ml-dsa's UseHint function has off by two error when r0 equals zero

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS 4.0

5.5 · CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:N/VC:N/VI:L/VA:N/SC:N/SI:N/SA:N/E:P · score source

EPSS

No dated EPSS signal available.

As published by the source; long descriptions are shortened.

Summary There's a bug in the usehint function where it adds 1 instead of subtracting 1 when the decomposed low bits r0 equal exactly zero. FIPS 204 Algorithm 40 is pretty clear that r0 0 means strictly positive, but the current code treats zero as positive. This causes valid signatures to potentially fail verification when this edge case gets hit. Details The issue is in ml-dsa/src/hint.rs in the usehint function. Here's what FIPS 204 Algorithm 40 says: Line 3 uses r0 0 (strictly greater than zero), and line 4 uses r0 <= 0 (less than or equal, which includes zero). So when r0 = 0, the spec says to subtract 1. But the current implementation does this: The problem is r0.0 <= gamma2 includes zero. When r0 = 0, this condition is true (since 0 <= gamma2), so it adds 1. But according to the spec, r0 = 0 should fall into the r0 <= 0 case and subtract 1 instead. The result is +1 when it should be -1, which is an off by two error mod m. PoC Take MLDSA 44 where γ2 = 95,232 and m = 44. If…

Published by the source

2 Feb 2026, 20:33 UTC

Last modified by the source

2 Feb 2026, 20:33 UTC

Source last checked

26 Oct 2026, 15:39 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-h37v-hp6w-2pp8

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# Patch Next advisory brief: GHSA-h37v-hp6w-2pp8

Advisory: ml-dsa's UseHint function has off by two error when r0 equals zero
IDs: GHSA-h37v-hp6w-2pp8
Record: https://patchnext.com/advisories/ghsa-h37v-hp6w-2pp8 (revision 1, source checked 26 Oct 2026, 15:39 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Medium (CVSS 5.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: ml-dsa 0.1.0-rc.5. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io ml-dsa: affected <= 0.1.0-rc.4; fixed in 0.1.0-rc.5

## Suggested inspection (read-only)
- crates.io ml-dsa: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on ml-dsa.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-h37v-hp6w-2pp8 (GitHub-reviewed advisory GHSA-h37v-hp6w-2pp8 lists affected ranges: crates.io ml-dsa >=0 <=0.1.0-rc.4)

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