CVE-2021-21705: PHP URL Validation Information Disclosure

CVE-2021-21705 Overview

CVE-2021-21705 is an improper input validation vulnerability affecting PHP's URL validation functionality. When using the filter_var() function with the FILTER_VALIDATE_URL parameter, a URL containing an invalid password field can be incorrectly accepted as valid. This flaw can lead to improper URL parsing, potentially causing applications to contact incorrect servers or make erroneous access control decisions.

Critical Impact

Applications relying on PHP's filter_var() for URL validation may accept malformed URLs, leading to Server-Side Request Forgery (SSRF) scenarios, incorrect server connections, or flawed authorization decisions.

Affected Products

• PHP versions 7.3.x below 7.3.29
• PHP versions 7.4.x below 7.4.21
• PHP versions 8.0.x below 8.0.8
• NetApp Clustered Data ONTAP
• Oracle SD-WAN Aware 8.2

Discovery Timeline

• 2021-10-04 - CVE-2021-21705 published to NVD
• 2024-11-21 - Last updated in NVD database

Technical Details for CVE-2021-21705

Vulnerability Analysis

This vulnerability stems from inadequate validation logic within PHP's filter_var() function when processing URLs with the FILTER_VALIDATE_URL filter. The function fails to properly validate the password component of a URL, allowing malformed URLs to pass validation checks that should have been rejected.

When an application uses filter_var($url, FILTER_VALIDATE_URL) to sanitize user-supplied URLs, the function may return true for URLs containing invalid characters or formatting in the password field. This creates a security gap where downstream URL parsing functions may interpret the malformed URL differently than expected, potentially redirecting requests to unintended destinations.

The vulnerability is classified under CWE-20 (Improper Input Validation), reflecting the core issue of insufficient input sanitization in security-critical validation routines.

Root Cause

The root cause lies in the URL parsing and validation logic within PHP's ext/filter extension. The validation routine does not comprehensively check all components of the URL structure, specifically failing to enforce proper character restrictions on the password portion of the userinfo component (the user:password@ section before the hostname).

According to RFC 3986, certain characters in the userinfo component must be percent-encoded. PHP's FILTER_VALIDATE_URL did not properly enforce these restrictions, allowing invalid password fields to slip through validation.

Attack Vector

The attack vector is network-based and requires no authentication or user interaction. An attacker can exploit this vulnerability by providing a crafted URL to any PHP application that relies on filter_var() with FILTER_VALIDATE_URL for URL sanitization before making outbound requests or access control decisions.

The vulnerability can be exploited by submitting a specially crafted URL with an invalid password field to an application endpoint. When the URL passes validation but is later parsed by other functions or external libraries, the inconsistent interpretation can lead to:

• Server-Side Request Forgery (SSRF): The application may connect to an attacker-controlled server instead of the intended destination
• Authorization Bypass: Access control checks based on URL hostname validation may be circumvented
• Data Exfiltration: Sensitive data intended for one server could be sent to an attacker-controlled endpoint

Technical details and proof-of-concept information can be found in the PHP Bug Report #81122.

Detection Methods for CVE-2021-21705

Indicators of Compromise

• Unusual outbound HTTP/HTTPS requests to unexpected destinations from PHP applications
• Web application logs showing malformed URLs with unusual characters in authentication components
• Failed connections or connection timeouts following URL validation success in application logs

Detection Strategies

• Audit PHP application code for usage of filter_var() with FILTER_VALIDATE_URL parameter
• Implement web application firewall (WAF) rules to detect malformed URL patterns in request parameters
• Monitor outbound network traffic from web servers for connections to unexpected IP addresses or domains
• Review PHP version inventory across infrastructure to identify vulnerable installations

Monitoring Recommendations

• Enable verbose logging for PHP applications handling external URLs
• Configure network monitoring to alert on outbound connections from web servers to untrusted destinations
• Implement application-level logging for all filter_var() calls involving URL validation
• Set up alerts for PHP error logs indicating URL parsing inconsistencies

How to Mitigate CVE-2021-21705

Immediate Actions Required

• Upgrade PHP to version 7.3.29 or later for 7.3.x branch
• Upgrade PHP to version 7.4.21 or later for 7.4.x branch
• Upgrade PHP to version 8.0.8 or later for 8.0.x branch
• Review application code to identify all instances of URL validation using filter_var()
• Apply vendor patches from NetApp and Oracle if using affected products

Patch Information

PHP has released patched versions addressing this vulnerability. Organizations should upgrade to the following minimum versions:

• PHP 7.3.x: Upgrade to 7.3.29 or later
• PHP 7.4.x: Upgrade to 7.4.21 or later
• PHP 8.0.x: Upgrade to 8.0.8 or later

Additional vendor advisories are available from NetApp Security Advisory and the Oracle Critical Patch Update January 2022. Linux distributions have also released updates; see the Gentoo GLSA 2022-09-20 for distribution-specific guidance.

Workarounds

• Implement additional URL validation using parse_url() to verify all URL components after filter_var() checks
• Use allowlisting for permitted URL schemes, hosts, and ports in application logic
• Deploy network-level controls to restrict outbound connections from PHP applications to approved destinations only
• Consider using URL validation libraries that provide more comprehensive parsing and validation

# Check current PHP version and identify if vulnerable
php -v

# Example: Verify PHP version is patched (should be >= 7.4.21 for 7.4.x branch)
php -r "echo 'PHP Version: ' . phpversion() . PHP_EOL; echo version_compare(phpversion(), '7.4.21', '>=') ? 'Patched' : 'Vulnerable';"