What’s Vulnerable › Vulnerabilities

CVE-2021-23953

• Severity: medium (CVSS 4.3 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.1% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2021-02-26 · updated 2026-06-17

Description

If a user clicked into a specifically crafted PDF, the PDF reader could be confused into leaking cross-origin information, when said information is served as chunked data. This vulnerability affects Firefox < 85, Thunderbird < 78.7, and Firefox ESR < 78.7.

Affected products

• Mozilla Firefox: before 85.0 — fixed in 85.0
• Mozilla Firefox ESR: before 78.7 — fixed in 78.7
• Mozilla Thunderbird: before 78.7 — fixed in 78.7

How to fix

Upgrade to a fixed version: Mozilla Firefox 85.0; Mozilla Firefox ESR 78.7; Mozilla Thunderbird 78.7. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1683940
• https://www.mozilla.org/security/advisories/mfsa2021-03/
• https://www.mozilla.org/security/advisories/mfsa2021-04/
• https://www.mozilla.org/security/advisories/mfsa2021-05/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1683940
• https://www.mozilla.org/security/advisories/mfsa2021-03/
• https://www.mozilla.org/security/advisories/mfsa2021-04/
• https://www.mozilla.org/security/advisories/mfsa2021-05/

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