CVE-2025-67639: 
Java vulnerability analysis and mitigation

CVE-2025-67639 is a Cross-Site Request Forgery (CSRF) vulnerability in the Jenkins login form, tracked as SECURITY-1166. It affects Jenkins weekly versions 2.540 and earlier, and LTS versions 2.528.2 and earlier, allowing attackers to trick authenticated users into logging in to the attacker's account. The vulnerability was disclosed on December 10, 2025, as part of a broader Jenkins security advisory. It carries a CVSS v3.1 base score of 3.5 (Low) (Jenkins Advisory, GitHub Advisory).

The root cause is classified as CWE-352 (Cross-Site Request Forgery): Jenkins does not require a CSRF token (crumb) for the URL that handles interactive login HTTP requests. Because the login endpoint lacks CSRF protection, an attacker can craft a malicious web page or link that, when visited by a logged-in or active-session user, submits a login request on their behalf — effectively forcing the victim to authenticate into the attacker's account (a "login CSRF" or account fixation scenario). Exploitation requires the attacker to have a low-privilege account on the Jenkins instance and requires user interaction (the victim must visit a crafted page) (Jenkins Advisory, GitHub Advisory).

Successful exploitation allows an attacker to force a victim user to be logged into the attacker's Jenkins account, potentially exposing any actions the victim performs (such as triggering builds, accessing credentials, or viewing sensitive pipeline data) under the attacker's session context. The primary impact is a limited confidentiality exposure, as the victim's subsequent interactions with Jenkins may inadvertently reveal information to the attacker. Integrity and availability are not directly impacted by this vulnerability (Jenkins Advisory, GitHub Advisory).

1. Reconnaissance: Identify a target Jenkins instance running version 2.540 or earlier (weekly) or 2.528.2 or earlier (LTS) that is accessible over the network.
2. Obtain attacker account: Register or use an existing low-privilege account on the target Jenkins instance to obtain valid attacker credentials.
3. Craft malicious login request: Create an HTML page or link containing a form or script that automatically submits a POST request to the Jenkins login endpoint (e.g., /j_spring_security_check) with the attacker's username and password, without a CSRF token.
4. Deliver to victim: Trick the target user (e.g., via phishing email or malicious link) into visiting the crafted page while they have an active browser session.
5. Session fixation achieved: The victim's browser submits the login request, logging them into the attacker's account. Any subsequent actions the victim takes on Jenkins (viewing credentials, triggering builds) are performed under the attacker's account context, potentially exposing sensitive information to the attacker (Jenkins Advisory).

• Logs: Jenkins access logs showing POST requests to /j_spring_security_check without a valid CSRF crumb/token parameter, especially from unexpected or external IP addresses.
• Logs: Authentication log entries showing a user account switching sessions unexpectedly, or the same account logging in from multiple distinct IP addresses in a short timeframe.
• Network: Unusual cross-origin POST requests to the Jenkins login endpoint originating from non-Jenkins domains or external referrers visible in HTTP access logs.

Jenkins has released fixed versions that validate CSRF tokens when processing login requests: Jenkins weekly 2.541 and Jenkins LTS 2.528.3. Administrators should upgrade to these versions as the primary remediation. As a temporary workaround if upgrading is not immediately possible, administrators can disable the security fix by setting the system property hudson.security.AuthenticationProcessingFilter2.skipCSRFCheck=true, though this is not recommended for production environments. Restricting access to the Jenkins login page to trusted networks can also reduce exposure (Jenkins Advisory).