CVE-2026-92238 in Thunderbird
Summary
by MITRE • 09/16/2026
A maliciously constructed mail header could lead to multiple fields being parsed as one, or potential memory safety violations. 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 resides within the email client's parsing logic for mail headers, specifically affecting how multi-line header fields are processed according to RFC standards. Email protocols allow certain header values to span multiple lines by using a specific folding mechanism where a newline followed by whitespace indicates continuation of the previous field. In this instance, the application fails to correctly distinguish between legitimate line continuations and malformed or maliciously crafted sequences that attempt to merge distinct logical fields into a single parsed entity. This parsing error creates an ambiguity in how the mail client interprets the structure of incoming messages, leading to incorrect segmentation of header data which can have severe downstream consequences for security controls and application stability.
From a technical perspective, this flaw represents a classic case of improper input validation during the tokenization phase of message processing. When a maliciously constructed header is received, the parser may incorrectly concatenate separate fields or misinterpret boundary markers, resulting in memory safety violations such as buffer overflows or out-of-bounds reads. These types of errors occur because the software does not adequately verify that the length and structure of parsed data align with expected bounds before writing to memory buffers. Such vulnerabilities are particularly dangerous because they can be triggered simply by receiving a specially crafted email, requiring no user interaction beyond opening or rendering the message in some configurations. This characteristic makes it an attractive vector for remote code execution attacks if the memory corruption allows for arbitrary write primitives or control flow hijacking.
The operational impact of this vulnerability is significant as it compromises both the integrity and availability of the mail client environment. Beyond the potential for remote code execution, incorrect header parsing can lead to spoofing vulnerabilities where attackers might bypass authentication checks or content filters that rely on specific header fields such as From, Reply-To, or X-Originating-IP. If an attacker can merge headers, they may manipulate how security gateways or local policies interpret message origin and routing information. Furthermore, the memory safety violations pose a direct threat to system stability, potentially causing application crashes that lead to denial of service conditions for end users who rely on continuous access to their email services.
This issue aligns with CWE-20 Improper Input Validation and CWE-120 Buffer Copy without Checking Size of Input in C/C++, reflecting the fundamental failure to sanitize and bound user-controlled data before processing. In terms of offensive security frameworks, this vulnerability facilitates techniques associated with ATT&CK T1566 Phishing, as it enables more sophisticated social engineering attacks by allowing attackers to craft emails that evade standard detection mechanisms or exploit client-side rendering bugs. The ability to manipulate header parsing effectively undermines the trust model inherent in email communication protocols.
Mitigation for this risk involves applying the vendor-provided patches released in Thunderbird versions 156 and 140.16, which contain corrected logic for handling multi-line headers and improved bounds checking during memory allocation operations. Organizations should ensure that all endpoints running affected versions are updated immediately to close these attack surfaces. Additionally, implementing strict email filtering policies at the network perimeter can help block malformed messages before they reach client applications. Security teams should also monitor for indicators of compromise related to unusual process behavior or memory access patterns within the mail client executable, as exploitation attempts may leave traces in system logs even if full remote code execution is not achieved. Regular auditing of third-party software dependencies and prompt application of security updates remain critical defense-in-depth strategies against such parsing-based vulnerabilities.