VulnerabilityPublished 28 Oct 2026

CVE-2026-116266: MIME-tools (Perl module) · Interpretation Conflict

Affects MIME-tools (Perl module)Source CVE Database V5

Not known to be exploited : upgrade to 5.521 soon.

Data as of 28 Oct 2026, 17:57 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.

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

A fixed version 5.521 of MIME::Head is available that addresses this issue. Users should upgrade to version 5.521 or later to remediate this vulnerability. No other mitigation or temporary workaround is indicated.

Description

MIME::Head versions from 5.518 up to but not including 5.521 for Perl improperly decode multipart boundaries containing partially encoded words without marking them as ambiguous. This allows a remote sender to craft a multipart boundary that is interpreted differently by MIME-tools and other mail readers, potentially hiding attachments from filters relying on ambiguous_content() checks while still displaying them in recipients' mail clients.

Impact

A remote attacker can craft email messages with multipart boundaries that bypass filters relying on MIME-tools' ambiguous_content() method, potentially allowing hidden attachments to evade detection. This can lead to security controls missing malicious or unwanted attachments, increasing the risk of malware delivery or data exfiltration via email.

Summary

The vulnerability in MIME::Head versions >=5.518 and <5.521 arises because the multipart_boundary() method decodes RFC 2047 encoded words anywhere in the boundary string but only triggers an ambiguity flag if the entire boundary is a single encoded word. This leads to inconsistent interpretation of multipart boundaries, as encoded words are not allowed in parameters per RFC standards. Consequently, other mail readers split the message on the literal boundary value, while MIME-tools treats the decoded value as the boundary. This discrepancy can be exploited by a remote sender to hide attachments from filters that rely on ambiguous_content() returning true, 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

metacpan.org/release/DSKOLL/MIME-tools-5.521/changes (opens in a new tab)

Community reviews

Loading reviews…

Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 28 Oct 2026, 17:46 UTC

WeaknessesCWE-436

Technical details

References

Community reviews

Related threats