Vulnerability in Mozilla Firefox
CVE-2021-38507
The Opportunistic Encryption feature of HTTP2 (RFC 8164) allows a connection to be transparently upgraded to TLS while retaining the visual properties of an HTTP connection, including being same-origin with unencrypted connections on port 80. However, if a second encrypted port on the same IP address (e.g. port 8443) did not opt-in to opportunistic encryption; a network attacker could forward a connection from the browser to port 443 to port 8443, causing the browser to treat the content of port 8443 as same-origin with HTTP. This was resolved by disabling the Opportunistic Encryption feature, which had low usage. This vulnerability affects Firefox < 94, Thunderbird < 91.3, and Firefox ESR < 91.3.
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.5 (Medium). Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:U/C:N/I:H/A:N.
EPSS exploit prediction
EPSS: 0.008 (55.1th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 55th percentile — 55.1% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.008 · newest: 0.008 · change: 0.000
Affected products
- Mozilla Firefox — versions unspecified
- Mozilla Firefox Esr — versions unspecified
- Mozilla Firefox Esr
- Mozilla Thunderbird — versions unspecified
- Debian Linux — versions 9.0, 10.0, 11.0
Weakness classification (CWE)
References
- www.mozilla.org/security/advisories/mfsa2021-49/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2021-50/ (Vendor Advisory)
- www.mozilla.org/security/advisories/mfsa2021-48/ (Vendor Advisory)
- bugzilla.mozilla.org/show_bug.cgi (Permissions Required, Issue Tracking, Vendor Advisory)
- www.debian.org/security/2021/dsa-5026 (vendor-advisory, Third Party Advisory)
- lists.debian.org/debian-lts-announce/2021/12/msg00030.html (mailing-list, Mailing List, Third Party Advisory)
- www.debian.org/security/2022/dsa-5034 (vendor-advisory, Third Party Advisory)
- lists.debian.org/debian-lts-announce/2022/01/msg00001.html (mailing-list, Mailing List, Third Party Advisory)
- security.gentoo.org/glsa/202202-03 (vendor-advisory, Third Party Advisory)
- security.gentoo.org/glsa/202208-14 (vendor-advisory, Third Party Advisory)
Frequently asked questions
- What is CVE-2021-38507?
- CVE-2021-38507 is a medium-severity vulnerability in Mozilla Firefox, classified under Origin Validation Error. CVSS score: 6.5/10. Published 2021-12-08.
- How severe is CVE-2021-38507?
- Medium severity. CVSS v3 base score is 6.5 out of 10.