PYSEC-2026-3832

Vulnerability from pysec - Published: 2026-09-10 09:44 - Updated: 2026-09-10 11:02
VLAI
Details

Summary

eml_parser strips parenthesised CFWS comments from Received: headers using a regex-based fix-point loop. The loop has quadratic time complexity in the number of nested parens. A single Received: header containing 5,000 nested parens causes ~1.3 seconds of CPU saturation per parsed message; runtime quadruples per doubling of nesting depth.

Impact

This represents a CPU exhaustion DoS in any pipeline that processes attacker-supplied EML files. An attacker can create relatively small EML files that will take multiple seconds to parse.

This is particularly problematic for synchronous email-processing pipelines (gateways, sandboxes, real-time triage) where worker latency directly translates to queue backpressure and possible service-level outages.

Patches

Since version 3.0.2, eml_parser uses a linear-time algorithm to remove the comments from Received: headers.

Severity
7.5 (High)
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Impacted products
Name purl
eml-parser pkg:pypi/eml-parser
Aliases
To clipboard 

{
  "affected": [
    {
      "package": {
        "ecosystem": "PyPI",
        "name": "eml-parser",
        "purl": "pkg:pypi/eml-parser"
      },
      "ranges": [
        {
          "events": [
            {
              "introduced": "0"
            },
            {
              "fixed": "3.0.2"
            }
          ],
          "type": "ECOSYSTEM"
        }
      ],
      "versions": [
        "0.9",
        "1.0",
        "1.1",
        "1.10",
        "1.11",
        "1.11.1",
        "1.11.2",
        "1.11.4",
        "1.11.5",
        "1.11.6",
        "1.11.7",
        "1.12.0",
        "1.13.0",
        "1.14.0",
        "1.14.1",
        "1.14.2",
        "1.14.3",
        "1.14.4",
        "1.14.5",
        "1.14.6",
        "1.14.7",
        "1.14.8",
        "1.15.0",
        "1.16.0",
        "1.17.0",
        "1.17.1",
        "1.17.2",
        "1.17.3",
        "1.17.4",
        "1.17.5",
        "1.3",
        "1.4",
        "1.5",
        "1.6",
        "1.7",
        "1.8",
        "1.9",
        "2.0.0",
        "2.0.1",
        "3.0.0",
        "3.0.1"
      ]
    }
  ],
  "aliases": [
    "CVE-2026-55620",
    "GHSA-g7gc-gmgp-wgqg"
  ],
  "details": "### Summary\n\n`eml_parser` strips parenthesised CFWS comments from `Received:` headers using a regex-based fix-point loop. The loop has quadratic time complexity in the number of nested parens. A single `Received:` header containing 5,000 nested parens causes ~1.3 seconds of CPU saturation per parsed message; runtime quadruples per doubling of nesting depth.\n\n### Impact\n\nThis represents a CPU exhaustion DoS in any pipeline that processes attacker-supplied EML files. An attacker can create relatively small EML files that will take multiple seconds to parse.\n\nThis is particularly problematic for synchronous email-processing pipelines (gateways, sandboxes, real-time triage) where worker latency directly translates to queue backpressure and possible service-level outages.\n\n### Patches\n\nSince version 3.0.2, `eml_parser` uses a linear-time algorithm to remove the comments from `Received:` headers.",
  "id": "PYSEC-2026-3832",
  "modified": "2026-09-10T11:02:05.624878Z",
  "published": "2026-09-10T09:44:56.755416Z",
  "references": [
    {
      "type": "WEB",
      "url": "https://github.com/GOVCERT-LU/eml_parser/security/advisories/GHSA-g7gc-gmgp-wgqg"
    },
    {
      "type": "WEB",
      "url": "https://github.com/GOVCERT-LU/eml_parser/pull/90"
    },
    {
      "type": "WEB",
      "url": "https://github.com/GOVCERT-LU/eml_parser/commit/746a69f86443eb0b6a47f77db3cfe727c21f92b3"
    },
    {
      "type": "PACKAGE",
      "url": "https://github.com/GOVCERT-LU/eml_parser"
    },
    {
      "type": "WEB",
      "url": "https://github.com/GOVCERT-LU/eml_parser/releases/tag/v3.0.2"
    },
    {
      "type": "PACKAGE",
      "url": "https://pypi.org/project/eml-parser"
    },
    {
      "type": "ADVISORY",
      "url": "https://github.com/advisories/GHSA-g7gc-gmgp-wgqg"
    },
    {
      "type": "ADVISORY",
      "url": "https://nvd.nist.gov/vuln/detail/CVE-2026-55620"
    }
  ],
  "severity": [
    {
      "score": "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H",
      "type": "CVSS_V3"
    }
  ],
  "summary": "eml_parser vulnerable to DoS via deeply nested parens in Received headers"
}



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Forecast uses a logistic model when the trend is rising, or an exponential decay model when the trend is falling. Fitted via linearized least squares.

Sightings

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  • Not confirmed: The user expressed doubt about the validity of the vulnerability.
  • Not patched: The vulnerability was not observed as successfully patched by the user who reported the sighting.



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