high · 7.5 Patched

Quadratic complexity in WordDecoder.DecodeHeader in mime

CVE-2026-42504 · GHSA-h524-452v-82p9 · EUVD-2026-34039 · GO-2026-5038 · BIT-golang-2026-42504 · published 2 Jun 2026

How serious is it

  • Severity: high, CVSS score 7.5 out of 10.
  • No exploit prediction (EPSS) score yet.
  • Not on the CISA list of exploited vulnerabilities.
  • No public exploit in Exploit-DB.

Is your app affected by CVE-2026-42504?

Paste this prompt into Claude Code, Cursor or any AI coding tool: it runs a free Shippable scan of your project and fixes what it finds. Built with Lovable, Bolt or v0? Scan your live app. Or upload your package file.

Who is affected

  • stdlib (Go) < 1.25.11
  • stdlib (Go) >= 1.26.0-0, < 1.26.4

How to check

  1. Run go list -m stdlib in your project to see the installed version, including copies pulled in by other packages.
  2. Or paste your lockfile into the checker: it lists every package with a known vulnerability, this one included.

How to fix

Copies a prompt for Claude Code, Cursor or Codex: it checks your lockfiles for this and upgrades it.
  1. On < 1.25.11: Upgrade stdlib to 1.25.11 or later, commit the lockfile and redeploy.
  2. On >= 1.26.0-0, < 1.26.4: Upgrade stdlib to 1.26.4 or later, commit the lockfile and redeploy.
  3. If a vulnerable version was live, check your logs and rotate the secrets that server could read.

The advisory

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

Check your whole app

Dependencies are only half of it. Free and read-only: before you push, run npx shippable-dev scan localhost:3000 in your project folder, no account needed. For your live app, you prove it's yours with a one-line tag first.

References

Merged from CVE List, OSV, ENISA EUVD. Last updated 1 Nov 2026. All vulnerabilities · RSS · JSON

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