# CVE-2026-55620: eml_parser: DoS via deeply nested parens in Received headers

> Attackers can cause denial of service by submitting emails with deeply nested parentheses, saturating CPU resources in applications using the eml_parser library.

- **Severity:** High 7.5 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 25 Aug 2026
- **Updated:** 26 Aug 2026
- **Also known as:** GHSA-g7gc-gmgp-wgqg, PYSEC-2026-3832

## What is CVE-2026-55620?

CVE-2026-55620 is a high-severity denial of service vulnerability in GOVCERT-LU eml_parser, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

### Official description

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.

Weakness: CWE-770 (Allocation of Resources Without Limits or Throttling), CWE-1124 (Excessively Deep Nesting)

## Which versions are affected by CVE-2026-55620?

3 open source packages ship an affected version, including eml_parser and eml-parser.


| Package | Ecosystem | Affected | Fixed in |
| --- | --- | --- | --- |
| `eml_parser` | PyPI | All versions | 3.0.2 |
| `eml-parser` | PyPI | All versions | 3.0.2 |
| `eml-parser` | PyPI | All versions | 3.0.2 |

## How do I fix CVE-2026-55620?

Upgrade eml_parser to 3.0.2 or later.

## Is CVE-2026-55620 being exploited?

There is no known exploitation of CVE-2026-55620 in the wild and no public exploit code.

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-55620 and fix it if it is affected.

What it is: CVE-2026-55620 is a high-severity denial of service vulnerability in GOVCERT-LU eml_parser, published on 25 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-55620 in the wild and no public exploit code.

Affected packages and fixed versions:
- eml_parser (PyPI): affected all versions; fixed in 3.0.2
- eml-parser (PyPI): affected all versions; fixed in 3.0.2
- eml-parser (PyPI): affected all versions; fixed in 3.0.2

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-55620/ (Markdown: https://insights.sbomify.com/CVE-2026-55620/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-55620
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-55620
- OSV: https://osv.dev/vulnerability/CVE-2026-55620

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-55620/
Summaries are generated and may contain mistakes; the official description above is the record.