CVE-2026-55586: Sumatrapdfreader Sumatrapdf

Medium severity, CVSS 6.6. EPSS: 0.2% chance of exploitation in the next 30 days.

SumatraPDF is a multi-format reader for Windows. In 3.6.1 and earlier, a crafted CHM file can supply malformed LZX Huffman code lengths to make_decode_table in ext/CHMLib/lzx.c. In the long-code branch, the function writes new internal nodes through next_symbol before validating that the canonical Huffman table has overflowed. The PRETREE case can write beyond the 104-entry PRETREE_table into adjacent heap state in struct LZXstate when reached through chm_open, chm_retrieve_object, LZXdecompress, and BUILD_TABLE. This produces heap memory corruption in the parser process, while arbitrary code execution has not been demonstrated. No fixed version is available as of this review.

Affected products

• Sumatrapdfreader Sumatrapdf: up to and including 3.6.1rel

Published 2026-08-20. Last modified 2026-09-18.