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

High 7.5 Published 2 Jun 2026 Updated 29 Oct 2026

Attackers can cause excessive CPU consumption and denial of service by sending maliciously crafted MIME headers to applications using the Go standard library.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-42504?

CVE-2026-42504 is a high-severity denial of service vulnerability in Go standard library mime, published on 2 Jun 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

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

Weakness: CWE-407

Which versions are affected by CVE-2026-42504?

2 open source packages ship an affected version, including golang and stdlib.

Package	Affected	Fixed in
`golang` Bitnami	All versions	`1.25.11, 1.26.4`
`stdlib` Go	All versions	`1.25.11, 1.26.4`

Linux distribution packages (21)

`cri-o` Red Hat:openshift:4.13::el8	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el8`
`cri-o-debuginfo` Red Hat:openshift:4.13::el8	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el8`
`cri-o-debugsource` Red Hat:openshift:4.13::el8	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el8`
`cri-o` Red Hat:openshift:4.13::el9	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el9`
`cri-o-debuginfo` Red Hat:openshift:4.13::el9	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el9`
`cri-o-debugsource` Red Hat:openshift:4.13::el9	All versions	`0:1.26.5-33.rhaos4.13.git1088e36.el9`
`openshift` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift-hyperkube` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift-kube-apiserver` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift-kube-controller-manager` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift-kube-scheduler` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift-kubelet` Red Hat:openshift:4.21::el10	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el10`
`openshift` Red Hat:openshift:4.21::el8	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el8`
`openshift-hyperkube` Red Hat:openshift:4.21::el8	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el8`
`openshift` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`openshift-hyperkube` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`openshift-kube-apiserver` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`openshift-kube-controller-manager` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`openshift-kube-scheduler` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`openshift-kubelet` Red Hat:openshift:4.21::el9	All versions	`0:4.21.0-202608140338.p2.g7ffd39d.assembly.stream.el9`
`inspektor-gadget` openSUSE:Tumbleweed	All versions	`0.53.2-1.1`

How do I fix CVE-2026-42504?

Upgrade golang to a fixed version for your release line: 1.25.11 or 1.26.4.

Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.

Is CVE-2026-42504 being exploited?

There is no known exploitation of CVE-2026-42504 in the wild and no public exploit code.

Sources

Also known as AZL-89360, AZL-89366, AZL-98718, BELL-CVE-2026-42504, BIT-golang-2026-42504, CLEANSTART-2026-EL32711 and 1001 more.