CVE-2026-91807 in PDF Editor
Summary
by MITRE • 09/23/2026
A heap-based out-of-bounds read vulnerability exists in Foxit PDF Editor/Reader’s handling of malformed image soft-mask data. Insufficient validation of the soft-mask data attribute during image parsing may cause an arithmetic underflow, resulting in an out-of-bounds read and application crash.
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Analysis
by VulDB Data Team • 09/23/2026
The identified security flaw resides within the core rendering engine of Foxit PDF Editor and Reader, specifically affecting the processing pipeline for Portable Document Format files that contain complex graphical elements. This vulnerability is classified as a heap-based out-of-bounds read, which corresponds to CWE-125 in the Common Weakness Enumeration standard. The root cause lies in the insufficient validation logic applied when parsing image soft-mask data attributes within PDF structures. Soft masks are used in PDF documents to define transparency and alpha channel information for images, allowing for sophisticated visual effects such as fading or masking specific portions of an image. When a maliciously crafted PDF file is processed, the application fails to adequately verify the integrity and bounds of these soft-mask data streams before attempting to access them during rendering operations.
During the parsing phase, the software performs arithmetic calculations based on values extracted from the malformed soft-mask attributes. Due to the lack of rigorous boundary checks, an attacker can supply specific input parameters that trigger an arithmetic underflow condition. This mathematical error results in a miscalculated memory address or buffer size that is significantly smaller than intended but wraps around to a large positive value due to integer overflow behavior typical in many programming languages and architectures. Consequently, the application attempts to read data from a heap-allocated buffer using this incorrect index or length parameter. Since the calculated offset exceeds the actual allocated boundaries of the memory region, the program performs an out-of-bounds read operation. This action accesses memory locations that are not part of the intended object, leading to the retrieval of unintended data stored in adjacent memory blocks.
The operational impact of this vulnerability is primarily characterized by application instability and potential denial of service conditions. When the rendering engine attempts to process these invalid memory references, it typically encounters access violations or segmentation faults within the operating system's memory management subsystem. This results in an immediate crash of the Foxit PDF Editor or Reader application, disrupting user workflows and potentially causing data loss if unsaved documents are involved. While out-of-bounds reads do not inherently allow for arbitrary code execution like buffer overflows might, they can leak sensitive information from the process memory space depending on what adjacent data is exposed. Furthermore, in complex rendering scenarios, such crashes can be leveraged as part of a larger attack chain to destabilize the host system or facilitate further exploitation if combined with other vulnerabilities within the same software stack.
From an offensive security perspective, this vulnerability aligns with techniques observed in malicious PDF campaigns where crafted documents are used to trigger exceptions and crash applications for denial-of-service purposes or to probe memory layouts during exploit development phases. It relates to ATT&CK technique T1498, Network Denial of Service, specifically when the goal is to disrupt availability through application crashes triggered by malformed inputs. The vulnerability highlights a critical gap in input validation practices within document parsing libraries, emphasizing the need for strict adherence to specification limits and robust error handling mechanisms that prevent arithmetic errors from propagating into memory access decisions.
To mitigate this risk, users should ensure that Foxit PDF Editor and Reader are updated to the latest version released by Foxit Software, which includes patches addressing these input validation deficiencies. Administrators managing enterprise environments should implement application whitelisting or sandboxing solutions to limit the potential impact of any successful exploitation attempts. Additionally, security awareness training for end-users regarding the risks of opening untrusted PDF files remains a vital defense layer against social engineering attacks that often precede such technical exploits. Developers and maintainers must prioritize rigorous fuzz testing of document parsers and enforce strict bounds checking on all arithmetic operations involving memory allocation sizes or array indices derived from external file inputs to prevent similar underflow conditions in the future.