eml_parser vulnerable to DoS via deeply nested parens in Received headers

Exploitation

No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.

CVSS

7.5 · CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

EPSS

• CVE-2026-55620: 0.63% estimated exploitation activity in the next 30 days; percentile 50.23 (FIRST EPSS, 2026-11-03). This is a model estimate, not proof of exploitation or customer exposure.

As published by the source; long descriptions are shortened.

Summary emlparser 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, emlparser uses a linear-time algorithm to remove the comments from Received: headers.

Published by the source

25 Aug 2026, 18:27 UTC

Last modified by the source

25 Aug 2026, 18:27 UTC

Source last checked

26 Oct 2026, 13:50 UTC

1. Revision 1 · created · 26 Oct 2026

   Published from ghsa record GHSA-g7gc-gmgp-wgqg

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# Patch Next advisory brief: CVE-2026-55620

Advisory: eml_parser vulnerable to DoS via deeply nested parens in Received headers
IDs: CVE-2026-55620, GHSA-g7gc-gmgp-wgqg
Record: https://patchnext.com/advisories/cve-2026-55620 (revision 1, source checked 26 Oct 2026, 13:50 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: High (CVSS 7.5)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: eml-parser 3.0.2. A vendor fix does not mean a deployment is fixed.

## Affected components
- PyPI eml-parser: affected < 3.0.2; fixed in 3.0.2

## Suggested inspection (read-only)
- `python -m pip show eml-parser`: prints the installed version. Run with the environment that serves production active.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on eml-parser.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://github.com/advisories/GHSA-g7gc-gmgp-wgqg (GitHub-reviewed advisory GHSA-g7gc-gmgp-wgqg lists affected ranges: PyPI eml-parser >=0 <3.0.2)

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