CVE-2016-2221 in WordPress
Summary
by MITRE
Open redirect vulnerability in the wp_validate_redirect function in wp-includes/pluggable.php in WordPress before 4.4.2 allows remote attackers to redirect users to arbitrary web sites and conduct phishing attacks via a malformed URL that triggers incorrect hostname parsing, as demonstrated by an https:example.com URL.
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Analysis
by VulDB Data Team • 08/21/2022
The CVE-2016-2221 vulnerability represents a critical open redirect flaw in WordPress core functionality that emerged from improper URL validation mechanisms within the wp_validate_redirect function. This vulnerability existed in WordPress versions prior to 4.4.2 and exploited a fundamental parsing error in how the system handled malformed URLs, particularly those beginning with https:example.com patterns that bypassed normal hostname validation checks. The flaw allowed attackers to craft malicious URLs that would appear legitimate to users while redirecting them to completely different domains, creating a dangerous vector for social engineering and phishing campaigns.
The technical implementation of this vulnerability stemmed from insufficient input sanitization in the pluggable.php file where the wp_validate_redirect function failed to properly validate URL schemes and hostnames. When processing URLs that contained malformed schemes such as https:example.com, the function would incorrectly parse the hostname portion, treating example.com as the actual destination rather than recognizing the malformed structure. This parsing error occurred because the validation logic did not adequately handle edge cases where URL schemes were improperly formatted, allowing attackers to manipulate the redirect behavior without proper authorization or authentication.
The operational impact of CVE-2016-2221 was significant as it enabled attackers to conduct sophisticated phishing operations by redirecting users from trusted WordPress domains to malicious sites designed to capture credentials or personal information. Attackers could leverage this vulnerability in various ways including crafting malicious links in forum posts, comments, or email communications that would redirect unsuspecting users to fake login pages or sites hosting malware. The vulnerability was particularly dangerous because it could be exploited across multiple WordPress installations without requiring specific user interaction beyond clicking a malicious link, making it a scalable threat for cybercriminals conducting large-scale phishing campaigns.
This vulnerability aligns with CWE-601 Open Redirect vulnerability classification and maps to attack techniques in the ATT&CK framework under T1566 Phishing and T1071.1001 Application Layer Protocol: Web Protocols. The flaw demonstrates how seemingly minor input validation issues in core web applications can create substantial security risks when exploited at scale. Organizations running vulnerable WordPress installations faced immediate exposure to credential theft, data exfiltration, and reputation damage as attackers could seamlessly redirect users through trusted domains. The vulnerability also highlighted the importance of proper URL validation in web applications and the potential for attackers to bypass standard security controls through creative manipulation of protocol parsing.
Mitigation strategies for CVE-2016-2221 required immediate patching of WordPress installations to version 4.4.2 or later, which included enhanced URL validation logic that properly handled malformed URL schemes and hostnames. Security administrators should have implemented additional monitoring of redirect behaviors and URL patterns within their WordPress installations to detect potential exploitation attempts. The vulnerability underscored the necessity of comprehensive input validation across all web application components and reinforced the importance of regular security updates and vulnerability assessments to prevent similar issues from affecting critical infrastructure components.