# CVE-2026-42504: Quadratic complexity in WordDecoder.DecodeHeader in mime

Status: published. Published: 2026-06-02. Updated: 2026-11-01.

Decoding a maliciously-crafted MIME header containing many invalid encoded-words can consume excessive CPU.

## Remediation

No remediation reported in collected sources

## Affected products and versions

- Go standard library / mime (mime): ≥ 0; < 1.25.11 (affected, semver); ≥ 1.26.0-0; < 1.26.4 (affected, semver). Source: CNA: Go

## Severity and prioritization

- CVSS 3.1: 7.5 (high); CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H; source: ADP: CISA-ADP

- CISA KEV: No listing in collected sources

- EPSS: 0.56% as of 2026-11-01; percentile 46.64%

- SSVC: exploitation none; automatable yes; technical impact partial; source ADP: CISA-ADP

- Weaknesses: CWE-407

## Exploit evidence

No exploit evidence in collected sources. This is not proof of no exploitation.

## References

- https://go.dev/issue/79217 (CNA: Go)

- https://go.dev/cl/774481 (CNA: Go)

- https://groups.google.com/g/golang-announce/c/tKs3rmcBcKw (CNA: Go)

- https://pkg.go.dev/vuln/GO-2026-5038 (CNA: Go)

## Provenance

- CNA: Go: https://www.cve.org/CVERecord?id=CVE-2026-42504; retrieved 2026-11-02; source updated 2026-11-01

- FIRST EPSS: https://api.first.org/data/v1/epss?cve=CVE-2026-42504; retrieved 2026-11-02; source updated Not reported

Revision: f76141579f81362055d31a7a096b5db8d7bfc959d847b0b0e1c6a0635e9d38cb

Canonical: https://y2.dev/cve/CVE-2026-42504