Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-63209: 0.57% estimated exploitation activity in the next 30 days; percentile 47.36 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Signed Integer Overflow in s2.NewDict Summary 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. Package - Name: github.com/klauspost/compress/s2 - Ecosystem: Go - Affected versions: <= v1.18.6 (all versions with NewDict function) Severity High (7.5) — CVSS v3.1: AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Vulnerability Type - CWE-190: Integer Overflow or Wraparound - CWE-787: Out-of-bounds Write (via unsafe.Pointer) Description The NewDict() function reads a uvarint from dictionary data and converts it to int: When r exceeds MaxInt64 (9223372036854775807), int(r) overflows to a negative value. Since a negative number is never greater than a positive length, the validation…

Published by the source

20 Oct 2026, 14:33 UTC

Last modified by the source

20 Oct 2026, 14:33 UTC

Source last checked

26 Oct 2026, 15:32 UTC

1. Revision 3 · amended · 26 Oct 2026

   Amended from ghsa record GHSA-259r-337f-4rfw

2. Revision 2 · amended · 24 Oct 2026

   Amended from osv record GO-2026-5841

3. Revision 1 · created · 24 Oct 2026

   Published from osv record GHSA-259r-337f-4rfw

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# Patch Next advisory brief: CVE-2026-63209

Advisory: Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress
IDs: CVE-2026-63209, GHSA-259r-337f-4rfw
Record: https://patchnext.com/advisories/cve-2026-63209 (revision 3, source checked 26 Oct 2026, 15:32 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: github.com/klauspost/compress 1.18.7. A vendor fix does not mean a deployment is fixed.

## Affected components
- Go github.com/klauspost/compress: affected <= 1.18.6; fixed in 1.18.7

## Suggested inspection (read-only)
- Go github.com/klauspost/compress: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on github.com/klauspost/compress.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/GHSA-259r-337f-4rfw (OSV record GHSA-259r-337f-4rfw lists affected ranges: Go github.com/klauspost/compress >=0 <1.18.7 (fixed))
- Verified: https://osv.dev/vulnerability/GO-2026-5841 (OSV record GO-2026-5841 lists affected ranges: Go github.com/klauspost/compress >=1.16.0 <1.18.7 (fixed))
- Verified: https://github.com/advisories/GHSA-259r-337f-4rfw (GitHub-reviewed advisory GHSA-259r-337f-4rfw lists affected ranges: Go github.com/klauspost/compress >=0 <=1.18.6)

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