# CVE-2026-116265: MIME::tools RFC 2231 boundary ambiguity check bypass

Source: https://junglewise.ai/threats/cve-2026-116265-mime-field-paramval-versions-before-5-521-for-perl-do-not-flag  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-10-28. Updated: 2026-10-28  
License: CC BY 4.0 with a link back

CVE-2026-116265 · Severity: info · Published 2026-10-28

MIME::tools is a Perl library used by mail systems to parse email messages. A vulnerability allows attackers to craft emails where the multipart boundary is encoded in a way that bypasses security checks, potentially hiding attachments from email filters while the recipient sees them normally. This could allow malicious attachments to evade antivirus or content filtering systems.

Technologies: [Perl MIME-tools](https://junglewise.ai/threats/technologies/mime-tools). Vendors: [Perl](https://junglewise.ai/threats/vendors/perl).

## Quick answers

**Is CVE-2026-116265 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-116265 being exploited in the wild.

**How severe is CVE-2026-116265?** Informational severity, published 28 October 2026.

**Which products and versions are affected?** Perl MIME-tools before 5.521.

**Is there a fix for CVE-2026-116265?** Yes: fixed in MIME-tools 5.521. Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-116265 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-116265?** Check whether you run Perl MIME-tools at an affected version. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## How to fix

- Fixed in MIME-tools 5.521

## Technical details

The boundary ambiguity validation in MIME::Field::ParamVal only checks plain "boundary=" parameters but misses boundaries split using RFC 2231 continuations (boundary*0=, boundary*1=, etc.) that are reassembled after checks run. Additionally, encoded words or trailing whitespace in RFC 2231-encoded boundaries go unflagged, though multipart_boundary() later decodes or strips them. A remote attacker can exploit this to make MIME::Parser interpret a different boundary than other mail clients, allowing attachments to bypass filters using ambiguous_content().

## Affected products

- Perl MIME-tools before 5.521

## Timeline

- 2026-10-28: disclosed
- 2026-10-27: patched

## References

- [https://cpan.org/modules](https://cpan.org/modules)
- [https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes](https://metacpan.org/release/DSKOLL/MIME-tools-5.521/changes)

## Related threats

- [MIME-tools MIME::Parser duplicate boundary detection bypass with comments](https://junglewise.ai/threats/cve-2026-116272-mime-field-paramval-versions-before-5-521-for-perl-do-not-flag-a)
- [MIME-tools FileUnder symlink following directory reuse](https://junglewise.ai/threats/cve-2026-116271-mime-parser-fileunder-versions-before-5-521-for-perl-reuse-a-pre)
- [MIME-tools RFC 2231 language tag parsing empty filename](https://junglewise.ai/threats/cve-2026-116269-mime-field-paramval-versions-before-5-521-for-perl-turn-an-rfc)
- [MIME-tools ParamVal regex backtracking CPU exhaustion](https://junglewise.ai/threats/cve-2026-116268-mime-field-paramval-versions-before-5-521-for-perl-allow-cpu)