VulnerabilityPublished 28 Oct 2026

CVE-2026-116266: MIME::Head versions from 5.518 before 5.521 for Perl decode a partly encoded multipart boundary

Source GCVE Database

Authoritative record:CVE-2026-116266· High

Not known to be exploited : upgrade to 5.521 soon.

Data as of 28 Oct 2026, 19:42 UTC

Severity

High

Rated High by Threat Radar's AI analysis · CVSS not yet scored by NVD

Patch: Available – fixed in 5.521

Exploitation evidence

Disclosed: evidence found.
CVE published 28 Oct 2026
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-c5m8-q98x-42wx

What to do

Upgrade MIME-tools (Perl module) to 5.521.

Affected software

Product	Affected	Fixed in
MIME-tools	Affected >=5.518 <5.521	Fixed in 5.521
CPANMIME-tools`pkg:cpan/MIME-tools`Other threats	Affected >=5.518 <5.521	Fixed in 5.521

Check your own hosts for these packages with threat-finder (opens in a new tab), our free open-source scanner.

AI-suggested

If you cannot patch yet, users should be cautious with attachments in emails from untrusted sources and consider additional filtering or scanning mechanisms that do not rely solely on ambiguous_content() detection.

Description

MIME::Head versions from 5.518 up to but not including 5.521 for Perl improperly decode multipart boundaries that are partly RFC 2047 encoded without marking the parse as ambiguous. This allows a remote sender to craft a multipart boundary that bypasses filters relying on ambiguous_content() detection, potentially hiding attachments from such filters while still displaying them in the recipient's mail client.

Impact

This flaw can be exploited by remote senders to evade attachment filtering mechanisms that depend on ambiguous_content() detection, potentially allowing malicious attachments to bypass security filters. The recipient's mail client will still display the attachment, increasing the risk of undetected malicious content delivery.

Summary

The vulnerability in MIME::Head (Perl module) versions 5.518 before 5.521 involves the multipart_boundary() method decoding RFC 2047 encoded words anywhere in the boundary string without triggering the ambiguity check unless the entire boundary is a single encoded word. This leads to a scenario where a multipart boundary like 'abc=?us-ascii?Q?def?=' is interpreted as 'abcdef' without ambiguity detection. Since encoded words are not allowed in parameters per RFC standards, other mail readers split messages on the literal boundary value, causing inconsistent parsing. A remote attacker can exploit this to hide attachments from filters that rely on ambiguous_content() to detect such anomalies, while the recipient's mail client still shows the attachment.

CWE-436Interpretation Conflict (opens in a new tab)
Class weaknessOWASP Top 10 A06:2025: Insecure Design (opens in a new tab)
Product A handles inputs or steps differently than Product B, which causes A to perform incorrect actions based on its perception of B's state.
OWASP Cheat Sheets
REST Assessment (opens in a new tab)
REST Security (opens in a new tab)
Cross-Site Request Forgery Prevention (opens in a new tab)
Secure Design Principles (opens in a new tab)
OWASP ASVS 4.0.3
V13.2.5 (opens in a new tab)
Attack patterns
HTTP Request Splitting (opens in a new tab)
HTTP Response Smuggling (opens in a new tab)
HTTP Response Splitting (opens in a new tab)
From MITRE CWE, MITRE CAPEC and OWASP (Top 10, OpenCRE).

References

Community reviews

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 28 Oct 2026, 19:42 UTC

WeaknessesCWE-436

Technical details

References

Community reviews

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