Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.8.4, the CSS selector parser in soupsieve contains a regular expression vulnerable to catastrophic backtracking when processing an attribute selector with an unterminated quoted value in soupsieve/css_parser.py, allowing an attacker who can supply untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup .select() / .select_one() to cause CPU exhaustion and denial of service. This issue is fixed in version 2.8.4.
Weakness
The product does not properly control the allocation and maintenance of a limited resource.
Affected Software
| Name | Vendor | Start Version | End Version |
|---|---|---|---|
| Soup_sieve | Facelessuser | * | 2.8.4 (excluding) |
| Red Hat Hardened Images | RedHat | python-attrs-main-26.1.0-3.hum1 | * |
| Red Hat Hardened Images | RedHat | python-rpds-py-main-2026.6.3-1.hum1 | * |
Potential Mitigations
Mitigation of resource exhaustion attacks requires that the target system either:
The first of these solutions is an issue in itself though, since it may allow attackers to prevent the use of the system by a particular valid user. If the attacker impersonates the valid user, they may be able to prevent the user from accessing the server in question.
The second solution is simply difficult to effectively institute – and even when properly done, it does not provide a full solution. It simply makes the attack require more resources on the part of the attacker.
Related Attack Patterns
- https://cwe.mitre.org/data/definitions/147.html
- https://cwe.mitre.org/data/definitions/227.html
- https://cwe.mitre.org/data/definitions/492.html
References
- https://github.com/facelessuser/soupsieve/commit/eb4397618709186c109400448c6043b728217dc3
- https://github.com/facelessuser/soupsieve/releases/tag/2.8.4
- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-836r-79rf-4m37
- https://github.com/facelessuser/soupsieve/security/advisories/GHSA-836r-79rf-4m37