Buffer overflow in Sumatrapdfreader Sumatrapdf
CVE-2026-55586
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.
Vulnerability class: Buffer Overflow
Published · last modified .
CVSS v3 metric
CVSS v3 base score 6.6 (Medium). Vector: CVSS:3.1/AV:L/AC:L/PR:N/UI:R/S:U/C:L/I:H/A:L.
EPSS exploit prediction
EPSS: 0.002 (6.2th percentile), scored .
Very low probability of exploitation in the next 30 days; routine patching cadence is appropriate. 6th percentile — 6.2% of CVEs in the catalogue have a lower EPSS than this one. How to read EPSS.
EPSS over last 30 days · oldest: 0.001 · newest: 0.002 · change: +0.000
Affected products
- Sumatrapdfreader Sumatrapdf — versions <= 3.6.1rel
Weakness classification (CWE)
References
Frequently asked questions
- What is CVE-2026-55586?
- CVE-2026-55586 is a medium-severity vulnerability in Sumatrapdfreader Sumatrapdf, classified under Improper Restriction of Operations within the Bounds of a Memory Buffer. CVSS score: 6.6/10. Published 2026-08-20.
- How severe is CVE-2026-55586?
- Medium severity. CVSS v3 base score is 6.6 out of 10.