Input validation error in Mozilla Thunderbird - CVE-2025-5986

Detailed vulnerability description

The vulnerability allows a remote attacker to gain access to sensitive information or perform a denial of service (DoS) attack.

The vulnerability exists due to insufficient validation of user-supplied input when handling mailbox:/// links. A remote attacker can create a specially crafted email mailbox:/// links and trigger unsolicited downloads of .pdf files to the user's desktop or home directory without prompting, even if auto-saving is disabled. Additionally, this behavior can be use to leak Windows credentials via SMB links when the email is viewed in HTML mode.

Note, viewing the email in HTML mode is enough to load external content.

How to mitigate CVE-2025-5986

Sources

• https://www.mozilla.org/en-US/security/advisories/mfsa2025-49/
• https://www.mozilla.org/en-US/security/advisories/mfsa2025-50/
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=1958580
• https://bugzilla.mozilla.org/buglist.cgi?bug_id=1968012