CVE-2026-49476 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 allocates unbounded memory when compiling large comma-separated selector lists, allowing an attacker who can supply a crafted selector string to soupsieve.compile() or Beautiful Soup .select() / .select_one() to allocate hundreds of megabytes of heap memory from a relatively small input and cause 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 the CSS selector parsing functionality within the Beautiful Soup 4 library ecosystem, representing a classic example of an unbounded memory allocation flaw that can be exploited for denial of service attacks. This issue resides in the parser's handling of comma-separated selector lists where the memory consumption grows exponentially with input size rather than remaining proportional to the actual selector complexity. The vulnerability manifests when attackers provide crafted CSS selector strings that contain large numbers of comma-separated selectors, causing the soupsieve.compile() function and associated Beautiful Soup selection methods to consume excessive heap memory. The flaw specifically impacts versions prior to 2.8.4, indicating that this was a recognized issue that required defensive coding measures to prevent malicious input from triggering resource exhaustion conditions.
The technical nature of this vulnerability aligns with common software security patterns where parsing functions fail to implement proper input validation or resource limiting mechanisms. When the CSS selector parser encounters large comma-separated lists, it allocates memory without bounds checks, allowing an attacker to craft inputs that trigger massive heap allocations from relatively small input strings. This behavior constitutes a memory consumption vulnerability that can be classified under CWE-772, specifically related to missing resource exhaustion checks during parsing operations. The issue occurs because the parser does not implement proper recursion depth limits or memory allocation caps when processing complex selector structures, particularly those involving multiple comma-separated elements that compound the memory requirements exponentially.
The operational impact of this vulnerability extends beyond simple denial of service conditions as it can potentially disrupt applications that rely on Beautiful Soup for HTML parsing and content extraction. Attackers can exploit this weakness by supplying malicious CSS selectors to the soupsieve.compile() function or through Beautiful Soup's .select() and .select_one() methods, causing applications to consume hundreds of megabytes of memory from minimal input payloads. This makes the vulnerability particularly dangerous in web applications or services where user-provided content is processed through CSS selector parsing, as it can be exploited to exhaust system resources and cause application crashes or unresponsiveness. The fix implemented in version 2.8.4 likely includes bounded memory allocation limits and proper input validation that prevents exponential growth in heap usage during CSS selector compilation.
Mitigation strategies for this vulnerability should focus on immediate version upgrades to 2.8.4 or later, which contain the necessary defensive coding measures against unbounded memory allocation. Security practitioners should also implement input sanitization mechanisms that validate CSS selector complexity before processing and establish resource monitoring systems to detect unusual memory consumption patterns. Organizations using Beautiful Soup in production environments should conduct security reviews of all code paths that accept user-provided CSS selectors or HTML content, ensuring that proper bounds checking is implemented at multiple layers of the application stack. This vulnerability demonstrates the importance of implementing defensive programming practices around parsing functions and highlights the need for regular security updates to address memory consumption flaws that can be exploited for resource exhaustion attacks. The ATT&CK framework categorizes this type of vulnerability under privilege escalation and resource exhaustion techniques, where attackers leverage parsing weaknesses to consume system resources and disrupt service availability.