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eml_parser is a Python module designed for parsing EML files—the standard format used by email clients to store individual messages. In security operations centers (SOCs), these parsers are critical for inspecting untrusted or potentially malicious email attachments and messages for threats.
Prior to version 3.0.2, the `HeaderParser.header_fetch_parse` function in `eml_parser/parser.py` relies on email.utils.getaddresses()—a function from the CPython standard library—to parse email headers that contain address information, such as To, Cc, Bcc, From, Reply-To, and Sender.
The vulnerability arises from how this standard library function handles CFWS (Comment / Folding White Space) constructs, a syntactic element defined in RFC 5322 that allows optional whitespace and comments within email addresses. While normal usage involves minimal nesting, an attacker can craft a malicious EML file containing deeply nested CFWS comment structures—for example, parentheses within parentheses within parentheses, repeated thousands of times.
When `email.utils.getaddresses()` processes such input, the recursive descent parser in the standard library enters uncontrolled recursion due to insufficient depth limits. This rapidly exhausts the Python interpreter’s call stack memory. Upon exhaustion, the application raises a `RecursionError` exception.
Critically, eml_parser versions before 3.0.2 do not implement specific exception handling for this error within the header parsing routine. The exception propagates upward and causes an abrupt termination of the entire message parsing process. This results in a Denial of Service condition where the parser fails completely rather than gracefully degrading or returning partial data.
For security pipelines that rely on continuous processing of high volumes of email traffic to detect phishing, malware, or other threats, a single malformed header can halt the ingestion and analysis of subsequent messages if not properly isolated by upstream error handling mechanisms. This vulnerability is classified under CWE-674 (Uncontrolled Recursion) and CWE-20 (Improper Input Validation), and is fixed in version 3.0.2.
DailyCVE Form:
Platform: Python/eml_parser
Version: < 3.0.2
Vulnerability: Uncontrolled Recursion
Severity: Medium (CVSS 5.3)
Date: 2026-08-25
Prediction: Already Patched (3.0.2)
What Undercode Say:
Check installed eml_parser version pip show eml_parser Upgrade to patched version pip install --upgrade eml_parser>=3.0.2 Verify the fix python -c "import eml_parser; print(eml_parser.<strong>version</strong>)"
Analytics: SOC pipelines processing untrusted EML files are the primary targets. An attacker can send a single email with a deeply nested CFWS header (e.g., 5,000+ nested parentheses) to trigger the RecursionError. The impact is mitigated by the fact that systems relying on eml_parser already need to handle exceptions from malformed messages in general.
Exploit: (Educational Purposes!)
import eml_parser
Malicious payload: deeply nested parentheses in a header
malicious_header = "To: " + "(" 5000 + "[email protected]" + ")" 5000
This would trigger RecursionError in versions < 3.0.2
eml_parser.decode_email(malicious_header)
Note: The exploit works by crafting an EML file where an address-bearing header (To, Cc, From, etc.) contains an excessively deep nesting of CFWS comments. The standard library’s recursive descent parser exhausts the call stack and raises RecursionError, which is not caught.
Protection:
- Upgrade to eml_parser version 3.0.2 or later, which catches the error and falls back to a regex-based parser.
- Wrap all calls to `eml_parser.decode_email()` or `eml_parser.decode_email_bytes()` in a `try/except` block to handle `RecursionError` gracefully.
- Deploy upstream input validation or rate limiting to reject emails with excessively nested headers before they reach the parser.
Impact:
- Denial of Service: Attackers can disrupt SOC pipelines that process untrusted EML files.
- Pipeline Failure: A single malformed email can abort parsing of the entire message queue.
- Operational Disruption: Worker latency, queue backpressure, and possible service-level outages in synchronous gateways, sandboxes, and real-time triage pipelines.
- Mitigated Risk: Callers already need to handle exceptions from malformed messages, so proper error handling can prevent complete failure.
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Sources:
Reported By: github.com
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