CVE-2021-29991

Mozilla: Header Splitting possible with HTTP/3 Responses

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Severity

Important

Red Hat Product Security

7.5

CVSS 3.1

Review impact separately from score.

Published · August 18, 2021

Updated · November 21, 2025

Description

The Mozilla Foundation Security Advisory describes this flaw as:

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.

CVSS score applicability

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.

Mitigation

No mitigation is specified in collected source.

CVSS / CWE

CVSS 3.1 · 7.5
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:H/A:N

CWE-79 · Improper Neutralization of Input During Web Page Generation ('Cross-site Scripting')