eml_parser serves as a python module for parsing eml files and returning various information found in the e-mail as well as computed information. Prior to 3.0.2, eml_parser.routing.noparenthesis in eml_parser/routing.py removes parenthesized CFWS comments from Received: headers with a regex-based fix-point loop whose running time is quadratic in the nesting depth. A single Received: header with 5,000 nested parentheses causes approximately 1.3 seconds of CPU saturation per parsed message, and doubling the nesting depth approximately quadruples the running time. An attacker can submit relatively small EML files that consume multiple seconds of processing time, causing worker latency, queue backpressure, and possible service-level outages in synchronous gateways, sandboxes, and real-time triage pipelines. This issue is fixed in version 3.0.2.

CVE-2026-55620 affects the eml_parser Python library (versions prior to 3.0.2), which is used for parsing EML email files. The vulnerability exists in the eml_parser.routing.noparenthesis function in routing.py, where a regex-based fix-point loop processes parenthesized CFWS comments in Received headers with quadratic time complexity relative to nesting depth. An attacker can craft a small EML file containing deeply nested parentheses (e.g., 5,000 levels) to cause approximately 1.3 seconds of CPU saturation per message, with processing time quadrupling as nesting depth doubles. This ReDoS (Regular Expression Denial of Service) vulnerability can lead to worker latency, queue backpressure, and service-level outages in synchronous gateways, sandboxes, and real-time triage pipelines. The issue has been patched in eml_parser version 3.0.2. The fix is available via a GitHub commit and documented in a security advisory on GitHub.