CVE-2026-107737 in SumatraPDF
Summary
by MITRE • 10/09/2026
SumatraPDF is a multi-format reader for Windows. In 3.6.1 and earlier, and in pre-release 3.7.0.20369 when WebView2 is absent or cannot initialize, ParseProtoUrl() accepts the signed host component of an its:// URL and FindHtmlWindowById() uses it directly as an index into gHtmlWindows. Opening a crafted CHM through the IE fallback backend with a negative or otherwise out-of-range window identifier can cause an out-of-bounds pointer read followed by an invalid object callback dereference and process termination. 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 the Windows operating system, supporting various file types including PDF, EPUB, MOBI, CHM, XPS, CBZ, and CBR. The software architecture relies on different rendering backends depending on the file format and available system components. For certain formats like HTML-based help files (CHM), SumatraPDF may utilize an Internet Explorer fallback backend if specific dependencies such as WebView2 are absent or fail to initialize properly. This architectural decision introduces a critical attack surface when handling untrusted input within these legacy rendering contexts, particularly regarding how URL parameters and window identifiers are processed during the parsing phase of document loading operations.
The core technical flaw resides in the interaction between the ParseProtoUrl() function and the FindHtmlWindowById() routine within the IE fallback backend logic. When a crafted CHM file is opened that contains an its:// URL scheme, the application extracts the signed host component from this URL structure. Instead of performing strict validation to ensure the extracted value represents a valid, non-negative integer index suitable for array access, the software passes this raw identifier directly into FindHtmlWindowById(). This function subsequently uses the provided identifier as a direct index into the global gHtmlWindows data structure, which maintains references to open HTML windows within the application session.
This lack of bounds checking leads to an out-of-bounds pointer read when the supplied window identifier is negative or exceeds the valid range of indices for the gHtmlWindows array. In C and C++ based applications, accessing memory outside the allocated boundaries of a data structure results in undefined behavior. Specifically, reading from invalid memory locations can retrieve corrupted pointers or garbage values that are then interpreted as object references. The subsequent step involves an invalid object callback dereference, where the application attempts to invoke methods on these malformed objects. This sequence inevitably triggers a process termination due to access violation exceptions, manifesting as a denial of service condition for the end user.
From a vulnerability classification perspective, this issue aligns with CWE-125 Out-of-bounds Read and CWE-94 Improper Control of Generation of Code (Code Injection) in its broader context of unsafe memory manipulation leading to execution flow disruption. The attack vector is classified under ATT&CK technique T1203 Exploitation for Defense Evasion or potentially T1189 Drive-by Compromise if delivered via malicious web content, although the primary impact here is denial of service rather than arbitrary code execution due to the lack of a fixed version and the nature of the crash. The absence of input validation on numeric indices derived from external sources represents a fundamental failure in secure coding practices regarding array boundary checks.
The operational impact of this vulnerability allows an attacker who can trick a user into opening a maliciously crafted CHM file or HTML document to cause immediate application crashes. While current reports indicate no fixed version is available as of the review date, users are advised to exercise extreme caution when viewing documents from untrusted sources. Mitigation strategies include disabling the IE fallback backend if possible by ensuring WebView2 is correctly installed and initialized, thereby bypassing the vulnerable code path entirely. Alternatively, administrators can restrict file associations for CHM files or implement application whitelisting policies to prevent unauthorized execution of document viewers with known vulnerabilities until a patched release becomes available.