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CVE-2021-29991

8.1 · HIGH

Firefox incorrectly accepted a newline in a HTTP/3 header, interpretting it as two separate headers.

Published Nov 3, 2021CWE-444EPSS 0.91%

Overview

CVE-2021-29991 is a high-severity vulnerability. It was published on November 3, 2021 and has a CVSS 3.1 base score of 8.1 (HIGH).

This vulnerability has a CVSS 3.1 base score of 8.1, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

Technical 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.

Remediation

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

References

Refbugzilla.mozilla.org/show_bug.cgi?id=1724896Refwww.mozilla.org/security/advisories/mfsa2021-37/Refbugzilla.mozilla.org/show_bug.cgi?id=1724896Refwww.mozilla.org/security/advisories/mfsa2021-37/

How CyberStrike helps

Frequently asked questions

What is CVE-2021-29991?

CVE-2021-29991 is a high-severity vulnerability. It was published on November 3, 2021 and has a CVSS 3.1 base score of 8.1 (HIGH).

How severe is CVE-2021-29991?

This vulnerability has a CVSS 3.1 base score of 8.1, rated HIGH. It can be exploited remotely over the network. No authentication or special privileges are required for exploitation.

How do I fix or remediate CVE-2021-29991?

Check the references section for vendor advisories, patches, and mitigation guidance. If immediate patching is not possible, review the CVSS vector to understand the attack surface and apply compensating controls such as network segmentation or access restrictions.

How can CyberStrike help with CVE-2021-29991?

CyberStrike is an open-source, AI-augmented pentesting engine. Rather than flag a version number, it tests whether an issue like CVE-2021-29991 is actually reachable in your own applications and APIs, then verifies each finding before reporting it. That tells you if the weakness is a real, exploitable risk in your environment or a false positive you can safely deprioritize.

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