CVE-2021-29991
This allowed for a header splitting attack against servers using HTTP/3.
Does this matter?
High impact if exploited, but EPSS currently rates exploitation as unlikely (0.91%). Schedule it in the normal patch cycle and watch for a rise in EPSS or a public exploit.
Description
Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers. This allowed for a header splitting attack against servers using HTTP/3. This vulnerability affects Firefox < 91.0.1 and Thunderbird < 91.0.1.
- CVSS 3.1
- 8.1 HIGHCVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
- EPSS
- 0.91% probability · 60th percentile
- Public exploits
- None in Exploit-DB
- CISA KEV
- Not listed
- Weakness
- CWE-444
- Affected
- mozilla/firefox · mozilla/thunderbird
- Source
- security@mozilla.org
EPSS 0.9% since 26 October 2026; no change recorded yet. Points are recorded when the score first appears and whenever it moves by a percentage point or more.
References
- https://bugzilla.mozilla.org/show_bug.cgi?id=1724896Permissions Required
- https://www.mozilla.org/security/advisories/mfsa2021-37/Vendor Advisory
- https://bugzilla.mozilla.org/show_bug.cgi?id=1724896Permissions Required
- https://www.mozilla.org/security/advisories/mfsa2021-37/Vendor Advisory
Source: NVD record, EPSS from FIRST.org, KEV from CISA, exploits from Exploit-DB. Refreshed daily. Download this record as JSON.