • Jean-Marie Jacquet
We gratefully acknowledge the following for insightful discussions and reviews 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
  • John Preuß Mattsson
  • Britta Hale
  • Werner Staub
  • Haowen Song
  • Chengxin Huang
  • Steve Luo
  • Kubilay Ahmet Küçük
  • Iman Schrock
  • Sophie Schmieg
  • Jakub Maria Plutowski
  • Patrick Duggan
  • Nathanael Ritz
  • Deb Cooley
We also gratefully acknowledge the following who gave feedback on previous state-of-the-art that we utilize as the basis:
  • Tuomas Aura
  • Ionut Mihalcea
  • Thomas Fossati
  • Hannes Tschofenig
  • Yaron Sheffer
  • Laurence Lundblade
  • Giridhar Mandyam
  • Christopher Patton
  • Jonathan Hoyland
  • Richard Barnes
Several others at the IETF, IRTF, CCC, and GA4GH 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. Muhammad Usama Sardar is funded by German Research Foundation ("Deutsche Forschungsgemeinschaft.")