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.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
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.
Which versions are affected by CVE-2026-55620?
3 open source packages ship an affected version, including eml_parser and eml-parser.
| Package | 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.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55620 being exploited?
There is no known exploitation of CVE-2026-55620 in the wild and no public exploit code.
Sources
Also known as GHSA-g7gc-gmgp-wgqg, PYSEC-2026-3832.