# GHSA-h37v-hp6w-2pp8: ml-dsa's UseHint function has off by two error when r0 equals zero

> An off-by-two error in the ml-dsa library causes valid signatures to fail verification, leading to denial of service for users relying on this cryptographic implementation.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 2 Feb 2026

## What is GHSA-h37v-hp6w-2pp8?

GHSA-h37v-hp6w-2pp8 is a vulnerability in ml-dsa, published on 2 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

### Summary

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:

```rust
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 <= gamma2` is `0 <= 95232` which is true
- So it returns `(0 + 1) % 44 = 1`

But FIPS 204 says:
- `r0 > 0` is `0 > 0` which is false
- `r0 ≤ 0` is `0 ≤ 0` which 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:

```rust
if h && r0.0 > 0 && r0.0 <= gamma2 {
    Elem::new((r1.0 + 1) % m)
} else if h {
    Elem::new((r1.0 + m - 1) % m)
}
```

## Which versions are affected by GHSA-h37v-hp6w-2pp8?

2 open source packages ship an affected version, including ml-dsa.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `ml-dsa` | crates.io | All versions | 0.1.0-rc.5 |
| `ml-dsa` | crates.io | All versions | 0.1.0-rc.5 |

## How do I fix GHSA-h37v-hp6w-2pp8?

Upgrade ml-dsa to 0.1.0-rc.5 or later.

## Is GHSA-h37v-hp6w-2pp8 being exploited?

There is no known exploitation of GHSA-h37v-hp6w-2pp8 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for GHSA-h37v-hp6w-2pp8 and fix it if it is affected.

What it is: GHSA-h37v-hp6w-2pp8 is a vulnerability in ml-dsa, published on 2 Feb 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of GHSA-h37v-hp6w-2pp8 in the wild and no public exploit code.

Affected packages and fixed versions:
- ml-dsa (crates.io): affected all versions; fixed in 0.1.0-rc.5
- ml-dsa (crates.io): affected all versions; fixed in 0.1.0-rc.5

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/GHSA-h37v-hp6w-2pp8/ (Markdown: https://insights.sbomify.com/GHSA-h37v-hp6w-2pp8/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/GHSA-h37v-hp6w-2pp8

---
From sbomify Insights: https://insights.sbomify.com/GHSA-h37v-hp6w-2pp8/
Summaries are generated and may contain mistakes; the official description above is the record.