CVE-2026-24850: Ml dsa ML-DSA Signature Verification Accepts Signatures Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-5x2r-hc65-25f9",
  "modified" : "2026-01-28T16:44:47Z",
  "published" : "2026-01-28T16:44:47Z",
  "aliases" : [ "CVE-2026-24850" ],
  "summary" : "ML-DSA Signature Verification Accepts Signatures with Repeated Hint Indices",
  "details" : "**Affected Crate:** `ml-dsa`  \n**Affected Versions:** v0.1.0-rc.2 (and commits since `b01c3b7`)  \n**Severity:** Medium  \n**Reporter:** Oren Yomtov (Fireblocks)\n\n## Summary\n\nThe 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.\n\n**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.\n\n## Vulnerability Details\n\n### Root Cause\n\nThe vulnerability is located in the `monotonic` helper function in `ml-dsa/src/hint.rs`:\n\n```rust\nfn monotonic(a: &[usize]) -> bool {\n    a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] <= *x)\n}\n```\n\nThe comparison operator `<=` allows equal consecutive values, meaning duplicate hint indices are not rejected. The correct implementation should use strict less-than (`<`):\n\n```rust\nfn monotonic(a: &[usize]) -> bool {\n    a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] < *x)\n}\n```\n\n### Regression Analysis\n\n- **Original correct code** (commit `1d3a1d1` - \"Add support for ML-DSA (#877)\"): Used `<` (strict)\n- **Bug introduced** (commit `b01c3b7` - \"Make ML-DSA signature decoding follow the spec (#895)\"): Changed to `<=`\n\nThe 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.\n\n### Technical Impact\n\nThis 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.\n\nPer 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:\n\n1. **Signature Malleability:** Multiple distinct byte sequences verify as valid for the same message/key pair\n2. **Protocol-Level Vulnerabilities:** Systems that rely on signature uniqueness (e.g., for transaction deduplication, replay protection, or signature-based identifiers) may be vulnerable\n3. **Interoperability Issues:** Non-compliant signatures may be rejected by other conforming implementations\n\n### Affected Security Levels\n\nAll ML-DSA parameter sets are affected:\n- ML-DSA-44 (NIST Security Level 2)\n- ML-DSA-65 (NIST Security Level 3)\n- ML-DSA-87 (NIST Security Level 5)\n\n## Proof of Concept\n\nSee the file [`poc_mldsa_repeated_hint.rs`](https://gist.github.com/orenyomtov/fb4616eb77d33017f41a71b30aa41a04) for a standalone proof of concept that demonstrates the vulnerability.\n\nThe PoC uses test vectors from the Wycheproof test suite that specifically test for this invalid encoding:\n\n- **Test Vector Source:** [Wycheproof ML-DSA Test Vectors](https://github.com/C2SP/wycheproof/blob/master/testvectors_v1/mldsa_44_verify_test.json)\n- Test Case ID 18: \"signature with a repeated hint\"\n- Expected Result: `invalid`\n- Actual Result: `valid` (BUG)\n\n## Remediation\n\nUpdate the `monotonic` function in `ml-dsa/src/hint.rs` to use strict less-than comparison:\n\n```rust\nfn monotonic(a: &[usize]) -> bool {\n    a.iter().enumerate().all(|(i, x)| i == 0 || a[i - 1] < *x)\n}\n```\n\n## Design Intent: ML-DSA is NOT Intended to Allow Malleability\n\nWhile some cryptographic libraries intentionally permit signature malleability for compatibility or performance reasons, **ML-DSA is explicitly designed to prevent it**:\n\n1. **FIPS 204 Specification:** ML-DSA is designed to be strongly unforgeable under chosen message attacks (SUF-CMA). This security property explicitly prevents signature malleability.\n\n2. **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.\n\n3. **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.\n\n4. **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.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "crates.io",
      "name" : "ml-dsa"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0.0.4"
      }, {
        "fixed" : "0.1.0-rc.4"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/signatures/security/advisories/GHSA-5x2r-hc65-25f9"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-24850"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/signatures/issues/894"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/signatures/pull/895"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/signatures/commit/400961412be2e2ab787942cf30e0a9b66b37a54a"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/RustCrypto/signatures/commit/b01c3b73dd08d0094e089aa234f78b6089ec1f38"
  }, {
    "type" : "WEB",
    "url" : "https://csrc.nist.gov/pubs/fips/204/final"
  }, {
    "type" : "WEB",
    "url" : "https://datatracker.ietf.org/doc/html/rfc9881"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/C2SP/wycheproof"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/C2SP/wycheproof/blob/master/testvectors_v1/mldsa_44_verify_test.json"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/C2SP/wycheproof/blob/master/testvectors_v1/mldsa_65_verify_test.json"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/C2SP/wycheproof/blob/master/testvectors_v1/mldsa_87_verify_test.json"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/RustCrypto/signatures"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-347" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-01-28T16:44:47Z",
    "nvd_published_at" : "2026-01-28T01:16:15Z"
  }
}