CVE-2020-4048: WordPress Open Redirect Vulnerability

CVE-2020-4048 Overview

CVE-2020-4048 is an open redirect vulnerability in WordPress that affects the wp_validate_redirect() function and URL sanitization logic. Due to improper input validation, an attacker can craft arbitrary external links that lead to unintended open redirects when clicked by users. This type of vulnerability is commonly exploited in phishing attacks, where victims are redirected from a trusted WordPress site to malicious external destinations.

Critical Impact

Attackers can abuse this vulnerability to redirect authenticated WordPress users to malicious sites, potentially facilitating phishing attacks, credential theft, or malware distribution by leveraging the trust users place in the legitimate WordPress domain.

Affected Products

• WordPress versions prior to 5.4.2 (and corresponding minor releases)
• Fedora Project Fedora 32 and 33
• Debian Linux 8.0, 9.0, and 10.0

Discovery Timeline

• June 12, 2020 - CVE-2020-4048 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2020-4048

Vulnerability Analysis

This open redirect vulnerability stems from insufficient URL sanitization in WordPress's wp_validate_redirect() function. The function is designed to validate redirect URLs and ensure they point to safe destinations within the same domain. However, due to a flaw in how certain characters and URL patterns are processed, attackers can bypass the validation logic and craft URLs that appear to be internal redirects but actually point to external malicious domains.

The vulnerability requires user interaction—a victim must click on a maliciously crafted link. When authenticated users click such links, they may be redirected away from the trusted WordPress site to attacker-controlled destinations. This can be particularly dangerous in scenarios where the redirect leads to credential harvesting pages that mimic the original WordPress login.

Root Cause

The root cause lies in the wp_validate_redirect() function's failure to properly sanitize the input URL before validation. The function trimmed whitespace and control characters from the location parameter but did not apply wp_sanitize_redirect() to handle a wider variety of potentially dangerous characters. This gap allowed specially crafted URLs to bypass the redirect validation checks.

Attack Vector

The attack requires network access and involves crafting a malicious URL that exploits the insufficient sanitization in wp_validate_redirect(). An attacker must convince a logged-in WordPress user to click on the crafted link, which could be distributed via phishing emails, social engineering, or embedded in user-generated content. When clicked, the user is redirected to an external site controlled by the attacker, despite the URL initially appearing to belong to the legitimate WordPress domain.

// Security patch from WordPress - src/wp-includes/pluggable.php
 	 * @return string redirect-sanitized URL
 	 */
 	function wp_validate_redirect( $location, $default = '' ) {
-		$location = trim( $location, " \t\n\r\0\\x08\\x0B" );
+		$location = wp_sanitize_redirect( trim( $location, " \t\n\r\0\\x08\\x0B" ) );
 		// Browsers will assume 'http' is your protocol, and will obey a redirect to a URL starting with '//'.
 		if ( substr( $location, 0, 2 ) == '//' ) {
 			$location = 'http:' . $location;

Source: GitHub Commit 6ef777e9a022bee2a80fa671118e7e2657e52693

Detection Methods for CVE-2020-4048

Indicators of Compromise

• Unusual redirect patterns in web server access logs pointing to external domains
• Presence of crafted URLs with special characters or encoding designed to bypass validation
• User reports of unexpected redirects to unfamiliar external sites
• Phishing reports where the initial link appeared to originate from the WordPress site

Detection Strategies

• Monitor web server logs for redirect responses (HTTP 301/302) to unexpected external domains
• Implement web application firewall (WAF) rules to detect and block suspicious URL patterns targeting WordPress redirect functions
• Review referrer headers for anomalies indicating redirect chain exploitation
• Deploy SentinelOne agents to detect post-exploitation activity if redirects lead to malware delivery

Monitoring Recommendations

• Enable detailed logging for WordPress core functions, particularly authentication and redirect operations
• Set up alerts for high volumes of redirect requests to external domains
• Monitor for spikes in 302 redirect responses from WordPress installations
• Track user-reported phishing incidents that reference legitimate WordPress URLs

How to Mitigate CVE-2020-4048

Immediate Actions Required

• Update WordPress to version 5.4.2 or the appropriate patched minor release for your version branch
• Verify the patch has been applied by checking the wp_validate_redirect() function in src/wp-includes/pluggable.php
• Review access logs for any historical exploitation attempts
• Inform users about potential phishing attempts leveraging this vulnerability

Patch Information

WordPress has released security patches addressing this vulnerability across all supported version branches. The primary fix is in WordPress 5.4.2, with backported fixes available in versions 5.3.4, 5.2.7, 5.1.6, 5.0.10, 4.9.15, 4.8.14, 4.7.18, 4.6.19, 4.5.22, 4.4.23, 4.3.24, 4.2.28, 4.1.31, 4.0.31, 3.9.32, 3.8.34, and 3.7.34. The fix ensures that wp_sanitize_redirect() is called on the input URL before validation occurs, preventing bypass attempts. Refer to the WordPress 5.4.2 Security Release and GitHub Security Advisory GHSA-q6pw-gvf4-5fj5 for complete details.

Workarounds

• Implement a web application firewall (WAF) with rules to detect and block open redirect attempts
• Use security plugins that provide additional URL validation and redirect protection
• Consider restricting redirect functionality to authenticated administrators only where feasible
• Deploy Content Security Policy headers to limit redirect destinations

# Configuration example - Apache mod_rewrite rule to block suspicious redirects
# Add to .htaccess to provide additional protection layer
RewriteEngine On
RewriteCond %{QUERY_STRING} redirect_to=.*https?:// [NC,OR]
RewriteCond %{QUERY_STRING} redirect_to=.*%2F%2F [NC]
RewriteRule .* - [F,L]