SumatraPDF Reader up to 3.6.1 CHMLib ext/CHMLib/lzx.c make_decode_table memory corruption

Summaryinfo

A vulnerability was found in SumatraPDF Reader SumatraPDF up to 3.6.1. It has been declared as problematic. The affected element is the function make_decode_table of the file ext/CHMLib/lzx.c of the component CHMLib. Executing a manipulation can lead to memory corruption. The identification of this vulnerability is CVE-2026-55586. The attack can only be executed locally. There is no exploit available.

Detailsinfo

A vulnerability was found in SumatraPDF Reader SumatraPDF up to 3.6.1. It has been declared as problematic. This vulnerability affects the function make_decode_table of the file ext/CHMLib/lzx.c of the component CHMLib. The manipulation with an unknown input leads to a memory corruption vulnerability. The CWE definition for the vulnerability is CWE-119. The product performs operations on a memory buffer, but it can read from or write to a memory location that is outside of the intended boundary of the buffer. As an impact it is known to affect confidentiality, integrity, and availability. CVE summarizes:

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.

The advisory is shared for download at github.com. This vulnerability was named CVE-2026-55586 since 06/17/2026. The exploitation appears to be easy. The attack needs to be approached locally. Successful exploitation requires user interaction by the victim. There are known technical details, but no exploit is available. The current price for an exploit might be approx. USD $0-$5k (estimation calculated on 08/20/2026).

There is no information about possible countermeasures known. It may be suggested to replace the affected object with an alternative product.

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Productinfo

Vendor

• SumatraPDF Reader

Name

• SumatraPDF

Version

• 3.6.0
• 3.6.1

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