Is CVE-2026-63209 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-63209 being exploited in the wild.

How severe is CVE-2026-63209?

High severity, with a CVSS score of 7.5, published 29 September 2026.

Which products and versions are affected?

github.com/klauspost/compress (Go) <= 1.18.6, SEMVER:introduced=0,fixed=1.18.7; Klauspost Compress before 1.18.7.

Is there a fix for CVE-2026-63209?

Yes. github.com/klauspost/compress (Go): upgrade to 1.18.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-63209 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-63209?

Check whether you run github.com/klauspost/compress (Go) or Klauspost Compress at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-63209: klauspost compress signed integer overflow in s2.NewDict

Q: Is there a fix for CVE-2026-63209?

Yes. github.com/klauspost/compress (Go): upgrade to 1.18.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

Q: How is CVE-2026-63209 attacked?

It is attacked over the network, in a software library.

Q: Am I affected by CVE-2026-63209?

Check whether you run github.com/klauspost/compress (Go) or Klauspost Compress at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-63209 · Severity: high · CVSS 7.5 · Published 2026-09-29

The compress library provides data compression algorithms used by applications to reduce file and network data sizes. A signed integer overflow vulnerability in the s2 compression format allows an attacker to crash an application by supplying a malicious dictionary that triggers an out-of-bounds memory access when encoding data. This results in denial of service via process crash.

Technologies: github.com/klauspost/compress (Go). Vendors: Go.

Quick answers

Is CVE-2026-63209 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-63209 being exploited in the wild.
How severe is CVE-2026-63209?: High severity, with a CVSS score of 7.5, published 29 September 2026.
Which products and versions are affected?: github.com/klauspost/compress (Go) <= 1.18.6, SEMVER:introduced=0,fixed=1.18.7; Klauspost Compress before 1.18.7.
Is there a fix for CVE-2026-63209?: Yes. github.com/klauspost/compress (Go): upgrade to 1.18.7 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-63209 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-63209?: Check whether you run github.com/klauspost/compress (Go) or Klauspost Compress at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

github.com/klauspost/compress (Go): upgrade to 1.18.7 or later
```
go get github.com/klauspost/compress@v1.18.7
```

Technical details

A signed integer overflow in s2.NewDict() allows bypass of repeat index validation when a uvarint-encoded repeat value exceeds MaxInt64. The overflowed negative repeat value subsequently causes an out-of-bounds memory access via unsafe.Pointer arithmetic in Dict.Encode(), resulting in SIGSEGV. The vulnerability requires attacker-controlled dictionary input and is patched in version 1.18.7.

Affected products

klauspost compress before 1.18.7

Identifiers

CVE-2026-63209
GHSA-259r-337f-4rfw
GO-2026-5841

Timeline

2026-09-29: disclosed
2026-06-30: patched: version 1.18.7 released

References

This page as Markdown. Free to reuse under CC BY 4.0 with a link back.