Mozilla Thunderbird up to 159 cleartext transmission

Summaryinfo

A vulnerability categorized as problematic has been discovered in Mozilla Thunderbird up to 159. Affected is an unknown function. Such manipulation leads to cleartext transmission. This vulnerability is documented as CVE-2026-126136. The attack can be executed remotely. There is not any exploit available. It is advisable to upgrade the affected component.

Detailsinfo

A vulnerability was found in Mozilla Thunderbird up to 159 and classified as problematic. Affected by this issue is an unknown functionality. The manipulation with an unknown input leads to a cleartext transmission vulnerability. Using CWE to declare the problem leads to CWE-319. The product transmits sensitive or security-critical data in cleartext in a communication channel that can be sniffed by unauthorized actors. Impacted is confidentiality. CVE summarizes:

For Exchange accounts using EWS or Microsoft Graph with OAuth2 authentication, Thunderbird sent the OAuth2 access token to the server even when the configured server URL used unencrypted HTTP. An attacker able to observe network traffic could obtain the token and use it to access the user's mailbox until the token expired. An attacker positioned on the network during account setup could cause such a URL to be configured. This vulnerability was fixed in Thunderbird 160.

The advisory is available at bugzilla.mozilla.org. This vulnerability is handled as CVE-2026-126136 since 11/10/2026. The exploitation is known to be difficult. The attack may be launched remotely. No form of authentication is required for exploitation. 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 11/10/2026). This vulnerability is assigned to T1040 by the MITRE ATT&CK project.

Upgrading to version 160 eliminates this vulnerability.

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Productinfo

Type

• Mail Client Software

Vendor

• Mozilla

Name

• Thunderbird

Version

• 159

License

• open-source

Website

• Vendor: https://www.mozilla.org/

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