CVE-2026-92239 in Thunderbird
Summary
by MITRE • 09/16/2026
A maliciously constructed IMAP line could cause an out-of-bounds buffer read. This vulnerability was fixed in Thunderbird 156 and Thunderbird 140.16.
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Analysis
by VulDB Data Team • 09/16/2026
The identified vulnerability represents a critical memory safety issue within the Internet Message Access Protocol (IMAP) client implementation of Mozilla Thunderbird, specifically affecting versions prior to 140.16. IMAP is a standard protocol used by email clients to retrieve messages from mail servers, and it involves parsing complex text-based command structures sent by users or automated scripts. The flaw resides in the parser logic responsible for handling specific line formats within this protocol stream. When Thunderbird processes an incoming message header or body part that contains a specially crafted IMAP response string, the internal buffer management routines fail to correctly validate the length of the input data against the allocated memory space. This lack of rigorous bounds checking allows the application to read beyond the boundaries of the intended buffer region.
From a technical perspective, this is classified as an out-of-bounds read vulnerability, which aligns with Common Weakness Enumeration (CWE) identifier CWE-125: Out-of-bounds Read. In such scenarios, the program attempts to access memory locations that are not allocated for its use. While an out-of-bounds read typically does not allow direct arbitrary code execution like a buffer overflow write vulnerability, it poses significant risks related to information disclosure and application stability. The attacker can potentially leak sensitive data residing in adjacent memory regions, such as internal pointers, cryptographic keys, or other user credentials stored within the process address space. Furthermore, accessing invalid memory addresses often triggers segmentation faults or access violations, leading to immediate application crashes. This capability enables denial-of-service attacks where a remote server hosting maliciously crafted email content can force the client software to terminate unexpectedly, disrupting productivity and potentially causing data loss if unsaved work is present in other open applications within the same session context.
The operational impact of this vulnerability extends beyond simple instability. In enterprise environments or among users who rely on Thunderbird for critical communications, a crash caused by parsing malicious email content can interrupt workflows and create opportunities for social engineering attacks where attackers might exploit the confusion following a crash to trick users into disabling security features or reinstalling compromised versions of the software. Additionally, if the leaked memory contents contain session tokens or authentication cookies associated with other services accessed via the browser engine embedded within Thunderbird, there is a risk of cross-application data leakage. This highlights the importance of strict isolation between different components in modern email clients and underscores how vulnerabilities in one subsystem can compromise the broader security posture of the application.
Mitigation strategies primarily involve applying the vendor-provided patch available in Thunderbird version 140.16 or later, which corrects the buffer boundary checks within the IMAP parser to ensure that all read operations remain strictly within allocated memory limits. Organizations should enforce automated update policies to ensure all client machines are running patched versions promptly after release. For users unable to upgrade immediately due to compatibility constraints with specific add-ons or plugins, temporary mitigations include disabling automatic email fetching for untrusted accounts and manually reviewing incoming messages from unknown sources before they are fully processed by the parser. Security teams should also monitor endpoint detection systems for unusual process termination events related to Thunderbird, which may indicate exploitation attempts in progress. Regular vulnerability scanning of deployed software inventories is essential to identify instances where this specific version gap exists across an organization's infrastructure.