#VU111055 Input validation error in Mozilla Thunderbird - CVE-2025-5986

Vulnerability identifier: #VU111055

Vulnerability risk: Medium

CVSSv4.0: 1.2 [CVSS:4.0/AV:N/AC:L/AT:N/PR:N/UI:A/VC:L/VI:N/VA:L/SC:N/SI:N/SA:N/E:U/U:Green]

CVE-ID: CVE-2025-5986

CWE-ID: CWE-20

Exploitation vector: Network

Exploit availability: No

Vulnerable software:
Mozilla Thunderbird
Client/Desktop applications / Messaging software

Vendor: Mozilla

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.

Mitigation
Install updates from vendor's website.

Vulnerable software versions

Mozilla Thunderbird: before 128.11.1

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External links
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

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Q & A

Can this vulnerability be exploited remotely?

Yes. This vulnerability can be exploited by a remote non-authenticated attacker via the Internet.

Is there known malware, which exploits this vulnerability?

No. We are not aware of malware exploiting this vulnerability.