# cosmian_kyber: AVX2 backend skips Fujisaki-Okamoto implicit rejection, enabling chosen-ciphertext key recovery

Advisory record · `RUSTSEC-2026-0288`

- **Identifiers:** `RUSTSEC-2026-0288`
- **State:** Confirmed
- **Severity:** CVSS 7.4 High
- **Critical:** No
- **Published:** 16 Aug 2026
- **Updated:** 18 Sept 2026
- **Reviewed by a person:** No

Published by OSV at 16 Aug 2026, 12:00 UTC. agentprime has not analysed this yet, so whether it affects any app is not known.

## Affected packages

- `cosmian_kyber` (crates.io): affected `>=0.0.0-0`; no fixed version yet

## Machine-written summary

Summary written by Claude Haiku from the source advisory. Not reviewed by a person.

> cosmian_kyber builds with the avx2 feature on x86_64 skip Fujisaki-Okamoto implicit rejection during decapsulation. An attacker who can submit ciphertexts under a reused key pair and observe the resulting shared secrets can recover the static secret key. No fixed version is listed; avoid the avx2 feature and do not reuse key pairs across decapsulations.

Written 8 Nov 2026.

## Not analysed yet

We haven’t published our analysis of this Advisory yet. Once a person has reviewed it, this page will say what’s vulnerable, what has to be true for your app to be affected, and how to check.

## Severity and exploitation

- 7.4 High, CVSS 3.1: `CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:N`, from OSV

- **CISA KEV:** Not listed.
- **EPSS:** No score.

## Timeline

- 31 Oct 2026, 15:17 UTC: First seen by agentprime
- 31 Oct 2026, 15:17 UTC: [Confirmed by OSV](https://osv.dev/vulnerability/RUSTSEC-2026-0288)

## Sources

- [OSV: RUSTSEC-2026-0288](https://osv.dev/vulnerability/RUSTSEC-2026-0288)
- <https://crates.io/crates/cosmian_kyber>
- <https://rustsec.org/advisories/RUSTSEC-2026-0288.html>
- <https://github.com/Cosmian/kyber/pull/6>

Facts on this page come from the GitHub Advisory Database (CC-BY 4.0), OSV and the other sources listed, as published. We don’t reproduce full advisory descriptions, which can contain exploit code.

## Check whether this affects you

A Match means an installed version is in the affected range above. Most don’t affect the app; agentprime hasn’t analysed this one yet.

- With the agentprime MCP server (<https://api.agentprime.io/mcp>), ask your agent to check the project: `check_lockfile` sends a lockfile, `check_packages` a list of installed packages, and both return the Matches and the upgrades that clear them. `get_advisory` returns this record. Setup: <https://agentprime.io/mcp.md>
- In a browser: <https://agentprime.io/check/> reads your lockfile in the browser, never uploading it.
- Whether it touches your code, with file and line: the Claude Code plugin, <https://agentprime.io/plugin.md>
- As JSON: <https://api.agentprime.io/v1/advisories/RUSTSEC-2026-0288>, or `POST https://api.agentprime.io/v1/match` with your packages (<https://agentprime.io/api.md>)

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The web page: <https://agentprime.io/advisories/RUSTSEC-2026-0288/>. Everything for agents: <https://agentprime.io/llms.txt>.