Denial of Service Vulnerability in eml_parser

The Python library eml_parser contains a vulnerability (CVE-2026-55620) where the logic used to strip parenthesized comments from Received: headers employs a fix-point loop with quadratic time complexity. When the parser encounters an EML file containing a Received: header with deeply nested parentheses, the processing time scales exponentially with the nesting depth. A header with 5,000 levels of nesting requires approximately 1.3 seconds of CPU time to parse. Because this library is frequently used in email gateways, security sandboxes, and automated triage pipelines, an attacker can intentionally craft malicious emails to induce significant latency. In synchronous processing architectures, this exhaustion of CPU resources leads to queue backpressure, worker starvation, and potential service-level denial of service.

Impact

Successful exploitation results in a denial of service for email processing pipelines. Because the complexity is triggered per-message, an attacker sending a high volume of crafted EML files can permanently saturate available worker threads, preventing the delivery or analysis of legitimate communications. This affects any system relying on eml_parser versions prior to 3.0.2 to ingest or inspect inbound mail.

Recommendation

Prioritize the immediate upgrade of the eml_parser library to version 3.0.2 or higher to implement the linear-time parsing algorithm. For environments unable to patch immediately, implement strict validation of incoming EML header length and nesting depth at the edge, or place email ingestion services behind a rate-limiting reverse proxy to mitigate the impact of CPU-heavy requests. Monitor for anomalous CPU spikes in mail processing worker nodes associated with the eml_parser library.