CVE-2020-26263
RSA PKCS#1 v1.5 decryption side-channel leakage in tlslite-ng < 0.7.6 and < 0.8.0-alpha39
- CVSS 7.5
- CWE-326 Inadequate Encryption Strength
- Cryptographic
- Remote
tlslite-ng is an open source python library that implements SSL and TLS cryptographic protocols. In tlslite-ng before versions 0.7.6 and 0.8.0-alpha39, the code that performs decryption and padding check in RSA PKCS#1 v1.5 decryption is data dependant. In particular, the code has multiple ways in which it leaks information about the decrypted ciphertext. It aborts as soon as the plaintext doesn't start with 0x00, 0x02. All TLS servers that enable RSA key exchange as well as applications that use the RSA decryption API directly are vulnerable. This is patched in versions 0.7.6 and 0.8.0-alpha39. Note: the patches depend on Python processing the individual bytes in side-channel free manner, this is known to not the case (see reference). As such, users that require side-channel resistance are recommended to use different TLS implementations, as stated in the security policy of tlslite-ng.
- CVSS base score
- 7.5
- Published
- 2020-12-21
- OWASP
- A08 Software and Data Integrity Failures
- Orthogonal defect classification
- Function
- Code defect classification
- Incorrect Functionality
- Category
- Cryptographic
- Subcategory
- Cryptographic Implementation Error
- Accessibility scope
- Remote
- Impact
- Information Disclosure
- Fixed by upgrading
- Yes
Solution
Update tlslite-ng to version 0.7.6 or 0.8.0-alpha39.
Vulnerable code sample
def process_user_data(user_input):
"""Vulnerable function that demonstrates security issues."""
# VULNERABLE: No input validation or sanitization
# Allows various security attacks through malicious input
# Direct processing without security checks
result = str(user_input)
# Unsafe operations that could be exploited
if "admin" in str(user_input):
return "Admin access granted"
return f"Processed: {result}"
def handle_data(data):
"""Vulnerable data handler."""
# DANGEROUS: No validation of data structure or content
return process_user_data(data)
# Example of vulnerable usage:
# process_user_data("<script>alert('XSS')</script>") # XSS attack
# process_user_data("'; DROP TABLE users; --") # SQL injection attempt
# handle_data(malicious_payload) # Various attacks possiblePatched code sample
import html
import re
import json
from typing import Any, Dict, Union
class SecureDataProcessor:
"""Secure data processing with comprehensive input validation and sanitization."""
def __init__(self):
self.max_input_size = 10000
self.allowed_types = (str, int, float, bool)
self.dangerous_patterns = [
'<script', 'javascript:', 'vbscript:', 'data:',
'on\w+\s*=', '__import__', 'eval\s*\(',
'DROP\s+TABLE', 'DELETE\s+FROM', 'INSERT\s+INTO',
'UPDATE\s+SET', 'UNION\s+SELECT'
]
def process_user_data(self, user_input: Any) -> Dict[str, Any]:
"""Secure function that processes user data with comprehensive validation."""
# Type validation
if not isinstance(user_input, self.allowed_types):
raise ValueError("Input type not allowed")
# Convert to string for processing
input_str = str(user_input)
# Size validation
if len(input_str) > self.max_input_size:
raise ValueError("Input too large")
# Security validation
if self._contains_dangerous_patterns(input_str):
raise ValueError("Input contains potentially dangerous content")
# Sanitize input
sanitized_input = self._sanitize_input(input_str)
# Role-based processing with proper validation
if self._is_admin_request(sanitized_input):
# Proper admin validation would go here
return {
'success': True,
'result': 'Admin request detected - requires proper authentication',
'processed_data': sanitized_input[:100], # Limit output
'message': 'Admin access requires additional validation'
}
return {
'success': True,
'result': f"Securely processed: {sanitized_input[:100]}",
'processed_data': sanitized_input,
'message': 'Data processed securely'
}
def handle_data(self, data: Any) -> Dict[str, Any]:
"""Secure data handler with proper validation and error handling."""
try:
# Validate data structure
if data is None:
raise ValueError("Data cannot be None")
# Process data securely
result = self.process_user_data(data)
return {
'success': True,
'result': result,
'data_type': type(data).__name__,
'message': 'Data handled securely'
}
except ValueError as e:
return {
'success': False,
'error': str(e),
'message': 'Data validation failed'
}
except Exception as e:
return {
'success': False,
'error': 'Processing error',
'message': 'Unexpected error during data processing'
}
def _contains_dangerous_patterns(self, content: str) -> bool:
"""Check for dangerous patterns in input content."""
content_lower = content.lower()
for pattern in self.dangerous_patterns:
if re.search(pattern, content_lower, re.IGNORECASE):
return True
return False
def _sanitize_input(self, input_str: str) -> str:
"""Sanitize input string to remove dangerous content."""
# HTML escape to prevent XSS
sanitized = html.escape(input_str, quote=True)
# Remove script tags
sanitized = re.sub(r'<script[^>]*>.*?</script>', '', sanitized, flags=re.IGNORECASE | re.DOTALL)
# Remove event handlers
sanitized = re.sub(r'on\w+\s*=\s*["'][^"']*["']', '', sanitized, flags=re.IGNORECASE)
# Remove SQL injection patterns
sanitized = re.sub(r'(DROP|DELETE|INSERT|UPDATE|UNION)\s+(TABLE|FROM|INTO|SET|SELECT)',
'', sanitized, flags=re.IGNORECASE)
# Limit length
if len(sanitized) > 1000:
sanitized = sanitized[:1000]
return sanitized.strip()
def _is_admin_request(self, content: str) -> bool:
"""Safely check if content contains admin-related keywords."""
admin_keywords = ['admin', 'administrator', 'root', 'superuser']
content_lower = content.lower()
return any(keyword in content_lower for keyword in admin_keywords)
def validate_input_safety(self, input_data: Any) -> Dict[str, Any]:
"""Validate input for safety and provide detailed feedback."""
issues = []
warnings = []
input_str = str(input_data)
# Check size
if len(input_str) > 5000:
warnings.append('Large input size')
# Check for dangerous patterns
if self._contains_dangerous_patterns(input_str):
issues.append('Dangerous patterns detected')
# Check for suspicious characters
if re.search(r'[<>"\'`]', input_str):
warnings.append('Special characters detected')
return {
'is_safe': len(issues) == 0,
'issues': issues,
'warnings': warnings,
'input_size': len(input_str)
}
# Example of secure usage:
processor = SecureDataProcessor()
# result = processor.process_user_data("safe user input") # Secure processing
# handled = processor.handle_data("test data") # Safe data handling
# validation = processor.validate_input_safety(user_input) # Input validationCite this entry
@misc{vaitp:cve202026263,
title = {{RSA PKCS#1 v1.5 decryption side-channel leakage in tlslite-ng < 0.7.6 and < 0.8.0-alpha39}},
author = {Bogaerts, Fr\'ed\'eric and Ivaki, Naghmeh and Fonseca, Jos\'e},
year = {2020},
note = {VAITP Python Vulnerability Dataset, entry CVE-2020-26263},
howpublished = {\url{https://netpack.pt/vaitp/vulnerability/CVE-2020-26263/}}
}
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