CVE-2021-29991
HIGHCVSS 8.1/10EPSS 0.89%
Last modified
CVE-2021-29991 is a high-severity vulnerability rated 8.1/10 on the CVSS scale. 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. EPSS estimates a 0.89% chance of exploitation in the next 30 days.
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.
Metrics
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N
Weakness Enumeration
Affected Software
| Vendor | Product | Versions |
|---|---|---|
| Mozilla | Firefox | < 91.0.1 |
| Mozilla | Thunderbird | < 91.0.1 |
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
Timeline
- Published
- Last Modified
- Status
- Modified
Frequently Asked Questions
What is CVE-2021-29991?
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.
How severe is CVE-2021-29991?
CVE-2021-29991 has a CVSS score of 8.1/10 (HIGH severity). The EPSS model estimates a 0.89% probability of exploitation in the next 30 days.
How do I fix CVE-2021-29991?
Check the vendor references and advisories linked above for patched versions and mitigation guidance. You can also run a Strix scan to test if your systems are affected.
How Strix Helps
- How Strix found a critical auth bypass in etcdStrix autonomously discovered a critical authentication bypass in etcd, later designated CVE-2026-33413.
- Autonomous PentestingAI agents that find and validate exploitable vulnerabilities like this one across your applications.
- PR ReviewsPentest every pull request so vulnerable code is caught before it ships to production.
- AI Penetration TestingHow AI-driven penetration testing continuously covers your attack surface.
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Are you affected by CVE-2021-29991?
Run a free Strix scan to check your systems for this vulnerability.
Scan your code nowSource: NVD / NIST