Process crash via overflowing S2 repeat index
Published Sep 29, 2026 · Updated Sep 29, 2026
Integer overflow in Klauspost compress S2 before 1.18.7 allows remote attackers to crash a process through a crafted S2 dictionary. The s2.NewDict function converts an untrusted repeat-index uvarint to a signed integer before checking its bounds; a wrapped negative value passes validation and causes an out-of-bounds read during Dict.Encode. Exploitation requires an application to accept attacker-supplied dictionary data and then encode at least 32 bytes with that dictionary.
Summary
What happened
Integer overflow in Klauspost compress S2 before 1.18.7 allows remote attackers to crash a process through a crafted S2 dictionary. The s2.NewDict function converts an untrusted repeat-index uvarint to a signed integer before checking its bounds; a wrapped negative value passes validation and causes an out-of-bounds read during Dict.Encode. Exploitation requires an application to accept attacker-supplied dictionary data and then encode at least 32 bytes with that dictionary.
The record
- CVE
- CVE-2026-63209
- Published
- Sep 29, 2026
- Updated
- Sep 29, 2026
- Vendor
- Klauspost
- Product
- compress
- Classifications
- CWE-190, CWE-787, T1499.004
- Attack vector
- network
- Privileges
- unauthenticated
Timeline
How it unfolded
- Sep 29, 2026CVE publishedPublication date reported by the CVE source.
- Sep 29, 2026Record updatedLatest update available in the CVE record.
Exploitability
Present is not the same as exploitable
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Is a vulnerable build present?
Compare these published version ranges with your installed build and any vendor patches.
- Affected versionversion=< 1.18.7
What conditions does exploitation require?
What is affected?
Published CVSS scores
CVSS describes severity. EPSS estimates exploitation probability.
Attacks
What attackers are doing with it
Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.
Activity data is delayed. The latest report is Sep 28, 2026.
Weakness, pattern, technique
Public exploit references
- Vendor-published S2 dictionary crash proof of conceptproof of concept · unverified
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Technologies
Your stack
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Your stack
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