# RUSTSEC-2026-0211: Non-constant time Authentication Tag Check in AES-GCM Decryption

> Timing side-channel in AES-GCM tag check allows attackers to recover authentication tags and forge ciphertexts via decryption oracles.

- **Severity:** Not scored
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** No
- **Published:** 14 Jul 2026

## What is RUSTSEC-2026-0211?

RUSTSEC-2026-0211 is a vulnerability in libcrux-aesgcm, published on 14 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

AES-GCM decryption used an implementation for checking the provided
authentication tag against the recomputed authentication tag that was
intended to be constant-time, but resulted in non-constant-time code
generation in certain circumstances.
Note that `libcrux-aesgcm` does not give guarantees on constant-time
code generation and possible mitigations must be considered
on a best-effort basis.

## Impact
Users of `libcrux-aesgcm` that expose a decryption oracle to an
attacker, which allows repeatedly querying for the same ciphertext
were at risk from timing side-channel attacks, depending on their
compilation environment. In the worst case, this could lead to
recovery of the recomputed authentication tag by the attacker, which
enables ciphertext forgery.

## Mitigation
Starting from version `0.0.9` (published as `libcrux-aes@v0.0.9`),
AES-GCM decryption uses a different, best-effort constant-time
implementation of the tag check, which has been checked to reliably
lead to constant time code generation, at the moment.

## Which versions are affected by RUSTSEC-2026-0211?

1 open source package ship an affected version, including libcrux-aesgcm.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `libcrux-aesgcm` | crates.io | From 0.0.0-0 | None |

## How do I fix RUSTSEC-2026-0211?

No fix has been published for RUSTSEC-2026-0211 yet. Limit exposure of the affected software until one is available.

## Is RUSTSEC-2026-0211 being exploited?

There is no known exploitation of RUSTSEC-2026-0211 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for RUSTSEC-2026-0211 and fix it if it is affected.

What it is: RUSTSEC-2026-0211 is a vulnerability in libcrux-aesgcm, published on 14 Jul 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 RUSTSEC-2026-0211 in the wild and no public exploit code.

Affected packages and fixed versions:
- libcrux-aesgcm (crates.io): affected from 0.0.0-0; no fix listed

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/RUSTSEC-2026-0211/ (Markdown: https://insights.sbomify.com/RUSTSEC-2026-0211/index.md)
```

## Sources

- OSV: https://osv.dev/vulnerability/RUSTSEC-2026-0211

---
From sbomify Insights: https://insights.sbomify.com/RUSTSEC-2026-0211/
Summaries are generated and may contain mistakes; the official description above is the record.