CVE-2026-63209: Integer Overflow or Wraparound and Out-of-bounds Write in compress

Severity: high (CVSS 7.5 v3.1)
Exploited in the wild: not listed in CISA KEV
EPSS: 0.5% probability of exploitation in the next 30 days
Public exploit or detection template: none known
Published: 2026-09-29 · updated 2026-09-30
Weakness: CWE-190

Description

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

Go Github.Com/Klauspost/Compress: before 1.18.7 — fixed in 1.18.7
klauspost compress: before 1.18.7

How to fix

Upgrade to a fixed version: Go Github.Com/Klauspost/Compress 1.18.7. Check the vendor advisory for mitigations.

References

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