CVE-2026-116265 in MIME::Field::ParamVal
Summary
by MITRE • 10/28/2026
MIME::Field::ParamVal versions before 5.521 for Perl do not flag an ambiguous multipart boundary written as RFC 2231 continuations.
The boundary ambiguity checks look only at a plain boundary= parameter, and a boundary split into RFC 2231 sections (boundary*0=, boundary*1= and so on) is joined after they have run. A repeated section number is concatenated rather than reported as a duplicate. From version 5.518, an encoded word or trailing whitespace in such a boundary also goes unflagged, although multipart_boundary() decodes or strips it.
A remote sender can make MIME::Parser split a message on a different boundary than other mail readers, hiding an attachment from a filter that relies on ambiguous_content() while the recipient's mail client shows it.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability in MIME::Field::ParamVal prior to version 5.521 for Perl represents a critical parsing inconsistency within email processing libraries, specifically affecting how multipart message boundaries are validated and interpreted. The core technical flaw lies in the sequence of operations performed during the parsing of Content-Type headers containing boundary parameters. When an attacker crafts a MIME header with a boundary parameter that utilizes RFC 2231 encoding continuations, such as splitting the value across multiple fields like boundary0= and boundary1=, the library fails to apply ambiguity checks before these segments are concatenated into a single string. The existing validation logic is designed to inspect only plain, unencoded boundary parameters for duplicates or ambiguities that could lead to message parsing errors. However, because the RFC 2231 encoded sections are joined after this initial check has already passed, any maliciously constructed boundaries that rely on this encoding mechanism bypass the security controls entirely.
This implementation defect allows a remote attacker to exploit the discrepancy between how different mail user agents and content filters interpret MIME messages. Specifically, if an email client or filtering system relies on functions like ambiguous_content() to detect malformed multipart structures, it may fail to identify the ambiguity because the check occurs before the decoding step. Consequently, while the recipient's standard mail client correctly decodes the RFC 2231 continuations and displays all parts of the message including hidden attachments, a security filter processing the same email might split the message on a different boundary or ignore certain sections entirely due to its failure to recognize the encoded structure as valid. This divergence in parsing logic creates a significant blind spot for automated threat detection systems that depend on consistent MIME interpretation.
The operational impact of this vulnerability is severe, particularly in environments where email gateways perform deep packet inspection or attachment filtering based on MIME structure analysis. An attacker can leverage this flaw to hide malicious attachments from security filters while ensuring they remain visible and accessible to the end user. This technique effectively bypasses content-based antivirus scans, data loss prevention systems, and other sandboxing mechanisms that rely on accurate boundary detection to isolate and analyze file components. The issue was further exacerbated in versions starting from 5.518, where encoded words or trailing whitespace within such boundaries also went unflagged by the ambiguity checks, although the multipart_boundary() function would still decode or strip these elements during final processing. This inconsistency between validation and execution phases is a classic example of how parsing logic flaws can undermine security controls designed to prevent message injection or evasion attacks.
From a standards perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the application fails to adequately validate input data before using it in critical decision-making processes regarding message structure. It also relates to CWE-749 Exposed Dangerous Method or API if considered from the library's design flaw of exposing inconsistent parsing states. In terms of offensive security frameworks, this technique is consistent with ATT&CK tactic T1566 Phishing and specifically sub-technique T1566.002 Spearphishing Attachment, where attackers use obfuscation or encoding to evade detection mechanisms that rely on static analysis or simplified parsing rules. The ability to hide attachments from filters while maintaining visibility for the client is a sophisticated evasion strategy that exploits the complexity of MIME standards and the varying levels of strictness in different implementations.
Mitigation strategies must focus on updating the underlying library to version 5.521 or later, where these boundary ambiguity checks have been corrected to account for RFC 2231 encoded continuations before concatenation occurs. Organizations relying on Perl-based email processing systems should prioritize patching this component to restore consistent parsing behavior across all stages of message handling. Additionally, security teams should review their mail filtering configurations to ensure they are not solely dependent on MIME boundary detection without supplementary checks for encoding anomalies or header inconsistencies. Implementing defense-in-depth measures that include heuristic analysis and behavioral monitoring can help detect attempts to exploit such parsing discrepancies even if the primary filter is bypassed. Regular auditing of email gateway logs for unusual multipart structures or failed attachment extractions may also aid in identifying potential exploitation attempts targeting this specific vulnerability class.