Heap Memory Corruption in SumatraPDF's CHM File Handling

CVE-2026-55586

What is CVE-2026-55586?

A vulnerability in SumatraPDF, a versatile document reader for Windows, allows a specially crafted CHM file to exploit an issue in the LZX decompression algorithm. An attacker can supply malformed LZX Huffman code lengths that lead to heap memory corruption when the 'make_decode_table' function incorrectly processes the data. Specifically, the vulnerability occurs in the long-code branch where new internal nodes may be written without validating the canonical Huffman table. This results in overwriting memory beyond the designated boundaries, impacting the stability and security of the application. Although arbitrary code execution has not been demonstrated, the potential for exploitation remains a significant concern. As of now, there is no patched version available.

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