CVE-2026-42525: Jenkins Entra ID Plugin Phishing Vulnerability

CVE-2026-42525 Overview

CVE-2026-42525 is an Open Redirect vulnerability in the Jenkins Microsoft Entra ID (previously Azure AD) Plugin version 666.v6060de32f87d and earlier. The vulnerability stems from insufficient validation of redirect URLs after user authentication, which allows attackers to redirect authenticated users to malicious external sites. This weakness (CWE-601) can be exploited to conduct phishing attacks by tricking users into believing they are interacting with a legitimate Jenkins instance while actually being directed to attacker-controlled infrastructure.

Critical Impact

Attackers can exploit this vulnerability to conduct phishing attacks against Jenkins users by manipulating post-login redirect URLs to direct victims to malicious sites designed to harvest credentials or deliver malware.

Affected Products

• Jenkins Microsoft Entra ID Plugin version 666.v6060de32f87d and earlier
• Jenkins Microsoft Azure AD Plugin (legacy name) version 666.v6060de32f87d and earlier

Discovery Timeline

• 2026-04-29 - CVE-2026-42525 published to NVD
• 2026-04-29 - Last updated in NVD database

Technical Details for CVE-2026-42525

Vulnerability Analysis

This vulnerability is classified as CWE-601 (URL Redirection to Untrusted Site, also known as Open Redirect). The Jenkins Microsoft Entra ID Plugin fails to properly restrict or validate the redirect URL parameter used after successful authentication. When users authenticate via Microsoft Entra ID (Azure AD), the plugin processes a redirect parameter to return users to their intended destination. However, the lack of proper URL validation allows attackers to craft malicious authentication links that redirect users to external, attacker-controlled websites after login.

The attack requires user interaction—specifically, the victim must click on a crafted link and complete the authentication process. Once authenticated, the user is automatically redirected to the malicious URL specified by the attacker. This makes the attack particularly effective for phishing campaigns since the initial authentication occurs on legitimate Jenkins and Microsoft Entra ID infrastructure, lending credibility to the attack.

Root Cause

The root cause of this vulnerability is the absence of proper validation and sanitization of the redirect URL parameter in the authentication flow. The plugin does not implement adequate checks to ensure that post-login redirects only point to trusted or same-origin destinations. This allows arbitrary external URLs to be specified in the redirect parameter, which the plugin processes without restriction.

Attack Vector

The attack is network-based and requires social engineering to entice users to click on malicious links. An attacker constructs a URL to the Jenkins instance that includes a manipulated redirect parameter pointing to a phishing site. The attack flow typically proceeds as follows:

1. Attacker crafts a malicious Jenkins authentication URL with a redirect parameter pointing to an attacker-controlled domain
2. Victim clicks the link, believing they are accessing their legitimate Jenkins instance
3. Victim authenticates normally through Microsoft Entra ID
4. Upon successful authentication, the plugin redirects the victim to the attacker's phishing site
5. The phishing site may mimic Jenkins or request additional credentials, potentially capturing sensitive information

The vulnerability relies on the trust users place in the legitimate Jenkins authentication URL and the Microsoft Entra ID login process, making it effective for targeted phishing campaigns against development and operations teams.

Detection Methods for CVE-2026-42525

Indicators of Compromise

• Unusual redirect URLs in Jenkins authentication requests containing external domains
• User reports of unexpected redirects after Jenkins authentication
• Authentication logs showing successful logins followed by redirects to non-Jenkins URLs
• Phishing reports from users who accessed Jenkins through external links

Detection Strategies

• Monitor Jenkins access logs for authentication requests with suspicious redirect parameters containing external hostnames
• Implement web application firewall (WAF) rules to flag or block authentication URLs with redirect parameters pointing to external domains
• Review reverse proxy or load balancer logs for patterns of authentication requests with unusual redirect destinations
• Deploy endpoint detection to identify browser redirects from Jenkins to known malicious or suspicious domains

Monitoring Recommendations

• Enable verbose logging for the Microsoft Entra ID Plugin authentication events
• Configure alerts for authentication redirects to non-whitelisted domains
• Monitor security information feeds for phishing campaigns targeting Jenkins users
• Implement user awareness training to report suspicious Jenkins login experiences

How to Mitigate CVE-2026-42525

Immediate Actions Required

• Update the Jenkins Microsoft Entra ID Plugin to the latest patched version as soon as available
• Review recent authentication logs for evidence of exploitation attempts
• Alert users about potential phishing attacks leveraging this vulnerability
• Implement network-level controls to restrict outbound redirects from Jenkins

Patch Information

Jenkins has released a security advisory addressing this vulnerability. Administrators should consult the Jenkins Security Advisory #SECURITY-3760 for detailed patch information and upgrade instructions. Apply the latest version of the Microsoft Entra ID Plugin that includes the fix for redirect URL validation.

Workarounds

• Deploy a reverse proxy or WAF rule to strip or validate redirect parameters in Jenkins authentication URLs
• Implement Content Security Policy (CSP) headers to restrict navigation to trusted domains
• Consider temporarily disabling the Microsoft Entra ID Plugin if the risk is deemed unacceptable until patching is complete
• Educate users to access Jenkins only through bookmarked URLs rather than clicking links in emails or messages

# Example: Nginx configuration to strip potentially malicious redirect parameters
# Add to your Jenkins reverse proxy configuration
location /securityRealm/finishLogin {
    # Log all authentication redirect attempts for review
    access_log /var/log/nginx/jenkins_auth_redirects.log;
    
    # Validate redirect parameter points to trusted domain
    if ($arg_from !~ "^https?://(jenkins\.yourdomain\.com|yourdomain\.com)") {
        set $args "";
    }
    
    proxy_pass http://jenkins_backend;
}