Description

Triggering an error condition in certain MIME bodies would cause uninitialized memory to be used. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.

Executive Summary

Technical Details

Mitigations

References

Additional Metadata

{
  "score": "9.1",
  "pubDate": "2026-09-01T22:17:19.700Z",
  "pubdate": "2026-09-01T22:17:19.700Z",
  "executiveSummary": "A memory safety vulnerability exists in Thunderbird due to the improper handling of specific MIME body error conditions. This flaw leads to the use of uninitialized memory, potentially allowing for unpredictable application behavior or information disclosure.\nThe vulnerability affects Thunderbird versions 155, 140.15, and 153.2. By triggering specific error conditions during the parsing or processing of malformed MIME entities, an attacker may be able to manipulate the application's memory state.\nThe primary risk involves the exploitation of memory corruption, which can lead to arbitrary code execution or the leakage of sensitive data residing in the heap or stack. Successful exploitation typically requires an attacker to deliver a specially crafted email or MIME payload to the victim, which is then processed by the Thunderbird client. No explicit user interaction beyond viewing the malicious content may be necessary depending on the automated processing settings of the email client. Organizations should prioritize updating to the patched versions to mitigate potential remote exploitation risks.",
  "technicalDetails": "The vulnerability stems from insufficient validation and error handling logic within the Thunderbird MIME parsing component. When an email message containing a malformed or syntactically invalid MIME body is processed, an error condition is triggered. In the vulnerable code path, the application fails to correctly initialize or zero out memory buffers before they are accessed or returned to the calling function.\nThe root cause is a failure in memory management state transition during the error-handling routine. When the MIME parser encounters a specific, non-standard condition, it interrupts the normal data extraction flow. If the subsequent cleanup or error-recovery logic assumes that a data structure or buffer has been populated, but the initialization was skipped due to the triggered error, the application proceeds to read or manipulate uninitialized memory.\nExploitation flow typically involves an attacker crafting a MIME body that induces a predictable state during parsing. By carefully structuring MIME headers or multipart boundaries, an attacker can influence the heap layout or the state of memory objects that are later accessed by the application. Because the memory is uninitialized, it may contain remnants of previous operations, such as pointer addresses, cryptographic keys, or user data.\nAttackers can leverage this uninitialized memory access to bypass security primitives like Address Space Layout Randomization (ASLR). By forcing the application to read and inadvertently leak these memory contents through application responses or metadata manipulation, the attacker can gain sufficient information to construct an exploit payload. If the uninitialized memory is later used as a function pointer or an object reference, it may allow for redirection of control flow.\nThe vulnerable component is the Thunderbird MIME processing engine, which is responsible for decoding and organizing complex, multi-part email structures. The vulnerability is present across the affected versions: Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2. Exploitation does not require prior authentication or elevated privileges, as the vulnerability is triggered during the standard parsing phase of email reception and display. The network exposure is limited only by the requirement that the victim client successfully fetches or receives the malicious MIME message, making it a viable vector for remote attackers."
}