CVE-2026-92240 in Thunderbird
Summary
by MITRE • 09/15/2026
A malicious or compromised IMAP server can trigger an out-of-bounds read in the IMAP response parser by sending an untagged '* ID' response, crashing Thunderbird. The affected parsing path is reachable before authentication. This vulnerability was fixed in Thunderbird 156 and Thunderbird 140.16.
VulDB is the best source for vulnerability data and more expert information about this specific topic.
Analysis
by VulDB Data Team • 09/15/2026
The identified vulnerability represents a critical security flaw within the Internet Message Access Protocol (IMAP) client implementation of Mozilla Thunderbird, specifically located in the response parser component responsible for handling server-to-client communication streams. This issue arises from an insufficient validation mechanism when processing untagged ID responses, which are part of the IMAP protocol's capability negotiation and identification framework defined in RFC 2971. The core technical failure is classified as an out-of-bounds read, a type of memory safety error where the application attempts to access data at a memory address that falls outside the allocated buffer boundaries. This specific flaw is triggered when the parser encounters a malformed or maliciously crafted untagged ID response from the IMAP server, leading Thunderbird to attempt reading beyond the limits of its internal buffers.
The severity and operational impact of this vulnerability are significantly amplified by its reachability prior to user authentication. In standard IMAP workflows, clients establish a connection with the mail server before authenticating credentials, allowing for initial capability exchange and identification exchanges. Because this parsing path is accessible during the pre-authentication phase, an attacker controlling or compromising a malicious IMAP server can exploit this flaw without requiring valid login credentials from the victim. This characteristic transforms the vulnerability into a potential remote code execution vector if combined with other exploitation techniques, although in its current manifestation, it primarily results in application instability and denial of service conditions. The immediate consequence for end-users is that Thunderbird may crash unexpectedly upon connecting to such a server, resulting in data loss from unsaved work and disruption of email access services.
From a threat modeling perspective, this vulnerability aligns with CWE-125, which describes Out-of-bounds Read vulnerabilities where software reads memory beyond the intended boundary. The attack vector is categorized under ATT&CK technique T1078, Valid Accounts, as it leverages legitimate protocol interactions to achieve malicious outcomes, although in this specific case, no valid account is strictly required due to the pre-authentication nature of the flaw. It also relates to CWE-20, Improper Input Validation, indicating that the software failed to adequately verify or sanitize input data received from an external source before processing it. The exploitation scenario typically involves a compromised mail server acting as a man-in-the-middle or simply a rogue server configured by an attacker to send specially crafted packets designed to trigger the buffer overflow condition during the initial handshake and ID exchange phases of the IMAP session.
Mitigation strategies for this vulnerability primarily involve immediate software updates, as Mozilla has addressed the root cause in Thunderbird version 140.16. Organizations should enforce strict update policies to ensure all client installations are patched against this specific parsing error. In environments where automatic updating is not feasible or delayed, network-level controls can provide temporary relief by filtering IMAP traffic for known malicious signatures associated with malformed ID responses, although such deep packet inspection may impact performance and privacy. Additionally, administrators should review server configurations to prevent compromised internal servers from being accessed by client applications until the patching process is complete. Long-term resilience requires implementing strict input validation frameworks within email clients and adhering to secure coding practices that prioritize memory safety checks during protocol parsing operations to prevent similar out-of-bounds access issues in future releases.