MIME Header Parsing Denial of Service in Go

Executive Summary

This vulnerability involves decoding a maliciously-crafted MIME header that contains many invalid encoded-words. When such a header is processed, it can cause excessive CPU consumption.

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Impact Analysis

The impact of this vulnerability is that processing specially crafted MIME headers can lead to excessive CPU usage. This could degrade system performance or potentially lead to denial of service conditions if the CPU resources are exhausted.

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Compliance Impact

The vulnerability CVE-2026-42504 causes excessive CPU consumption due to a denial-of-service attack vector in the Go mime package when decoding maliciously crafted MIME headers.

There is no information provided in the available resources about how this vulnerability directly impacts compliance with common standards and regulations such as GDPR or HIPAA.

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Detection Guidance

This vulnerability involves excessive CPU consumption when decoding maliciously crafted MIME headers with many invalid encoded-words using the Go mime package's WordDecoder.DecodeHeader function.

To detect this vulnerability on your system, you can monitor CPU usage patterns for processes using the Go mime package, especially those handling MIME headers.

There are no specific commands provided in the resources to detect the vulnerability directly, but general monitoring commands include:

• Use system monitoring tools like 'top' or 'htop' to identify processes with unusually high CPU usage.
• Use network monitoring tools like 'tcpdump' or 'wireshark' to capture and analyze MIME headers in network traffic for suspiciously crafted encoded-words.
• If you have access to application logs, look for unusually slow processing or timeouts related to MIME header decoding.

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Mitigation Strategies

The primary mitigation step is to update the Go standard library mime package to a patched version where this vulnerability is fixed.

Specifically, upgrade to Go versions at least go1.25.11 or go1.26.4 or later, where the WordDecoder.DecodeHeader function has been improved to handle malicious inputs efficiently.

Until the update is applied, consider limiting or filtering incoming MIME headers that contain suspiciously many encoded-words to reduce the risk of CPU exhaustion.

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