What is Executable Regular Expression Error (CWE-624)?

TL;DR: Executable Regular Expression Error (CWE-624) is a critical security flaw where untrusted inputs can modify or execute code within regular expressions, leading to unauthorized actions. Escaping or quoting user inputs prevents this vulnerability.

What is Executable Regular Expression Error?

Executable Regular Expression Error (CWE-624) is a type of vulnerability where regular expressions can execute code or modify patterns based on user inputs. As defined by the MITRE Corporation under CWE-624, this error allows attackers to inject commands or alter regex behavior, leading to unauthorized actions and potential system compromise.

Quick Summary

Executable Regular Expression Error (CWE-624) is a critical security flaw where untrusted inputs can modify or execute code within regular expressions. This vulnerability enables attackers to manipulate patterns for malicious purposes, potentially leading to data theft, system compromise, and other severe consequences. Jump to: What is Executable Regular Expression Error? · Quick Summary · Overview · How It Works · Business Impact · Attack Scenario · Detection · Fix · Framework-Specific Fixes · Ask AI · Best Practices Checklist · FAQ · Vulnerabilities Related

Jump to: Quick Summary · Executable Regular Expression Error Overview · How Executable Regular Expression Error Works · Business Impact of Executable Regular Expression Error · Executable Regular Expression Error Attack Scenario · How to Detect Executable Regular Expression Error · How to Fix Executable Regular Expression Error · Framework-Specific Fixes for Executable Regular Expression Error · How to Ask AI to Check Your Code for Executable Regular Expression Error · Executable Regular Expression Error Best Practices Checklist · Executable Regular Expression Error FAQ · Vulnerabilities Related to Executable Regular Expression Error · References · Scan Your Own Site

Executable Regular Expression Error Overview

What: Executable Regular Expression Error (CWE-624) is a vulnerability where regular expressions can execute code or modify patterns based on user inputs.

Why it matters: This error allows attackers to inject commands or alter regex behavior, leading to unauthorized actions and potential system compromise. It impacts applications that use untrusted input in regex patterns without proper validation.

Where it occurs: In applications using languages like Perl, Python, Ruby, where regular expressions allow for user-controlled inputs.

Who is affected: Developers and security teams responsible for maintaining codebases with vulnerable regex implementations.

Who is NOT affected: Systems already employing strict validation rules to restrict pattern modifications by untrusted sources.

How Executable Regular Expression Error Works

Root Cause

The root cause of this vulnerability lies in the ability of regular expressions to execute code or modify patterns based on user inputs. When a user can control parts of a regex, they may inject commands that alter its behavior, leading to unauthorized actions.

Attack Flow

1. An attacker identifies a part of the application where regular expressions are used.
2. The attacker crafts input data to manipulate the regex pattern in a way that executes code or alters its functionality.
3. The manipulated regex is executed by the application, resulting in unintended behavior such as command execution or data manipulation.

Prerequisites to Exploit

• User-controlled inputs can influence regex patterns.
• Lack of proper validation and escaping mechanisms for user-provided strings.

Vulnerable Code

import re

def process_input(user_input):
    pattern = f"pattern={user_input}"
    # Vulnerable: Directly using user input in a regex pattern
    match = re.search(pattern, some_data)

This code is vulnerable because it directly uses untrusted user_input to modify the regex pattern.

Secure Code

import re

def process_input(user_input):
    safe_pattern = f"pattern={re.escape(user_input)}"
    # Safe: Escaping user input before using in a regex pattern
    match = re.search(safe_pattern, some_data)

This code is secure because it escapes the user_input before incorporating it into the regex pattern.

Business Impact of Executable Regular Expression Error

Confidentiality

• Data Exposure: Attackers can manipulate regular expressions to access sensitive data.
• Financial Losses: Unauthorized access to confidential information can lead to financial losses and legal penalties.

Integrity

• Data Corruption: Attackers may alter regex patterns to corrupt or modify application data, leading to incorrect processing of information.

Availability

• System Disruption: Manipulated regular expressions can cause system crashes or denial-of-service conditions by triggering unexpected behavior in the application.

Executable Regular Expression Error Attack Scenario

1. An attacker identifies a part of an application where user input affects regex patterns.
2. The attacker crafts a malicious input to modify the regex pattern, potentially injecting commands that execute unauthorized actions.
3. The manipulated regex is executed by the application, leading to unintended behavior such as data manipulation or command execution.

How to Detect Executable Regular Expression Error

Manual Testing

• Review code for direct use of user inputs in regular expressions.
• Check if user-provided strings are properly escaped before being used in patterns.

Automated Scanners (SAST / DAST)

Static analysis can detect the presence of untrusted inputs directly affecting regex patterns. Dynamic testing is required to confirm that these patterns are executed under real conditions.

PenScan Detection

PenScan’s scanner engines such as ZAP, Nuclei, Wapiti, Nikto, SSLyze, and Dalfox may identify instances where user input can influence regular expressions.

False Positive Guidance

False positives occur when a regex pattern looks risky but is actually safe due to context that the scanner cannot determine. Ensure patterns are not being manipulated in ways that could execute code or alter behavior unexpectedly.

How to Fix Executable Regular Expression Error

• Escape or quote user inputs before inserting them into regular expressions.
• Use language-specific features like Perl’s \Q and \E to prevent execution of special characters.
• Implement strict validation rules to restrict pattern modifications by untrusted sources.
• Ensure all regex patterns are validated for potential code injection vulnerabilities.

Framework-Specific Fixes for Executable Regular Expression Error

Python

import re

def process_input(user_input):
    safe_pattern = f"pattern={re.escape(user_input)}"
    match = re.search(safe_pattern, some_data)

This example demonstrates escaping user input before using it in a regex pattern.

How to Ask AI to Check Your Code for Executable Regular Expression Error

Executable Regular Expression Error Best Practices Checklist

✅ Escape or quote user inputs before inserting them into regular expressions. ✅ Use language-specific features like Perl’s \Q and \E to prevent execution of special characters. ✅ Implement strict validation rules to restrict pattern modifications by untrusted sources. ✅ Ensure all regex patterns are validated for potential code injection vulnerabilities. ✅ Regularly review and update security practices related to regex usage.

Executable Regular Expression Error FAQ

How does an executable regular expression error occur?

An executable regular expression error occurs when a regular expression contains user-controlled inputs that can execute code or modify patterns, leading to unauthorized actions.

What are the potential impacts of an executable regular expression error?

This vulnerability can lead to unauthorized code execution, data theft, and system compromise by allowing attackers to manipulate regular expressions for malicious purposes.

How can I detect an executable regular expression error in my application?

Use static analysis tools like SAST and DAST to identify patterns that allow user input to modify or execute regular expressions. Manual testing involves reviewing code for untrusted inputs interacting with regex features.

What are some common examples of vulnerable code related to executable regular expression errors?

Vulnerable code includes using user-provided strings directly in regex patterns without proper validation or escaping, allowing attackers to inject commands or alter pattern behavior.

How can I fix an executable regular expression error in my application?

Escape or quote user inputs before inserting them into regular expressions. Use language-specific features like Perl’s \Q and \E to prevent execution of special characters.

How can I ensure secure coding practices to avoid executable regular expression errors?

Always validate and escape user inputs before using them in regex patterns. Use strict validation rules to restrict pattern modifications by untrusted sources.

Vulnerabilities Related to Executable Regular Expression Error

References

• MITRE
• NVD

Scan Your Own Site

Manual code review catches what you know to look for. An automated scan catches what you didn’t. Scan your own website using PenScan to find Executable Regular Expression Error and other risks before an attacker does.