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How CVE-2026-55620 Works
eml_parser is a Python module designed to parse EML files and extract various information from emails. Prior to version 3.0.2, the module contained a critical vulnerability in how it processed `Received:` email headers.
Specifically, the function `eml_parser.routing.noparenthesis` in `eml_parser/routing.py` was responsible for removing parenthesized CFWS (comments) from `Received:` headers. To accomplish this, it employed a regex-based fix-point loop. A fix-point loop is an algorithm that repeatedly applies a regex operation until no further changes occur, which is a common but potentially dangerous pattern.
The core of the vulnerability lies in the quadratic time complexity of this loop. The runtime of the algorithm is proportional to the square of the nesting depth of parentheses within the header. This means that as an attacker increases the number of nested parentheses, the processing time grows exponentially.
The severity of this algorithmic flaw is demonstrated by concrete metrics. A single `Received:` header containing 5,000 nested parentheses causes approximately 1.3 seconds of CPU saturation per parsed message. Furthermore, the situation worsens dramatically with deeper nesting; doubling the nesting depth approximately quadruples the running time.
An attacker can exploit this by crafting relatively small EML files that contain a single, maliciously crafted `Received:` header. When a vulnerable eml_parser instance processes this file, it triggers the quadratic loop, consuming excessive CPU resources and causing a denial of service (DoS).
This vulnerability (CVE-2026-55620) has been assigned a CVSS v3.1 base score of 7.5 (High).
DailyCVE Form:
Platform: eml_parser
Version: < 3.0.2
Vulnerability: CPU Exhaustion DoS
Severity: High (7.5 CVSS)
date: 2026-08-25
Prediction: Already Patched (3.0.2)
What Undercode Say:
To understand the impact of this vulnerability in a production environment, one can simulate the attack. The following Python script demonstrates how to generate a malicious payload and measure its effect on a vulnerable version of the library.
Generate a malicious email header with 5000 nested parentheses
python3 -c "print('Received: from example.com (' + '(' 5000 + ')' 5000 + ')')" > malicious.eml
To test the CPU consumption, you can use the `time` command to measure the execution time of parsing this file with a vulnerable version of eml_parser.
Assuming a vulnerable eml_parser is installed
time python3 -c "import eml_parser; ep = eml_parser.EmlParser(); ep.decode_email(open('malicious.eml', 'rb').read())"
This will output the real, user, and sys time taken to parse the file, demonstrating the CPU saturation.
Exploit: (Educational Purposes!)
The exploit is trivial to execute. An attacker only needs to include a single `Received:` header in an EML file with a deeply nested parenthetical comment. The file itself can be very small, making it an efficient vector for a Denial of Service attack.
Received: from attacker.com (((((...5000 nested parens...)))))
When a vulnerable parser processes this header, the regex-based fix-point loop enters a state of quadratic runtime, consuming all available CPU on the worker thread and preventing it from processing other, legitimate emails. This can lead to queue backpressure and service-level outages in synchronous email-processing pipelines like gateways and sandboxes.
Protection:
The vulnerability is completely resolved in eml_parser version 3.0.2 and later. The fix replaces the vulnerable quadratic algorithm with a linear-time algorithm to remove comments from `Received:` headers.
– Immediate Action: Upgrade to eml_parser version 3.0.2 or higher.
– Verification: Check your project’s dependencies to ensure you are not using any version prior to 3.0.2.
– Workaround (if upgrade is not possible): As a temporary measure, implement a timeout or rate-limiting on email parsing tasks to prevent a single malicious email from consuming resources indefinitely. However, upgrading is the only complete and recommended solution.
Impact:
- CPU Exhaustion: An attacker can cause 100% CPU saturation on a worker for several seconds using a very small email file.
- Denial of Service (DoS): This vulnerability leads to a CPU exhaustion DoS, impacting the availability of the email processing service.
- Service Disruption: In synchronous pipelines (e.g., email gateways, sandboxes), this latency directly translates to queue backpressure, potentially causing widespread service outages and impacting all users.
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Sources:
Reported By: github.com
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