# CVE-2023-4421: Information disclosure in NSS

> Timing side-channel in NSS PKCS#1 v1.5 padding checks allows attackers to decrypt TLS sessions or forge signatures.

- **Severity:** Medium 6.5 (`CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 12 Dec 2023
- **Updated:** 4 Nov 2025
- **Also known as:** DEBIAN-CVE-2023-4421, GHSA-3hpv-hgvq-792m, OESA-2026-2612, UBUNTU-CVE-2023-4421

## What is CVE-2023-4421?

CVE-2023-4421 is a medium-severity vulnerability in Mozilla NSS, published on 12 Dec 2023. Any signed-in user can exploit it over the network, with no action from the victim.

### Official 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.

Weakness: CWE-203 (Observable Discrepancy)

## Which versions are affected by CVE-2023-4421?

It affects 1 product.


Products: Mozilla Nss.

## How do I fix CVE-2023-4421?

Upgrade to a fixed version.

## Is CVE-2023-4421 being exploited?

There is no known exploitation of CVE-2023-4421 in the wild and no public exploit code.

## Other vulnerabilities in Mozilla NSS

- [CVE-2021-43527](https://insights.sbomify.com/CVE-2021-43527/): Memory corruption in NSS (critical 9.8)
- [CVE-2020-12403](https://insights.sbomify.com/CVE-2020-12403/): Memory corruption in nss (critical 9.1)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2023-4421 and fix it if it is affected.

What it is: CVE-2023-4421 is a medium-severity vulnerability in Mozilla NSS, published on 12 Dec 2023. Any signed-in user can exploit it over the network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2023-4421 in the wild and no public exploit code.

Affected product: Mozilla NSS. No package-level version data is available, so check the vendor advisory linked from the reference page.

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/CVE-2023-4421/ (Markdown: https://insights.sbomify.com/CVE-2023-4421/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2023-4421
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2023-4421
- OSV: https://osv.dev/vulnerability/CVE-2023-4421

---
From sbomify Insights: https://insights.sbomify.com/CVE-2023-4421/
Summaries are generated and may contain mistakes; the official description above is the record.