CVE-2017-5443 in Firefox
Summary
by MITRE
An out-of-bounds write vulnerability while decoding improperly formed BinHex format archives. This vulnerability affects Thunderbird < 52.1, Firefox ESR < 45.9, Firefox ESR < 52.1, and Firefox < 53.
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Analysis
by VulDB Data Team • 11/26/2025
The vulnerability identified as CVE-2017-5443 represents a critical out-of-bounds write flaw that manifests during the decoding process of malformed BinHex format archives within multiple Mozilla-based applications. This issue stems from insufficient input validation mechanisms that fail to properly handle improperly structured BinHex data streams, creating opportunities for malicious actors to exploit memory corruption vulnerabilities. The affected software ecosystem includes Thunderbird versions prior to 52.1, Firefox Extended Support Release versions before 45.9 and 52.1, and standard Firefox versions before 53, indicating a widespread impact across Mozilla's browser and email client platforms. The vulnerability specifically targets the BinHex decoding component, which is used to handle binary data encoding formats commonly found in legacy systems and certain file attachments.
The technical exploitation of this vulnerability occurs when the application attempts to parse and decode BinHex formatted data that contains malformed structures or improper data boundaries. During the decoding process, the application's memory management routines fail to properly validate array bounds or buffer limits, allowing an attacker to craft specially crafted BinHex archives that trigger memory corruption. This out-of-bounds write condition can potentially overwrite adjacent memory locations, leading to arbitrary code execution or application crashes. The flaw operates at the decoding layer where input data is processed and converted into usable binary formats, making it particularly dangerous as it can be triggered through email attachments or file downloads that contain maliciously constructed BinHex data. According to CWE classification, this vulnerability maps to CWE-787 Out-of-bounds Write, which specifically addresses situations where a program writes to memory beyond the boundaries of a buffer.
The operational impact of CVE-2017-5443 extends beyond simple application instability, as successful exploitation can enable remote code execution capabilities that allow attackers to gain full control over affected systems. This vulnerability is particularly concerning in email environments where users may inadvertently open malicious attachments, as the attack vector requires no user interaction beyond the initial opening of the compromised BinHex file. The widespread nature of the affected software versions means that a significant portion of users remained vulnerable for extended periods, creating substantial risk exposure for organizations relying on these applications. Attackers could leverage this vulnerability to deliver malware payloads, establish persistent backdoors, or perform privilege escalation attacks on compromised systems, making it a high-priority security concern for both individual users and enterprise environments. The vulnerability's classification under the ATT&CK framework would align with techniques involving code injection and privilege escalation, as the memory corruption can be weaponized to execute arbitrary commands within the application context.
Mitigation strategies for CVE-2017-5443 primarily focus on immediate software updates and patches provided by Mozilla to address the underlying buffer handling issues in the BinHex decoding routines. Organizations should prioritize updating all affected versions of Thunderbird, Firefox ESR, and Firefox to their respective patched releases, as these updates contain fixed memory validation checks and improved input sanitization mechanisms. Additional defensive measures include implementing email filtering rules to block suspicious BinHex attachments, deploying sandboxing techniques to limit the impact of potential exploitation, and conducting security awareness training to educate users about the risks of opening unknown file attachments. Network-level protections such as web application firewalls and content filtering systems can also help reduce the attack surface by blocking malicious BinHex data streams before they reach vulnerable applications. The vulnerability demonstrates the importance of robust input validation and memory safety practices in software development, particularly for applications that process external data formats that may contain malformed or malicious content.