PYSEC-2026-3833

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PYSEC-2026-3833

Summary: eml_parser has parser DoS via deeply nested parentheses in e-mail headers

Details: ### Summary `eml_parser` uses the `email.utils.getaddresses()` function from the CPython standard library to parse e-mail headers that contain e-mail addresses (such as `To`, `Cc`, `Bcc`, `From`, `Reply-To`, `Sender`, ...). When the input header contains a deeply nested CFWS (comment / folding white space) construct, the recursive descent parser in the standard library exhausts the call stack. The resulting `RecursionError` is not caught by `eml_parser`, so the exception propagates and aborts parsing of the whole message. ### Impact SOC pipelines use `eml_parser` to process untrusted e-mails. An attacker can easily create an eml file that will trigger the `RecursionError` during parsing. The impact is mitigated by the fact that there are various other situations in which `eml_parser` will raise an exception when attempting to parse a malformed or pathological eml file. In particular, very deeply nested multipart e-mails also result in a `RecursionError` being raised by the library voluntarily. Therefore, systems relying on `eml_parser` already need to detect and handle errors emanating from the library in an appropriate way. ### Workarounds The issue can be avoided by wrapping the call to `eml_parser.decode_email` or `eml_parser.decode_email_bytes` in a `try/except` construct. ### Patches Since version 3.0.2, `eml_parser` will catch the error in the standard library parser and fall back to a simpler parser based on a regular expression.

References: https://github.com/GOVCERT-LU/eml_parser/security/advisories/GHSA-m66c-fw79-6359, https://github.com/GOVCERT-LU/eml_parser/pull/90, https://github.com/GOVCERT-LU/eml_parser/commit/746a69f86443eb0b6a47f77db3cfe727c21f92b3, https://github.com/GOVCERT-LU/eml_parser, https://github.com/GOVCERT-LU/eml_parser/releases/tag/v3.0.2, https://pypi.org/project/eml-parser, https://github.com/advisories/GHSA-m66c-fw79-6359, https://nvd.nist.gov/vuln/detail/CVE-2026-55619