CVE-2021-29991

high

Published — · Modified —

CVSS v3

—

CVSS v2

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VIR risk

8.0

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.

Predictions

Exploit likelihood

20%

Patch ETA

—

Heuristic predictions, AS-IS, for prioritization only.

Mitigations

vendor Authored 2026-05-27

Vendor advisory: debian — https://security-tracker.debian.org/tracker/CVE-2021-29991

vendor Authored 2026-05-27

Vendor advisory: suse — https://www.suse.com/security/cve/CVE-2021-29991.html

OS impact

OS	Version	Status	Fixed in
arch		fixed	`91.1.0-1`
sles		affected
debian	sid	fixed	`91.0.1-1`

References

https://www.suse.com/security/cve/CVE-2021-29991.html
https://security-tracker.debian.org/tracker/CVE-2021-29991

Verify integrity in audit chain (admin only). AS-IS.