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
EPSS: 0.009 (55.5th percentile) — read the EPSS interpretation.
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.
Affected products
- Mozilla Firefox — versions unspecified
- Mozilla Thunderbird — versions unspecified
Weakness classification (CWE)
References
- security@mozilla.org (x_refsource_MISC, Vendor Advisory)
- security@mozilla.org (Permissions Required, x_refsource_MISC)
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.