CVE-2026-42504Network attack vector

Remote CPU exhaustion via invalid encoded-word reparsing

Published Jun 2, 2026 · Updated Aug 28, 2026

Algorithmic complexity in Go standard library mime before 1.25.11 and 1.26.0-0 through 1.26.3 allows remote attackers to exhaust CPU. WordDecoder.DecodeHeader appends only the opening marker after an invalid encoded-word and restarts parsing, repeatedly rescanning overlapping input in near-quadratic time. Exposure requires an application to pass attacker-controlled MIME headers to this decoder; the bounded result is CPU exhaustion and service interruption.

CVSS severity7.5
High
EPSS probability0.56%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Algorithmic complexity in Go standard library mime before 1.25.11 and 1.26.0-0 through 1.26.3 allows remote attackers to exhaust CPU. WordDecoder.DecodeHeader appends only the opening marker after an invalid encoded-word and restarts parsing, repeatedly rescanning overlapping input in near-quadratic time. Exposure requires an application to pass attacker-controlled MIME headers to this decoder; the bounded result is CPU exhaustion and service interruption.

The record

CVE
CVE-2026-42504
Published
Jun 2, 2026
Updated
Aug 28, 2026
Vendor
Go standard library
Product
mime
Classifications
CWE-407, T1499.004
Attack vector
network
Privileges
unauthenticated

Timeline

How it unfolded

  1. Jun 2, 2026CVE publishedPublication date reported by the CVE source.
  2. Aug 28, 2026Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=0 <1.25.11
  2. Affected versionversion=1.26.0-0 <1.26.4

What conditions does exploitation require?

Attack vectornetwork
Required privilegesunauthenticated

What is affected?

Go standard library · mimeversion=0 <1.25.11; version=1.26.0-0 <1.26.4

Published CVSS scores

7.5CISA-ADPCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 9, 2026Sep 15, 2026
Latest reporting daySep 15, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesunauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-407Inefficient Algorithmic Complexity
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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