CVE-2026-116270 in MIME::Words
Summary
by MITRE • 10/28/2026
MIME::Words versions before 5.521 for Perl allow memory exhaustion via repeated "=?" sequences in decode_mimewords.
For each "=?" that does not start a complete encoded word, decode_mimewords appends an error message containing the whole input string to $@. A boundary of 8,000 "=?" pairs (16,000 bytes) adds 8,000 copies of itself to $@, about 128 MB.
From version 5.518, MIME::Parser decodes every multipart boundary this way, so a remote sender can exhaust the memory of the parsing process with one Content-Type header.
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Analysis
by VulDB Data Team • 10/28/2026
The vulnerability identified in versions of the Perl module MIME::Words prior to release 5.521 represents a significant resource exhaustion flaw rooted in improper input validation and inefficient error handling mechanisms within the decode_mimewords function. This component is responsible for parsing and decoding MIME encoded words, which are text strings used in email headers to represent non-ASCII characters using ASCII-compatible encoding schemes such as Base64 or Quoted-Printable. The core technical flaw arises from how the module processes malformed input sequences that begin with the equals question mark delimiter but do not constitute a complete valid encoded word structure. Instead of discarding these incomplete segments or handling them with bounded memory allocation, the implementation appends an error message to the Perl special variable $@ for each such invalid sequence encountered during parsing.
The operational impact of this design choice is severe due to the quadratic growth in memory consumption relative to the number of malformed sequences. Specifically, when decode_mimewords encounters a "=?" that does not start a complete encoded word, it generates an error message containing the entire input string processed so far and appends it to $. Consequently, if an attacker supplies a stream containing repeated invalid "=?" markers, each new marker triggers the creation of a larger error string because the accumulated input grows with every iteration. For instance, processing approximately 8,000 such pairs results in the accumulation of roughly 128 megabytes of data within $ alone. This exponential memory scaling transforms what might otherwise be a simple parsing failure into a critical denial-of-service condition capable of crashing the host system or exhausting available resources for other processes.
The risk is further amplified by the integration of MIME::Words with MIME::Parser, particularly from version 5.518 onwards. In this configuration, every multipart boundary encountered during email processing triggers the decoding routine via decode_mimewords. This architectural coupling allows a remote attacker to exploit the vulnerability through a single malicious Content-Type header in an incoming email message. By crafting a header that contains a sufficient number of malformed "=?" sequences, the sender can force the parsing process to consume excessive memory rapidly. Since MIME::Parser is often used by mail servers and applications to handle inbound messages, this vector enables remote code execution scenarios indirectly through resource exhaustion rather than direct command injection, effectively bypassing traditional input validation checks that focus on content type or payload size but neglect malformed encoding structures.
From a classification perspective, this vulnerability aligns with CWE-400, which covers Uncontrolled Resource Consumption, specifically manifesting as memory exhaustion. The attack vector is consistent with ATT&CK technique T1496, Resource Hijacking, where an adversary uses computing resources to disrupt service availability or perform cryptomining, although in this case the primary intent is likely denial of service through system instability. The flaw highlights a common pitfall in legacy Perl modules where error handling strategies prioritize diagnostic verbosity over resource safety, failing to implement bounds checking on accumulated data structures during iterative parsing operations.
Mitigation for this vulnerability requires an immediate upgrade to MIME::Words version 5.521 or later, which addresses the underlying logic errors in decode_mimewords by preventing the unbounded accumulation of error messages. For organizations unable to patch immediately due to dependency constraints, defensive coding practices should be implemented at the application layer. This includes implementing strict length limits on email headers before they are passed to MIME::Parser and configuring timeout mechanisms for parsing operations to prevent long-running processes from consuming excessive memory. Additionally, input sanitization routines can be deployed to strip or validate encoded word sequences prior to processing, ensuring that malformed structures do not reach the vulnerable decoding function. Regular security audits of mail handling components should also include fuzzing tests targeting MIME encoding edge cases to detect similar resource exhaustion flaws in related libraries.