CVE-2026-103680 in tnef
Summary
by MITRE • 10/01/2026
A flaw was found in tnef. A heap-based buffer overflow can occur in the find_free_number() function when generating numbered backup suffixes for duplicate filenames. When numbered backups are enabled and file overwriting is disabled, an attacker can supply a specially crafted Transport Neutral Encapsulation Format (TNEF) file with an excessive number of colliding attachment filenames, causing the numeric counter to write past the allocated memory buffer. This issue may result in an application crash, leading to a Denial of Service (DoS), or potentially arbitrary code execution.
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Analysis
by VulDB Data Team • 10/01/2026
The vulnerability identified in tnef represents a critical heap-based buffer overflow within the find_free_number function, which is responsible for generating numbered backup suffixes when handling duplicate filenames during file processing operations. This flaw emerges specifically under conditions where numeric backups are enabled and direct file overwriting is disabled, forcing the application to attempt creating unique identifiers by appending incremental numbers to existing filename strings. The root cause lies in insufficient bounds checking within the logic that manages these counter increments, allowing an attacker who can supply a specially crafted Transport Neutral Encapsulation Format TNEF file with an excessive number of colliding attachment filenames to trigger a write operation beyond the boundaries of the allocated memory buffer. This scenario exploits the assumption that the target buffer has sufficient space to accommodate the longest possible numeric suffix generated during the collision resolution process, a condition that is not adequately validated before the memory write occurs.
From a technical perspective, this vulnerability aligns with CWE-120 Buffer Copy without Checking Size of Input Classic and CWE-787 Out-of-bounds Write in terms its fundamental nature as an improper limitation on buffer size leading to memory corruption. The heap-based overflow allows for arbitrary data manipulation within the process memory space, which can corrupt adjacent heap metadata or overwrite critical control structures such as function pointers or exception handlers depending on the specific allocator implementation and surrounding memory layout. Such memory corruption is particularly dangerous because it provides a reliable mechanism for an attacker to gain unauthorized access to system resources or execute malicious code with the privileges of the affected application process. The vulnerability falls under MITRE ATT&CK technique T1203 Exploitation for Client Execution as it targets client-side applications that parse untrusted input, and potentially T1496 Resource Hijacking if exploited primarily for denial of service purposes through resource exhaustion or system instability caused by memory corruption.
The operational impact of this vulnerability is severe, presenting a dual threat vector to organizations relying on tnef for processing email attachments encoded in the Transport Neutral Encapsulation Format commonly associated with Microsoft Outlook rich text format messages and other proprietary data structures. In its most immediate manifestation, the buffer overflow can cause application crashes leading to Denial of Service conditions that disrupt productivity and potentially destabilize systems if the affected process is critical infrastructure or part of a larger service chain. However, the more significant risk lies in the potential for arbitrary code execution where an attacker leverages the memory corruption to inject and execute shellcode or other malicious payloads on the victim machine. This capability transforms what might otherwise be viewed as a simple parsing error into a high-severity remote code execution vulnerability if the application processes untrusted TNEF files received via email or downloaded from external sources without adequate sandboxing or input validation mechanisms in place prior to invoking tnef libraries.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements to prevent similar classes of vulnerabilities. The primary defense involves applying vendor-provided patches that update the find_free_number function to include rigorous bounds checking before performing any memory writes based on calculated suffix lengths developers should ensure that buffer sizes are dynamically verified against maximum possible output requirements including worst-case scenarios involving numerous filename collisions. Additionally organizations should implement input validation at entry points where TNEF files are ingested ensuring that attachment counts and filename structures conform to expected norms rather than accepting arbitrarily large or malformed inputs without scrutiny deploying application whitelisting solutions can further restrict the execution of untrusted binaries while network segmentation limits lateral movement in case a host is compromised. Regular security assessments including static analysis scanning for buffer overflow patterns and dynamic fuzzing testing against TNEF parsers will help identify similar weaknesses before they are exploited by adversaries seeking to gain initial access or escalate privileges within enterprise environments.