Mozilla Thunderbird up to 128.11.0/139.0.1 mailbox Link random values

Summaryinfo

A vulnerability classified as problematic was found in Mozilla Thunderbird up to 128.11.0/139.0.1. This affects an unknown function of the component mailbox Link Handler. Executing a manipulation can lead to random values. This vulnerability appears as CVE-2025-5986. The attack may be performed from remote. There is no available exploit. Upgrading the affected component is advised.

Detailsinfo

A vulnerability was found in Mozilla Thunderbird up to 128.11.0/139.0.1. It has been declared as problematic. This vulnerability affects an unknown function of the component mailbox Link Handler. The manipulation with an unknown input leads to a random values vulnerability. The CWE definition for the vulnerability is CWE-330. The product uses insufficiently random numbers or values in a security context that depends on unpredictable numbers. As an impact it is known to affect confidentiality. CVE summarizes:

A crafted HTML email using mailbox:/// links can trigger automatic, unsolicited downloads of .pdf files to the user's desktop or home directory without prompting, even if auto-saving is disabled. This behavior can be abused to fill the disk with garbage data (e.g. using /dev/urandom on Linux) or to leak Windows credentials via SMB links when the email is viewed in HTML mode. While user interaction is required to download the .pdf file, visual obfuscation can conceal the download trigger. Viewing the email in HTML mode is enough to load external content. This vulnerability affects Thunderbird < 128.11.1 and Thunderbird < 139.0.2.

The advisory is available at bugzilla.mozilla.org. This vulnerability was named CVE-2025-5986 since 06/10/2025. The exploitation appears to be difficult. The attack can be initiated remotely. No form of authentication is required for a successful exploitation. Successful exploitation requires user interaction by the victim. The technical details are unknown and an exploit is not available. The structure of the vulnerability defines a possible price range of USD $0-$5k at the moment (estimation calculated on 06/11/2025). This vulnerability is assigned to T1600.001 by the MITRE ATT&CK project.

The vulnerability scanner Nessus provides a plugin with the ID 238109 (Mozilla Thunderbird < 128.11.1), which helps to determine the existence of the flaw in a target environment.

Upgrading to version 128.11.1 or 139.0.2 eliminates this vulnerability.

The vulnerability is also documented in the vulnerability database at Tenable (238109). If you want to get best quality of vulnerability data, you may have to visit VulDB.

Productinfo

Type

• Mail Client Software

Vendor

• Mozilla

Name

• Thunderbird

Version

• 128.0
• 128.1
• 128.2
• 128.3
• 128.4
• 128.5
• 128.6
• 128.7
• 128.8
• 128.9
• 128.10
• 128.11.0
• 139.0.0
• 139.0.1

License

• open-source

Website

• Vendor: https://www.mozilla.org/

CPE 2.3info

• 🔍
• 🔍
• 🔍

CPE 2.2info

• 🔍
• 🔍
• 🔍

CVSSv4info

CVSSv3info

CVSSv2info

Exploitinginfo

Threat Intelligenceinfo

Countermeasuresinfo

Timelineinfo

Sourcesinfo

Entryinfo

If you want to get best quality of vulnerability data, you may have to visit VulDB.

Discussion