CVE-2026-63209: Klauspost Compress

High severity, CVSS 7.5. EPSS: 0.4% chance of exploitation in the next 30 days.

compress provides various compression algorithms. Prior to version 1.18.7, a signed integer overflow vulnerability in s2.NewDict() allows an attacker to bypass repeat index validation by supplying a dictionary with a uvarint-encoded repeat value exceeding MaxInt64. When Dict.Encode() is subsequently called, the overflowed negative repeat value causes an out-of-bounds memory access via unsafe.Pointer arithmetic, crashing the process with SIGSEGV. This issue has been patched in version 1.18.7.

Affected products

• Klauspost Compress: before 1.18.7 (fixed in 1.18.7)

Published 2026-09-29. Last modified 2026-09-30.