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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.
Medium5.3EPSS 0.85%
- Published
- Updated
- Reserved
- Assigned by
- mozilla
Description
Improved Host header checks to reject requests not sent to a well-known local hostname or IP, or the server-specified hostname.
Timeline
ID reserved
By mozilla
- 4 months later
· 4 months later
Published
- 2 years later
· 2 years later
Record updated
Changes after publication (a new score, a public exploit, EPSS crossing 1, 10 or 50 %, leaving KEV) are kept since November 2026; earlier ones are not known.
Scores
| Scored by | CVSS | Score | Vector |
|---|---|---|---|
| NVDNVD · Modified | 3.1 | Medium5.3 | CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:L/A:N |
- EPSS (exploitation in 30 days)
- 0.85% · top 42% of all CVEs (7 Nov 2026)
Loading the EPSS history…
The list uses the assigner's score when there is one, then CISA's, then NVD's: Medium 5.3.
Weakness
- Fixed a DNS rebinding issues by enforcing a stricter Host header check.
Affected products
- affected unspecified – < 0.30.0
References (2)
- https://github.com/mozilla/geckodriver/releases/tag/v0.30.0 (opens in a new tab)x_refsource_MISCx_transferred
- https://bugzilla.mozilla.org/show_bug.cgi?id=1652612 (opens in a new tab)x_refsource_MISCx_transferred
Sources
The same CVE in the primary databases and aggregators, to check against.
- CVE.org record (opens in a new tab)
- CVE List JSON (GitHub) (opens in a new tab)
- NVD (opens in a new tab)
- FIRST EPSS (opens in a new tab)
- OSV.dev (opens in a new tab)
- GitHub Advisories (opens in a new tab)
- CIRCL Vulnerability-Lookup (opens in a new tab)
Badge for a README or a ticket
[](https://trackcve.app/cve/CVE-2021-4138)Record last updated .