Quadratic complexity in WordDecoder.DecodeHeader in mime
Affects the Go standard library mime package before 1.25.11 and in the 1.26 line before 1.26.4. Fixed in Go 1.26.4 and 1.25.11.
- Published
- Last modified
- Where the record stands: published, rejected by its publisher, or superseded by another id.What do these mean? →
- published
- Whether CISA lists it in its Known Exploited Vulnerabilities catalog: proof that attackers are using it.What do these mean? →
- Not listed in CISA KEV
- How many real-world setups can hit the bug, from the vendor's advisory and CISA data. It does not know your setup.What do these mean? →
- Unclear
- Fix
- Fixed in 1.25.11, 1.26.4
- Whether working attack code is public (Exploit-DB, Metasploit, Nuclei or GitHub). Public code makes attacks easier.What do these mean? →
- None on Exploit-DB, Metasploit or GitHub
- Exploitation: none
- Automatable: yes
- Technical impact: partial
Summary
Decoding a maliciously-crafted MIME header containing many invalid encoded-words can consume excessive CPU.
Timeline
- CVE published
- CISA SSVC assessment
Exploitation: none · Automatable: yes · Technical impact: partial
- Record last updated
Affected versions
- mime
fixed in 1.25.11; From 1.26.0-0 · fixed in 1.26.4
References 4
go.dev
groups.google.com
pkg.go.dev
Questions about CVE-2026-42504
What does this mean?
Decoding a MIME header that contains many invalid encoded-words can use excessive CPU time, because the work grows quadratically with the size of the header.Source: groups.google.com
Am I affected, and how do I check?
The mime package's WordDecoder.DecodeHeader is affected in Go versions before 1.25.11 and in the 1.26 line before 1.26.4. Check the Go version a program was built with by running go version on the toolchain used to build it.Source: groups.google.com
What should I do about it?
Upgrade the Go toolchain to 1.26.4 or 1.25.11, the fixed releases, and rebuild programs that use the mime package.Source: groups.google.com
Is it being exploited?
It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked.Source: cve.now feed data