CVE-2021-29991

Severity: high (CVSS 8.1 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.9% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2021-11-03 · updated 2026-06-17
Weakness: CWE-444

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.

Affected products

Mozilla Firefox: before 91.0.1 — fixed in 91.0.1
Mozilla Thunderbird: before 91.0.1 — fixed in 91.0.1

How to fix

Upgrade to a fixed version: Mozilla Firefox 91.0.1; Mozilla Thunderbird 91.0.1. Check the vendor advisory for mitigations.

References

https://bugzilla.mozilla.org/show_bug.cgi?id=1724896
https://www.mozilla.org/security/advisories/mfsa2021-37/
https://bugzilla.mozilla.org/show_bug.cgi?id=1724896
https://www.mozilla.org/security/advisories/mfsa2021-37/

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