Is CVE-2026-42504 exploited in the wild?

No. Junglewise Threat Intelligence has no report of CVE-2026-42504 being exploited in the wild.

How severe is CVE-2026-42504?

Informational severity, with a CVSS score of 0, published 2 June 2026.

Which products and versions are affected?

stdlib (Go) SEMVER:introduced=0,fixed=1.25.11,introduced=1.26.0-0,fixed=1.26.4.

Is there a fix for CVE-2026-42504?

Yes. stdlib (Go): upgrade to 1.26.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

How is CVE-2026-42504 attacked?

It is attacked over the network, in a software library.

Am I affected by CVE-2026-42504?

Check whether you run stdlib (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

CVE-2026-42504: Google Go CPU exhaustion in mime WordDecoder.DecodeHeader

CVE-2026-42504 · Severity: info · CVSS 0 · Published 2026-06-02

A vulnerability in the Go programming language's standard library can allow an attacker to cause a denial-of-service condition. By sending a specially crafted email or web header, an attacker can force the application to consume excessive processor power, potentially slowing down or crashing the service. This affects any Go-based application that processes MIME headers, such as mail servers or web applications.

Technologies: stdlib (Go). Vendors: Go.

Quick answers

Is CVE-2026-42504 exploited in the wild?: No. Junglewise Threat Intelligence has no report of CVE-2026-42504 being exploited in the wild.
How severe is CVE-2026-42504?: Informational severity, with a CVSS score of 0, published 2 June 2026.
Which products and versions are affected?: stdlib (Go) SEMVER:introduced=0,fixed=1.25.11,introduced=1.26.0-0,fixed=1.26.4.
Is there a fix for CVE-2026-42504?: Yes. stdlib (Go): upgrade to 1.26.4 or later. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.
How is CVE-2026-42504 attacked?: It is attacked over the network, in a software library.
Am I affected by CVE-2026-42504?: Check whether you run stdlib (Go) at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

How to fix

stdlib (Go): upgrade to 1.26.4 or later
```
go get stdlib@v1.26.4
```

Technical details

A vulnerability exists in the 'mime' package's WordDecoder.DecodeHeader function due to quadratic complexity when handling invalid encoded-words. When the decoder encounters an invalid encoded-word, it consumes only the initial '=?' prefix and resumes parsing from that point. An attacker can exploit this by providing a header with many nested or repeated '=?' prefixes followed by a single terminal '?=', causing the decoder to repeatedly re-scan the input. This leads to excessive CPU consumption (Denial of Service). The issue is fixed in Go versions 1.25.11 and 1.26.4.

Affected products

Google Go before 1.25.11, 1.26.0 before 1.26.4

Identifiers

CVE-2026-42504
GO-2026-5038

Timeline

2026-05-05: disclosed: Issue opened on Go GitHub repository
2026-06-02: advisory: CVE-2026-42504 published
2026-06-02: patched: Fixed in Go 1.25.11 and 1.26.4

References

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