Vulnerability in Mozilla Firefox
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.
Vulnerability class: HTTP Request Smuggling
Published · last modified .
CVSS v3 metric
CVSS v3 base score 8.1 (High). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:H/I:H/A:N.
EPSS exploit prediction
EPSS: 0.009 (58.5th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 58th percentile — 58.5% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.009 · newest: 0.009 · change: -0.000
Affected products
- Mozilla Firefox — versions unspecified
- Mozilla Thunderbird — versions unspecified
Weakness classification (CWE)
References
- www.mozilla.org/security/advisories/mfsa2021-37/ (Vendor Advisory)
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required)
Frequently asked questions
- What is CVE-2021-29991?
- CVE-2021-29991 is a high-severity vulnerability in Mozilla Firefox, classified under Inconsistent Interpretation of HTTP Requests (HTTP Request/Response Smuggling). CVSS score: 8.1/10. Published 2021-11-03.
- How severe is CVE-2021-29991?
- High severity. CVSS v3 base score is 8.1 out of 10.