CWE-625: Permissive Regular Expression

What is Permissive Regular Expression?

• Overview: Permissive Regular Expression (CWE-625) occurs when a regular expression is used that does not sufficiently restrict the set of allowed values, leading to partial matches that might not fully validate the input as intended.

• Exploitation Methods:

• Attackers can exploit this by providing input that only partially matches the expected pattern, bypassing input validation.
• Common attack patterns include using inputs that contain malicious payloads effectively bypassing security checks due to improper use of regular expressions.

• Security Impact:

• Direct consequences include unauthorized access, data leakage, or execution of unintended actions due to insufficient input validation.
• Potential cascading effects include enabling further injection attacks or serving as a foothold for additional vulnerabilities.
• Business impact may include data breaches, loss of customer trust, legal repercussions, and financial losses.

• Prevention Guidelines:

• Specific code-level fixes include ensuring that regular expressions are correctly anchored to match the entire input string from start to finish.
• Security best practices involve defining strict character sets and boundaries for allowed input, avoiding overly broad patterns.
• Recommended tools and frameworks include static analysis tools that can identify weak regular expressions and use libraries that offer safer pattern matching functions.

Corgea can automatically detect and fix Permissive Regular Expression in your codebase. [Try Corgea free today](https://corgea.app).

Technical Details

Likelihood of Exploit: Not specified

Affected Languages: Perl, PHP

Affected Technologies: Not specified

Vulnerable Code Example

#!/usr/bin/perl
use strict;
use warnings;

sub validate_username {
    my (\$username) = @_;
    # Vulnerable regular expression: Allows too many characters
    # This regex is too permissive as it allows any length of alphanumeric strings and underscores
    if (\$username =~ /^[a-zA-Z0-9_]*\$/) {
        print "Username is valid.\n";
    } else {
        print "Username is invalid.\n";
    }
}

validate_username("valid_user123");
validate_username("invalid_user@123");

How to fix Permissive Regular Expression?

To fix a permissive regular expression vulnerability, you should:

1. Define a Clear Pattern: Restrict the allowed characters and the length of input.
2. Boundary Specification: Set explicit limits on the length of the input to prevent overly long strings.
3. Test Against a Comprehensive Set of Inputs: Ensure the regex matches only the intended patterns.

In this case, we will limit usernames to alphanumeric characters and underscores, and set a reasonable length constraint (e.g., 3 to 16 characters). This prevents overly permissive matching and potential misuse with special characters or excessive length.

Fixed Code Example

#!/usr/bin/perl
use strict;
use warnings;

sub validate_username {
    my (\$username) = @_;
    # Fixed regular expression: Restricts username to 3-16 characters, alphanumeric and underscores only
    # This regex now enforces a proper length and allowed character set
    if (\$username =~ /^[a-zA-Z0-9_]{3,16}\$/) {
        print "Username is valid.\n";
    } else {
        print "Username is invalid.\n";
    }
}

validate_username("valid_user123");
validate_username("invalid_user@123");

In the fixed code, the regular expression ^[a-zA-Z0-9_]{3,16}\$ ensures that the username consists only of alphabets, numbers, and underscores, and is between 3 to 16 characters long. This prevents potentially harmful inputs and adheres to a more secure and restricted pattern.