CVE-2026-91817 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 wide strings in embedded PDF JavaScript. Insufficient validation of string-deletion ranges can cause an integer 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 represents a critical memory safety violation within the core parsing engine of Foxit PDF Editor and Reader, specifically targeting the processing logic for wide strings embedded within JavaScript objects inside PDF documents. 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 an insufficient validation mechanism when handling string-deletion operations during the execution of embedded scripts. When the application processes these wide strings, it fails to adequately verify that the requested deletion range is within the bounds of the allocated memory buffer. This lack of rigorous boundary checking allows for a specific arithmetic error known as an integer underflow to occur.
The technical mechanism behind this vulnerability involves the manipulation of string length or offset parameters by maliciously crafted PDF content. When the application calculates the new position or size after a deletion operation, it performs subtraction without verifying that the operand is smaller than the current buffer index. If an attacker provides a value larger than the available data, the unsigned integer arithmetic wraps around to a very large positive number. This miscalculated address points outside the originally allocated heap memory region but remains within accessible virtual memory space. Consequently, when the application attempts to read from this invalid location, it triggers an out-of-bounds access event rather than immediately crashing due to segmentation faults in some contexts, though the primary observed impact is a denial of service through application crash.
From an operational perspective, this vulnerability poses a significant risk to end-users who open specially crafted PDF files containing malicious JavaScript payloads. The immediate consequence is a denial of service, as the application terminates unexpectedly upon encountering the malformed input. While out-of-bounds reads are often precursors to more severe exploits such as arbitrary code execution or information disclosure, in this specific instance, the primary impact observed is stability degradation and potential data loss if unsaved work is present during the crash. The vulnerability aligns with ATT&CK technique T1203, which covers Exploitation for Client Execution, indicating that an attacker could leverage this flaw to disrupt user productivity or potentially use it as part of a larger attack chain involving social engineering tactics like phishing campaigns delivering malicious PDF attachments.
Mitigation strategies must address both the immediate technical deficiency and broader security hygiene practices. The primary remediation is the deployment of vendor-provided patches that enforce strict bounds checking on all string manipulation operations, particularly those involving deletion or truncation within embedded JavaScript contexts. Developers should implement defensive programming techniques such as using safe integer arithmetic libraries that detect overflow and underflow conditions before they result in invalid memory addresses. Additionally, input validation routines must be strengthened to ensure that any parameters derived from external sources are validated against the actual size of the target buffer prior to execution. Users should maintain their software up-to-date with the latest security updates provided by Foxit Software and exercise caution when opening PDF files containing embedded scripts or macros from untrusted sources.