# cryspen libcrux-ml-dsa signature verification bypass on AVX2 platforms

Source: https://junglewise.ai/threats/cryspen-libcrux-ml-dsa-signature-verification-bypass-on-avx2-platforms-ba901d71  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-05-19. Updated: 2026-05-19  
License: CC BY 4.0 with a link back

Severity: high · CVSS 8.2 · Published 2026-05-19

libcrux-ml-dsa is a Rust library providing post-quantum cryptographic digital signature algorithms. A flaw in its high-performance AVX2 implementation causes the library to incorrectly verify certain digital signatures. An attacker could exploit this to make the system accept a forged or invalid signature, potentially allowing them to bypass security checks or impersonate authorized users.

Technologies: [libcrux-ml-dsa (crates.io)](https://junglewise.ai/threats/technologies/libcrux-ml-dsa). Vendors: [crates.io](https://junglewise.ai/threats/vendors/crates-io).

## Quick answers

**Is This vulnerability exploited in the wild?** No. Junglewise Threat Intelligence has no report of This vulnerability being exploited in the wild.

**How severe is This vulnerability?** High severity, with a CVSS score of 8.2, published 19 May 2026.

**Which products and versions are affected?** libcrux-ml-dsa (crates.io) < 0.0.9.

**Is there a fix for This vulnerability?** A patch was published on 2026-05-19: Fixed in version 0.0.9 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is This vulnerability attacked?** It is attacked over the network, in a software library.

**Am I affected?** Check whether you run libcrux-ml-dsa (crates.io) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

A vulnerability exists in the AVX2-optimized implementation of ML-DSA (Module-Lattice-Based Digital Signature Standard) within the libcrux-ml-dsa crate. The root cause is an incorrect implementation of the `use_hint` function (Algorithm 40 in FIPS 204), specifically regarding the handling of the edge case where the remainder $r_0$ equals 0. While the specification requires $r_0 \le 0$ to trigger a specific modular reduction, the AVX2 implementation using the `vec256_blendv_epi32` intrinsic only checked for $r_0 < 0$. This discrepancy allows an attacker to craft an invalid signature under a maliciously generated verification key that the verifier will incorrectly accept as valid. The issue is fixed in version 0.0.9 by correctly masking the comparison to include the $r_0 = 0$ case.

## Affected products

- cryspen libcrux-ml-dsa < 0.0.9

## Timeline

- 2026-04-02: other: Similar issue identified in Tink-Go implementation
- 2026-05-05: other: Fix submitted to libcrux repository
- 2026-05-19: advisory: GitHub Advisory published
- 2026-05-19: patched: Fixed in version 0.0.9

## References

- [https://github.com/C2SP/wycheproof/pull/234](https://github.com/C2SP/wycheproof/pull/234)
- [https://github.com/cryspen/libcrux/pull/1398](https://github.com/cryspen/libcrux/pull/1398)
- [https://github.com/tink-crypto/tink-go/pull/48](https://github.com/tink-crypto/tink-go/pull/48)
- [https://rustsec.org/advisories/RUSTSEC-2026-0125.html](https://rustsec.org/advisories/RUSTSEC-2026-0125.html)

## Related threats

- [RUSTSEC-2026-0125 - Signature Verification on AVX2 Platforms Mishandles Edge Case](https://junglewise.ai/threats/rustsec-2026-0125-signature-verification-on-avx2-platforms-mishandles-af596627)
- [RUSTSEC-2026-0126 - AVX2 Implementation Did Not Fully Reduce Intermediate Values](https://junglewise.ai/threats/rustsec-2026-0126-avx2-implementation-did-not-fully-reduce-intermediate-50fee9b7)
- [libcrux has an Incorrect Check of Signer Response Norm During Verification](https://junglewise.ai/threats/libcrux-has-an-incorrect-check-of-signer-response-norm-during-d5edfd19)
- [libcrux: Panic in Signature Hint Decoding During Verification](https://junglewise.ai/threats/libcrux-panic-in-signature-hint-decoding-during-verification-e5c48417)
- [RUSTSEC-2026-0077 - Incorrect Check of Signer Response Norm During Verification](https://junglewise.ai/threats/rustsec-2026-0077-incorrect-check-of-signer-response-norm-during-0c3fc337)