CVE-2026-24850

**Affected Crate:** `ml-dsa` **Affected Versions:** v0.1.0-rc.2 (and commits since `b01c3b7`) **Severity:** Medium **Reporter:** Oren Yomtov (Fireblocks) ## Summary The ML-DSA signature verification implementation in the RustCrypto `ml-dsa` crate incorrectly accepts signatures with repeated (duplicate) hint indices. According to the ML-DSA specification (FIPS 204 / RFC 9881), hint indices within each polynomial must be **strictly increasing**. The current implementation uses a non-strict monotonic check (`<=` instead of `<`), allowing duplicate indices. **Note:** This is a regression bug. The original implementation was correct, but commit `b01c3b7` ("Make ML-DSA signature decoding follow the spec (#895)", fixing issue #894) inadvertently changed the strict `<` comparison to `<=`, introducing the vulnerability. ## Vulnerability Details ### Root Cause The vulnerability is located in the `monotonic` helper function in `ml-dsa/src/hint.rs`: ```rust fn monotonic(a: &[usize]) -> bool { a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] <= *x) } ``` The comparison operator `<=` allows equal consecutive values, meaning duplicate hint indices are not rejected. The correct implementation should use strict less-than (`<`): ```rust fn monotonic(a: &[usize]) -> bool { a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] < *x) } ``` ### Regression Analysis - **Original correct code** (commit `1d3a1d1` - "Add support for ML-DSA (#877)"): Used `<` (strict) - **Bug introduced** (commit `b01c3b7` - "Make ML-DSA signature decoding follow the spec (#895)"): Changed to `<=` The commit message suggests it was intended to fix issue #894 and make decoding follow the spec, but the change to the `monotonic` function was in the wrong direction. The other changes in that commit (to `use_hint` function) may have been correct, but this specific change introduced signature malleability. ### Technical Impact This vulnerability allows **signature malleability** - the same logical signature can have multiple valid byte-level encodings. An attacker can take a valid signature and create additional "valid" signatures by duplicating hint indices. Per the ML-DSA specification (FIPS 204, Section 6.2 and Algorithm 26 `HintBitUnpack`), hint indices must be strictly increasing to ensure a unique, canonical encoding. Accepting non-canonical signatures can lead to: 1. **Signature Malleability:** Multiple distinct byte sequences verify as valid for the same message/key pair 2. **Protocol-Level Vulnerabilities:** Systems that rely on signature uniqueness (e.g., for transaction deduplication, replay protection, or signature-based identifiers) may be vulnerable 3. **Interoperability Issues:** Non-compliant signatures may be rejected by other conforming implementations ### Affected Security Levels All ML-DSA parameter sets are affected: - ML-DSA-44 (NIST Security Level 2) - ML-DSA-65 (NIST Security Level 3) - ML-DSA-87 (NIST Security Level 5) ## Proof of Concept See the file [`poc_mldsa_repeated_hint.rs`](https://gist.github.com/orenyomtov/fb4616eb77d33017f41a71b30aa41a04) for a standalone proof of concept that demonstrates the vulnerability. The PoC uses test vectors from the Wycheproof test suite that specifically test for this invalid encoding: - **Test Vector Source:** [Wycheproof ML-DSA Test Vectors](https://github.com/C2SP/wycheproof/blob/master/testvectors_v1/mldsa_44_verify_test.json) - Test Case ID 18: "signature with a repeated hint" - Expected Result: `invalid` - Actual Result: `valid` (BUG) ## Remediation Update the `monotonic` function in `ml-dsa/src/hint.rs` to use strict less-than comparison: ```rust fn monotonic(a: &[usize]) -> bool { a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] < *x) } ``` ## Design Intent: ML-DSA is NOT Intended to Allow Malleability While some cryptographic libraries intentionally permit signature malleability for compatibility or performance reasons, **ML-DSA is explicitly designed to prevent it**: 1. **FIPS 204 Specification:** ML-DSA is designed to be strongly unforgeable under chosen message attacks (SUF-CMA). This security property explicitly prevents signature malleability. 2. **NIST PQC Forum Discussion:** In February 2024, there was a discussion on the NIST PQC forum about potential malleability in ML-DSA's hint unpacking. The consensus was that ML-DSA is intended to be SUF-CMA, meaning any malleability issues should be considered bugs and fixed. 3. **No Documentation of Intentional Malleability:** There is no documentation in the RustCrypto `ml-dsa` crate, FIPS 204, or RFC 9881 suggesting that signature malleability is an acceptable or intentional property. 4. **Regression Bug:** The fact that the original implementation had strict ordering (`<`) and this was changed to non-strict (`<=`) in a "fix" commit suggests this was an unintentional regression, not a design decision.