CVE-2026-84641 in Thunderbird
Summary
by MITRE • 09/02/2026
A malicious IMAP server can trigger use-after-free and heap-memory disclosure by sending a crafted ID response. Heap contents can ultimately be persisted to prefs.js. This vulnerability was fixed in Thunderbird 155, Thunderbird 140.15, and Thunderbird 153.2.
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Analysis
by VulDB Data Team • 09/02/2026
The described vulnerability represents a critical security flaw within the Internet Message Access Protocol (IMAP) client implementation of Mozilla Thunderbird, specifically involving improper memory management during the processing of server responses. The core technical issue stems from a use-after-free condition triggered when the application processes an ID response sent by a malicious IMAP server. In this scenario, the software allocates heap memory to handle incoming data but fails to properly manage the lifecycle of that memory allocation after it is no longer needed or under specific error conditions. Consequently, if a compromised or adversarial server sends a specially crafted ID response, the application may attempt to access or manipulate memory regions that have already been freed and potentially reallocated for other purposes. This use-after-free flaw allows an attacker who controls the IMAP server to execute arbitrary code with the privileges of the user running Thunderbird, provided they can trick the victim into connecting to their malicious server.
Beyond the potential for remote code execution through memory corruption, this vulnerability also facilitates heap-memory disclosure. By carefully crafting the ID response and exploiting the timing of memory reallocation, an attacker can read sensitive data from the process's heap space. This information leak is particularly dangerous because it enables the extraction of internal application states, cryptographic keys, or other confidential data stored in memory. The severity of this disclosure is significantly amplified by the fact that these leaked heap contents can be persisted to the prefs.js file. Since prefs.js serves as a primary configuration storage for Thunderbird profiles containing user preferences and potentially cached authentication tokens or session identifiers, writing sensitive heap data into this file creates a persistent side channel. This means that even after the immediate exploitation attempt concludes, residual malicious artifacts or stolen secrets may remain on disk in a readable format, facilitating further attacks such as credential theft or profile manipulation without requiring continuous network access to the vulnerable server.
From an industry standard perspective, this vulnerability aligns with CWE-416, Use After Free, which describes situations where pointers are used after they have been freed, leading to undefined behavior and potential security breaches. Additionally, the heap-memory disclosure aspect relates closely to CWE-200, Exposure of Sensitive Information to an Unauthorized Actor. The attack vector involves a malicious server initiating contact with the client, which maps to MITRE ATT&CK technique T1571, Non-Standard Port, or more accurately in this context, leveraging legitimate protocols like IMAP for initial access and exploitation. The persistence mechanism via prefs.js suggests capabilities associated with data staging or exfiltration techniques where sensitive information is written to disk-based storage locations that are less monitored than active memory regions.
The operational impact of this vulnerability allows an attacker to achieve arbitrary code execution on the victim's system by leveraging the use-after-free condition, effectively compromising the integrity and confidentiality of the user's environment. The ability to disclose heap contents further erodes trust in the application’s security model, as it exposes internal workings that should remain opaque to external entities. Furthermore, the persistence of this data into prefs.js means that the compromise is not ephemeral; attackers can potentially retrieve stolen information at a later time or use the persisted data to maintain access if other authentication mechanisms are bypassed. This creates a multi-stage attack path where initial exploitation leads directly to long-term reconnaissance and potential lateral movement within the user's digital ecosystem.
Mitigation for this vulnerability requires immediate action by users to update their Thunderbird installation to version 155, or alternatively to versions 140.15 and 153.2, which contain the necessary patches to correct the memory management errors in the IMAP ID response handler. Security administrators should enforce automated patching policies for email clients across organizational networks to ensure that all instances are protected against this specific exploit chain. Additionally, users can mitigate risk by configuring their mail client settings to disable automatic connection to untrusted servers or by using network-level controls such as firewalls and intrusion detection systems to monitor and block suspicious IMAP traffic patterns indicative of crafted ID responses. Regular security audits and monitoring for unusual file modifications in profile directories like prefs.js can also help detect potential exploitation attempts that may have bypassed initial defenses, allowing for rapid incident response before significant data loss occurs.