What’s Vulnerable › Vulnerabilities

CVE-2019-9801

• Severity: medium (CVSS 5.3 v3.0)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 1.3% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2019-04-26 · updated 2026-06-17
• Weakness: CWE-20

Description

Firefox will accept any registered Program ID as an external protocol handler and offer to launch this local application when given a matching URL on Windows operating systems. This should only happen if the program has specifically registered itself as a "URL Handler" in the Windows registry. *Note: This issue only affects Windows operating systems. Other operating systems are unaffected.*. This vulnerability affects Thunderbird < 60.6, Firefox ESR < 60.6, and Firefox < 66.

Affected products

• Mozilla Firefox: before 66.0 — fixed in 66.0
• Mozilla Firefox ESR: before 60.6 — fixed in 60.6
• Mozilla Thunderbird: before 60.6 — fixed in 60.6

How to fix

Upgrade to a fixed version: Mozilla Firefox 66.0; Mozilla Firefox ESR 60.6; Mozilla Thunderbird 60.6. Check the vendor advisory for mitigations.

References

• https://bugzilla.mozilla.org/show_bug.cgi?id=1527717
• https://www.mozilla.org/security/advisories/mfsa2019-07/
• https://www.mozilla.org/security/advisories/mfsa2019-08/
• https://www.mozilla.org/security/advisories/mfsa2019-11/
• https://bugzilla.mozilla.org/show_bug.cgi?id=1527717
• https://www.mozilla.org/security/advisories/mfsa2019-07/
• https://www.mozilla.org/security/advisories/mfsa2019-08/
• https://www.mozilla.org/security/advisories/mfsa2019-11/

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