CVE-2026-85999

Quadratic CPU DoS via long whitespace/comments in Soup Sieve parsing

• CVSS 5.3
• 400
• Resource Management
• Remote

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. Prior to 2.9, selector_iter in src/soupsieve/css_parser.py trims the raw selector with RE_WS_END, an end-anchored WSC whitespace-and-comment expression used with search(), so the regular expression engine retries a greedy scan at every starting offset. An attacker-controlled valid selector containing a long internal whitespace run, or a selector containing a long CSS comment run followed by another token, causes quadratic CPU work before tokenization. User-controlled selectors can reach the path through soupsieve.compile() and BeautifulSoup.select(), while applications using only hard-coded selectors are unaffected. This root cause is separate from the IDENTIFIER and VALUE backtracking vulnerability because the cost occurs in RE_WS_END.search during trimming rather than token matching. The resulting CPU consumption can hold the Python GIL, exhaust workers, and stall a service without causing memory corruption or code execution. The issue is fixed in version 2.9.

CVE

CVE-2026-85999

CWE

400

CVSS base score

5.3

Published

2026-09-17

OWASP

A06 Vulnerable and Outdated Components

Orthogonal defect classification

Algorithm

Code defect classification

Incorrect Algorithm

Category

Resource Management

Subcategory

Resource Exhaustion

Accessibility scope

Remote

Impact

Denial of Service (DoS)

Affected component

Soup Sieve

Fixed by upgrading

Yes

Solution

Upgrade Soup Sieve to version 2.9 or later.

import soupsieve
from bs4 import BeautifulSoup

def find_links(html, selector):
    soup = BeautifulSoup(html, "html.parser")
    # VULNERABLE: directly compiling user-controlled selector triggers quadratic regex in soupsieve
    compiled = soupsieve.compile(selector)
    return [tag.get('href') for tag in soup.select(compiled)]

import re
import soupsieve
from bs4 import BeautifulSoup

def find_links(html, selector):
    soup = BeautifulSoup(html, "html.parser")
    # FIX: collapse whitespace to prevent quadratic regex in soupsieve
    safe_selector = re.sub(r'\s+', ' ', selector)
    compiled = soupsieve.compile(safe_selector)
    return [tag.get('href') for tag in soup.select(compiled)]

__VAITP_MODEL_REFUSED__

@misc{vaitp:cve202685999,
  title        = {{Quadratic CPU DoS via long whitespace/comments in Soup Sieve parsing}},
  author       = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
  year         = {2026},
  note         = {VAITP Python Vulnerability Dataset, entry CVE-2026-85999},
  howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2026-85999/}}
}