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CVE-2026-49476 — Soup Sieve: Memory Exhaustion via Large Comma-Separated Selector Lists in soupsieve

Affected: facelessuser soupsieve

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.

GHSA-2wc2-fm75-p42x — Soup Sieve has Memory Exhaustion via Large Comma-Separated Selector Lists

Affected: PyPI: soupsieve

### Summary

The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) allocates unbounded memory when compiling large comma-separated selector lists. An attacker who can supply a crafted CSS selector string to `soupsieve.compile()` or Beautiful Soup's `.select()` / `.select_one()` can cause the application to allocate hundreds of megabytes of heap memory from a relatively small input, leading to memory exhaustion and denial of service.

To be completely transparent, AI tools helped surface this issue. However, it was independently reproduced and carefully validated. Researchers follow responsible disclosure practices and originally shared this report privately.

A **500 KB** selector string triggers allocation of approximately **244 MB** of heap memory - a 488x— amplification ratio**.

### Details

**Affected code:** `soupsieve/css_parser.py`, lines ~204, 925, 1106

The soupsieve CSS parser splits comma-separated selector lists and creates one `CSSSelector` object per list item. Each `CSSSelector` object contains parsed selector data structures including `SelectorList`, `Selector`, and associated tag/attribute/pseudo-class metadata.

When a selector string such as `a,a,a,...` (with 250,000 comma-separated items) is passed to `sv.compile()`, the parser: