What’s Vulnerable › Vulnerabilities

CVE-2021-23969

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.2% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-02-26 · updated 2026-06-17

Description

As specified in the W3C Content Security Policy draft, when creating a violation report, "User agents need to ensure that the source file is the URL requested by the page, pre-redirects. If that’s not possible, user agents need to strip the URL down to an origin to avoid unintentional leakage." Under certain types of redirects, Firefox incorrectly set the source file to be the destination of the redirects. This was fixed to be the redirect destination's origin. This vulnerability affects Firefox < 86, Thunderbird < 78.8, and Firefox ESR < 78.8.

Affected products

• Debian Linux: 9.0, 10.0
• Mozilla Firefox: before 86.0 — fixed in 86.0
• Mozilla Firefox ESR: before 78.8 — fixed in 78.8
• Mozilla Thunderbird: before 78.8 — fixed in 78.8

How to fix

Upgrade to a fixed version: Mozilla Firefox 86.0; Mozilla Firefox ESR 78.8; Mozilla Thunderbird 78.8. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1542194
• https://lists.debian.org/debian-lts-announce/2021/03/msg00000.html
• https://security.gentoo.org/glsa/202104-09
• https://security.gentoo.org/glsa/202104-10
• https://www.debian.org/security/2021/dsa-4866
• https://www.mozilla.org/security/advisories/mfsa2021-07/
• https://www.mozilla.org/security/advisories/mfsa2021-08/
• https://www.mozilla.org/security/advisories/mfsa2021-09/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1542194
• https://lists.debian.org/debian-lts-announce/2021/03/msg00000.html
• https://security.gentoo.org/glsa/202104-09
• https://security.gentoo.org/glsa/202104-10

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