# CVE-2021-23991: Denial of service in Thunderbird

> An attacker can prevent encrypted email delivery to a recipient by exploiting invalid subkey handling in Thunderbird's OpenPGP implementation.

- **Severity:** Medium 6.8 (`CVSS:3.1/AV:N/AC:H/PR:N/UI:R/S:U/C:H/I:H/A:N`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** A user has to click or open something
- **Fix available:** Yes
- **Published:** 24 Jun 2021
- **Updated:** 3 Aug 2024
- **Also known as:** DEBIAN-CVE-2021-23991, GHSA-x5j6-p8w8-5r65, UBUNTU-CVE-2021-23991

## What is CVE-2021-23991?

CVE-2021-23991 is a medium-severity denial of service vulnerability in Mozilla Thunderbird, published on 24 Jun 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.

### Official description

If a Thunderbird user has previously imported Alice's OpenPGP key, and Alice has extended the validity period of her key, but Alice's updated key has not yet been imported, an attacker may send an email containing a crafted version of Alice's key with an invalid subkey, Thunderbird might subsequently attempt to use the invalid subkey, and will fail to send encrypted email to Alice. This vulnerability affects Thunderbird < 78.9.1.

## Which versions are affected by CVE-2021-23991?

It affects 1 product.


Products: Mozilla Thunderbird.

## How do I fix CVE-2021-23991?

Upgrade to a fixed version.

## Is CVE-2021-23991 being exploited?

There is no known exploitation of CVE-2021-23991 in the wild and no public exploit code.

## Other vulnerabilities in Mozilla Thunderbird

- [CVE-2026-92238](https://insights.sbomify.com/CVE-2026-92238/): Ambiguous parsing of mail headers (critical 9.8)
- [CVE-2026-84637](https://insights.sbomify.com/CVE-2026-84637/): Calendar invitation attachments could launch local executables (critical 9.8)
- [CVE-2021-43529](https://insights.sbomify.com/CVE-2021-43529/): Memory corruption in Thunderbird (critical 9.8)
- [CVE-2019-11703](https://insights.sbomify.com/CVE-2019-11703/): Memory corruption in Thunderbird (critical 9.8)
- [CVE-2019-11704](https://insights.sbomify.com/CVE-2019-11704/): Memory corruption in Thunderbird (critical 9.8)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2021-23991 and fix it if it is affected.

What it is: CVE-2021-23991 is a medium-severity denial of service vulnerability in Mozilla Thunderbird, published on 24 Jun 2021. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Exploitation: There is no known exploitation of CVE-2021-23991 in the wild and no public exploit code.

Affected product: Mozilla Thunderbird. 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-2021-23991/ (Markdown: https://insights.sbomify.com/CVE-2021-23991/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2021-23991
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2021-23991
- OSV: https://osv.dev/vulnerability/CVE-2021-23991

---
From sbomify Insights: https://insights.sbomify.com/CVE-2021-23991/
Summaries are generated and may contain mistakes; the official description above is the record.