CVE-2026-49477 in soupsieve
Summary
by MITRE • 07/15/2026
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.
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Analysis
by VulDB Data Team • 07/15/2026
The vulnerability in Soup Sieve affects a CSS selector library that serves as an extension for Beautiful Soup 4, a popular Python library for parsing HTML and XML documents. This particular flaw exists within the CSS selector parser component located in the soupsieve/css_parser.py file, where a regular expression pattern demonstrates susceptibility to catastrophic backtracking behavior. The vulnerability manifests when processing attribute selectors containing unterminated quoted values, creating a scenario where seemingly benign input can trigger exponential execution time growth during regex matching operations.
The technical implementation of this vulnerability stems from improper handling of regular expression patterns within the CSS parsing logic. When an attacker provides a malicious CSS selector string with an unterminated quoted attribute value, the parser's regex engine enters into a state of catastrophic backtracking where it repeatedly re-evaluates the same input positions multiple times. This creates an exponential time complexity that grows rapidly with input size, transforming what should be a simple parsing operation into a computationally intensive process that consumes excessive CPU resources.
The operational impact of this vulnerability extends beyond mere performance degradation to full denial of service conditions. An attacker who can control CSS selector strings passed to soupsieve.compile() or Beautiful Soup's .select() and .select_one() methods can cause significant system resource exhaustion by submitting carefully crafted malicious selectors. This allows for remote exploitation without requiring authentication, making it particularly dangerous in web applications where user input is processed through these parsing functions. The vulnerability affects all versions prior to 2.8.4, leaving systems running older versions exposed to potential abuse.
This vulnerability aligns with CWE-400, which specifically addresses uncontrolled resource consumption, and more precisely maps to CWE-1333, focusing on regular expressions that are vulnerable to catastrophic backtracking. From an adversarial perspective, this issue corresponds to ATT&CK technique T1496, which covers resource exhaustion attacks targeting availability. The fix implemented in version 2.8.4 addresses the root cause by modifying the regular expression patterns to prevent backtracking scenarios and implementing input validation that detects malformed attribute selectors before they can trigger the vulnerable parsing logic.
Organizations should prioritize updating to Soup Sieve version 2.8.4 or later to mitigate this vulnerability, as no other workarounds exist for the underlying parsing implementation. The fix involves strengthening the regex patterns used in CSS selector processing and implementing proper input sanitization that prevents malformed attribute values from reaching the vulnerable backtracking logic. Security teams should also monitor applications that utilize Beautiful Soup with user-supplied CSS selectors to ensure that all dependencies are updated to versions containing this patch, particularly in environments where external input is processed through HTML parsing functions.