Summary
ml-dsa's UseHint function has off by two error when r0 equals zero
There's a bug in the use_hint 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 use_hint function. Here's what FIPS 204 Algorithm 40 says:
3: if h = 1 and r0 > 0 return (r1 + 1) mod m
4: if h = 1 and r0 <= 0 return (r1 − 1) mod m
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:
if h && r0.0 <= gamma2 {
Elem::new((r1.0 + 1) % m)
} else if h && r0.0 >= BaseField::Q - gamma2 {
Elem::new((r1.0 + m - 1) % m)
}
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 use_hint(true, 0) is called:
Decompose(0)gives(r1=0, r0=0)- The condition
r0.0 <= gamma2is0 <= 95232which is true - So it returns
(0 + 1) % 44 = 1
But FIPS 204 says:
r0 > 0is0 > 0which is falser0 ≤ 0is0 ≤ 0which is true- So it should return
(0 - 1) mod 44 = 43
The function returns 1 when it should return 43.
This can happen in real signatures whenever any coefficient of the w' vector happens to be a multiple of 2γ2, which makes its decomposed r0 equal zero. It's not super common but it's definitely possible, and when it hits, verification will fail for a completely valid signature.
Impact
This is a FIPS 204 compliance bug that affects signature verification. When the edge case triggers, valid signatures get rejected. Since MLDSA is supposed to be used for high security post quantum cryptography, having verification randomly fail isn't great. It's also theoretically possible that the mismatch between what signing expects and what verification does could be exploited somehow, though that would need more looking into.
The fix is straightforward, just change the condition to explicitly check for positive values:
if h && r0.0 > 0 && r0.0 <= gamma2 {
Elem::new((r1.0 + 1) % m)
} else if h {
Elem::new((r1.0 + m - 1) % m)
}
Affected versions
Security releases
Kodem intelligence
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Frequently Asked Questions
- What is GHSA-H37V-HP6W-2PP8? GHSA-H37V-HP6W-2PP8 is a medium-severity security vulnerability in ml-dsa (rust), affecting versions <= 0.1.0-rc.4. It is fixed in 0.1.0-rc.5.
- Which versions of ml-dsa are affected by GHSA-H37V-HP6W-2PP8? ml-dsa (rust) versions <= 0.1.0-rc.4 is affected.
- Is there a fix for GHSA-H37V-HP6W-2PP8? Yes. GHSA-H37V-HP6W-2PP8 is fixed in 0.1.0-rc.5. Upgrade to this version or later.
- Is GHSA-H37V-HP6W-2PP8 exploitable, and should I be worried? Whether GHSA-H37V-HP6W-2PP8 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether GHSA-H37V-HP6W-2PP8 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix GHSA-H37V-HP6W-2PP8? Upgrade
ml-dsato 0.1.0-rc.5 or later.