CVE-2020-27511: Prototypejs Prototype DOS Vulnerability

CVE-2020-27511 Overview

A Regular Expression Denial of Service (ReDoS) vulnerability was discovered in the stripTags and unescapeHTML components of Prototype.js version 1.7.3. An attacker can exploit this vulnerability by crafting malicious HTML tags that cause the underlying regular expression engine to enter a catastrophic backtracking state, resulting in excessive CPU consumption and application unavailability.

Critical Impact

This vulnerability allows remote attackers to cause a denial of service condition by sending specially crafted HTML input, potentially rendering web applications unresponsive and affecting service availability for all users.

Affected Products

• Prototype.js version 1.7.3
• Web applications utilizing Prototype.js stripTags function
• Web applications utilizing Prototype.js unescapeHTML function

Discovery Timeline

• 2021-06-21 - CVE-2020-27511 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2020-27511

Vulnerability Analysis

This vulnerability exists within the string manipulation functions of Prototype.js, specifically in the stripTags and unescapeHTML methods. These functions use regular expressions to parse and process HTML content. The regular expressions employed are vulnerable to algorithmic complexity attacks, where certain input patterns cause the regex engine to perform an exponential number of backtracking operations.

When processing malformed or specially crafted HTML tags, the regex engine enters a state of catastrophic backtracking. This occurs because the regular expression patterns contain nested quantifiers or overlapping alternatives that create an exponential number of possible matching paths. The attack vector is network-based, requiring no authentication or user interaction, making it particularly dangerous for public-facing web applications.

Root Cause

The root cause lies in the design of the regular expression patterns used within the stripTags and unescapeHTML functions located in the Prototype.js string handling module. The regex patterns are not optimized to prevent excessive backtracking when parsing malformed HTML structures, leading to exponential time complexity under certain input conditions.

Attack Vector

The attack can be executed remotely over the network without requiring any privileges or user interaction. An attacker submits specially crafted HTML content to any input field or API endpoint that processes data using the vulnerable stripTags or unescapeHTML functions. When the application attempts to sanitize this input, the regex engine consumes excessive CPU resources, potentially locking up the server thread and causing denial of service.

The vulnerability mechanism involves sending HTML tags with specific structural characteristics that trigger catastrophic backtracking. This typically involves nested or malformed tag structures with repeated patterns that match multiple paths in the regular expression. For detailed technical analysis and proof-of-concept examples, refer to the GitHub PoC for CVE-2020-27511.

Detection Methods for CVE-2020-27511

Indicators of Compromise

• Abnormally high CPU utilization on web servers processing user-submitted HTML content
• Application response time degradation or timeouts when handling specific input patterns
• Server threads or processes becoming unresponsive during string sanitization operations
• Increased memory consumption in JavaScript runtime environments

Detection Strategies

• Monitor server CPU utilization for sustained spikes correlated with user input processing
• Implement request timeout monitoring to detect long-running string operations
• Deploy application performance monitoring (APM) tools to identify slow function execution in stripTags or unescapeHTML calls
• Analyze web application logs for repeated requests containing unusually large or complex HTML payloads

Monitoring Recommendations

• Configure alerting for CPU utilization thresholds exceeding normal operational baselines
• Implement input length and complexity limits at the application layer before string processing
• Enable detailed logging for string sanitization function execution times
• Monitor for patterns of repeated requests from single sources that may indicate ReDoS exploitation attempts

How to Mitigate CVE-2020-27511

Immediate Actions Required

• Identify all applications using Prototype.js version 1.7.3 in your environment
• Implement input validation and length restrictions before passing data to stripTags or unescapeHTML functions
• Consider replacing Prototype.js with actively maintained alternatives for HTML sanitization
• Deploy web application firewalls (WAF) with rules to detect and block malicious HTML patterns

Patch Information

Prototype.js is a legacy JavaScript framework with limited active development. Users should consult the Prototype.js official site for any available updates or security advisories. Given the framework's maintenance status, organizations should strongly consider migrating to modern, actively maintained JavaScript libraries for HTML sanitization and string manipulation tasks.

Workarounds

• Implement request timeout limits to prevent long-running regex operations from blocking server resources
• Add input length validation to reject excessively long or complex HTML strings before processing
• Use alternative HTML sanitization libraries that employ non-regex-based parsing or optimized regex patterns
• Deploy rate limiting to mitigate the impact of repeated exploitation attempts

# Example: Nginx configuration to limit request body size and processing time
# Add to server or location block

client_max_body_size 1m;
client_body_timeout 10s;
proxy_read_timeout 30s;

# Limit request rate per IP
limit_req_zone $binary_remote_addr zone=html_sanitize:10m rate=10r/s;
limit_req zone=html_sanitize burst=20 nodelay;