What’s Vulnerable › Vulnerabilities

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

• Severity: high (CVSS 7.5 v3.1)
• Exploited in the wild: not listed in CISA KEV
• EPSS: 0.6% probability of exploitation in the next 30 days
• Public exploit or detection template: none known
• Published: 2026-06-02 · updated 2026-11-01
• Weakness: CWE-407

Description

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

Affected products

• Go Stdlib: before 1.25.11; from 1.26.0-0 before 1.26.4 — fixed in 1.25.11, 1.26.4
• Go standard library mime: before 1.25.11; from 1.26.0-0 before 1.26.4 — fixed in 1.25.11, 1.26.4

How to fix

Upgrade to a fixed version: Go Stdlib 1.25.11, 1.26.4; Go standard library mime 1.25.11, 1.26.4. Check the vendor advisory for mitigations.

References

• https://go.dev/cl/774481
• https://go.dev/issue/79217
• https://groups.google.com/g/golang-announce/c/tKs3rmcBcKw
• https://pkg.go.dev/vuln/GO-2026-5038

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