CVE-2026-107731 in SumatraPDF
Summary
by MITRE • 10/09/2026
SumatraPDF is a multi-format reader for Windows. In 3.7.0.22298, four independently reachable range-validation variants in src/LitDoc.cpp allow file-controlled offsets and sizes to overflow, narrow to negative values, or wrap before incomplete bounds checks. The affected calculations include contentOffset, the directory expression dirOff64 + dirLen64, and the decoded-section offset + size, along with secondary-header range handling. Opening a crafted LIT file that reaches one of these variants can cause invalid pointer reads and deterministic application termination. No broader impact is claimed beyond the advisory-supported conditions. No fixed version is available as of this review.
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Analysis
by VulDB Data Team • 10/09/2026
SumatraPDF serves as a widely utilized multi-format document viewer for Windows, designed to handle various file types including PDF, EPUB, and MOBI with an emphasis on speed and minimal resource consumption. The software architecture relies heavily on parsing complex binary structures within these documents to render content accurately. In version 3.7.0.22298, a critical series of vulnerabilities was identified within the source code file src/LitDoc.cpp, which is responsible for handling LIT files, an older Microsoft Reader format. These vulnerabilities stem from insufficient validation of range parameters derived directly from untrusted input data contained within the target document. The core issue involves four distinct variants where file-controlled offsets and sizes are processed without adequate bounds checking prior to arithmetic operations or memory access decisions. This lack of rigorous validation allows maliciously crafted LIT files to exploit logical flaws in how numerical values are interpreted and manipulated by the application engine.
The technical nature of these vulnerabilities centers on integer overflow, underflow, and type narrowing issues that occur before incomplete bounds checks can prevent out-of-bounds accesses. Specifically, the calculations involving contentOffset, the directory expression calculated as dirOff64 plus dirLen64, the decoded-section offset combined with its size, and secondary-header range handling are susceptible to manipulation. When a crafted LIT file is opened, it provides values that cause these arithmetic operations to overflow or wrap around due to signed integer limitations. For instance, adding two large positive offsets might result in a negative value if the sum exceeds the maximum limit of a signed 32-bit integer and is stored in such a type without proper unsigned handling or pre-checks. Similarly, size values can narrow from larger data types to smaller ones, truncating critical information about memory allocation requirements. These mathematical anomalies lead to incorrect pointer arithmetic, where the application calculates an invalid memory address based on the corrupted offset or size parameters.
The operational impact of these flaws is primarily a denial-of-service condition characterized by deterministic application termination. When SumatraPDF processes one of these crafted LIT files and attempts to access memory at the calculated invalid addresses, it triggers exceptions such as segmentation faults or access violations. This results in the immediate crash of the viewer process, disrupting user workflow and potentially causing data loss if unsaved work is present. While the advisory suggests that no broader impact beyond application termination is currently confirmed, the underlying mechanism involves reading from arbitrary memory locations via invalid pointers. In more complex scenarios involving other parsers or different execution contexts within similar software architectures, such out-of-bounds reads can sometimes lead to information disclosure vulnerabilities where sensitive data residing in adjacent memory regions might be leaked to the attacker. However, for this specific instance, the primary risk remains system stability and availability rather than remote code execution or privilege escalation.
These vulnerabilities align closely with Common Weakness Enumeration categories that describe flaws in input validation and arithmetic handling. The root cause is best classified under CWE-190 Integer Overflow or Wraparound, as the core failure lies in numerical values exceeding their representable limits during calculation. Additionally, they fall under CWE-20 Improper Input Validation because the application fails to verify that user-controlled data meets expected constraints before use. From a threat modeling perspective using the MITRE ATT&CK framework, these issues relate to techniques involving resource exhaustion or denial of service through invalid input processing. Attackers can leverage this by distributing malicious LIT files via email attachments, file-sharing platforms, or compromised websites, relying on social engineering to trick users into opening them. Since SumatraPDF is often used for quick document previews, the ease with which a user might open an untrusted file increases the attack surface significantly.
Mitigation strategies must focus on both immediate remediation and long-term architectural improvements. As no fixed version was available at the time of this review, users are advised to exercise extreme caution when opening LIT files from unknown or untrusted sources. Implementing strict input validation within the parsing logic is essential; developers should ensure that all offsets and sizes derived from file headers are validated against maximum allowable bounds before any arithmetic operations are performed. Using unsigned integer types for size calculations can prevent negative value wrapping, while explicit checks for overflow conditions prior to addition or subtraction will mitigate range-validation errors. Furthermore, implementing sandboxing techniques where the document parser runs with restricted privileges can limit the impact of potential crashes and reduce the risk of memory corruption leading to more severe exploits. Regular updates to third-party libraries used by SumatraPDF and adopting static analysis tools that detect integer overflow patterns during development are critical steps toward preventing similar vulnerabilities in future releases.