CVE-2026-91811 in PDF Editor
Summary
by MITRE • 09/23/2026
A heap-based out-of-bounds write vulnerability exists in Foxit PDF Editor/Reader’s PRC parser due to insufficient validation of vertex indices in triangular fan texture meshes. Successful exploitation could result in memory corruption and an application crash.
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Analysis
by VulDB Data Team • 09/23/2026
The identified security flaw resides within the Portable Resource Format, or PRC, parsing module of Foxit PDF Editor and Reader applications. This component is responsible for interpreting complex vector graphics data embedded within PDF documents to render high-fidelity visual content on user displays. The specific vulnerability stems from a heap-based out-of-bounds write condition triggered by insufficient validation of vertex indices when processing triangular fan texture meshes. In computer graphics, triangular fans are an efficient method for rendering polygonal surfaces by defining a central vertex and a sequence of surrounding vertices that form adjacent triangles. When the parser processes these structures, it relies on index arrays to map logical positions in the mesh data structure to actual memory locations where geometric coordinates are stored.
The core technical deficiency lies in the lack of rigorous bounds checking for these indices before they are used as offsets into heap-allocated buffers. If a maliciously crafted PDF document contains PRC data with vertex indices that exceed the allocated size of the target buffer, the parser will write graphical coordinate data to memory locations beyond the intended boundary. This action constitutes an arbitrary heap overwrite, allowing an attacker to corrupt adjacent memory structures such as allocation headers, function pointers, or other critical application state variables. The immediate operational consequence of this corruption is often a denial of service manifested by an application crash due to segmentation faults or invalid memory access violations when the corrupted data is subsequently accessed during rendering operations.
However, the implications extend far beyond simple stability issues. Heap-based out-of-bounds writes are among the most dangerous classes of vulnerabilities because they provide a mechanism for arbitrary code execution under specific conditions. By carefully crafting the malicious PDF to control the content written outside the buffer boundaries, an attacker can overwrite function pointers or exception handling structures within the application's memory space. This technique aligns with Common Weakness Enumeration identifier CWE-787, which classifies out-of-bounds writes on heap objects as a critical integrity violation. Furthermore, this vulnerability maps to MITRE ATT&CK tactic T1203, specifically falling under exploitation for defense evasion or initial access vectors where malicious files are used to trigger code execution in vulnerable software components without user interaction beyond opening the file.
The risk is significantly amplified by the widespread adoption of Foxit PDF Editor and Reader as enterprise-grade tools for document management and review. Users frequently open documents received via email, downloaded from untrusted sources, or generated by third-party applications, assuming that standard security controls will prevent malicious content execution. The absence of strict input validation in the PRC parser means that a single crafted file can serve as an effective attack vector against multiple users within an organization if not properly mitigated. This highlights the importance of treating all external inputs to document parsers with extreme skepticism and implementing defense-in-depth strategies rather than relying solely on application-level security features.
To mitigate this vulnerability, immediate action must be taken by applying vendor-provided patches that address the insufficient validation logic in the PRC parser. Organizations should ensure that their Foxit installations are updated to versions where vertex index bounds checking has been rigorously implemented and tested against edge cases involving large or malformed triangular fan meshes. In environments where patching is not immediately feasible, network-level controls such as deep packet inspection appliances configured to detect suspicious PDF structures containing PRC data can provide a layer of protection by blocking potentially malicious documents before they reach the endpoint application. Additionally, restricting user privileges and employing application whitelisting policies can limit the impact if an exploitation attempt succeeds, reducing the likelihood of full system compromise through privilege escalation following memory corruption events.