CVE-2026-57305: Jenkins Assembla Plugin CSRF Vulnerability

CVE-2026-57305 Overview

CVE-2026-57305 is a cross-site request forgery (CSRF) vulnerability affecting the Jenkins Assembla Plugin version 1.4 and earlier. The flaw allows attackers to coerce authenticated Jenkins users into initiating connections to an attacker-specified URL using attacker-supplied credentials. The Jenkins project disclosed the issue in Jenkins Security Advisory 2026-06-24. The vulnerability is tracked under [CWE-352] and carries a CVSS v3.1 score of 5.4.

Critical Impact
Attackers can abuse the Assembla Plugin to send credentials to attacker-controlled endpoints, enabling credential capture and unauthorized connection initiation from the Jenkins controller.

Affected Products

Jenkins Assembla Plugin 1.4
Jenkins Assembla Plugin versions earlier than 1.4
Jenkins controllers with the Assembla Plugin installed

Discovery Timeline

2026-06-24 - Jenkins publishes security advisory SECURITY-3692
2026-06-24 - CVE-2026-57305 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-57305

Vulnerability Analysis

The Jenkins Assembla Plugin exposes an HTTP endpoint that accepts a URL, username, and password as parameters and initiates an outbound connection on behalf of the requester. The endpoint does not require POST requests or validate a CSRF crumb. An attacker who can trick a Jenkins user with Overall/Read permission into visiting a malicious page can force that user's browser to issue a state-changing request to the plugin endpoint.

The consequences include the Jenkins controller connecting to an attacker-specified host and transmitting attacker-supplied credentials. When valid credentials are submitted by the victim, the attacker captures whether those credentials are accepted by an arbitrary endpoint, enabling credential validation abuse and reconnaissance of internal services.

Root Cause

The vulnerability stems from missing CSRF protection on a form submission handler within the Assembla Plugin. The plugin allows the connection-test functionality to be invoked via GET requests without requiring a request-scoped crumb token. This violates Jenkins' standard CSRF defense model, which mandates POST requests and crumb validation for any action with side effects.

Attack Vector

Exploitation requires the attacker to lure an authenticated Jenkins user to a crafted web page or link. The victim's browser issues a request to the vulnerable plugin endpoint on the Jenkins controller, carrying the victim's session cookies. The Jenkins controller then connects to the attacker-specified URL using the attacker-supplied username and password. No prior knowledge of internal infrastructure is required, though the attacker must know or guess the Jenkins controller URL.

The vulnerability mechanism is documented in the Jenkins Security Advisory 2026-06-24. No public proof-of-concept code is currently available.

Detection Methods for CVE-2026-57305

Indicators of Compromise

Outbound HTTP or HTTPS connections from the Jenkins controller to unexpected external hosts originating from the Assembla Plugin component.
Jenkins access logs showing GET requests to Assembla Plugin endpoints containing url, username, and password query parameters.
Authentication failures or successes on internal services correlated with Jenkins controller source IPs.

Detection Strategies

Review the installed plugin inventory on all Jenkins controllers and flag instances of assembla at version 1.4 or earlier.
Correlate Jenkins HTTP access logs with egress firewall logs to identify connection attempts driven by the Assembla Plugin endpoint.
Hunt for referrer headers in Jenkins access logs that originate from external or untrusted domains targeting plugin URLs.

Monitoring Recommendations

Enable verbose access logging on the Jenkins web tier and forward logs to a centralized analytics platform for retention and querying.
Alert on outbound connections from the Jenkins controller to hosts outside an approved allowlist.
Monitor Jenkins audit logs for credential-related plugin actions performed without an associated user-initiated job execution.

How to Mitigate CVE-2026-57305

Immediate Actions Required

Identify all Jenkins controllers running the Assembla Plugin and inventory the installed version.
Restrict network egress from the Jenkins controller to only required destinations until a patched plugin version is deployed.
Confirm that Jenkins' built-in CSRF protection (Prevent Cross Site Request Forgery exploits) is enabled at the global security level.

Patch Information

Review the Jenkins Security Advisory 2026-06-24 for the latest remediation status. At the time of advisory publication, administrators should consult the advisory for the fixed plugin version and upgrade through the Jenkins Plugin Manager. If no fixed version is available, treat the plugin as unpatched and apply the workarounds below.

Workarounds

Disable or uninstall the Assembla Plugin until a fixed version is installed.
Limit Overall/Read permission to trusted users to reduce the population of potential CSRF victims.
Place the Jenkins controller behind authenticated proxies or VPN access to reduce drive-by CSRF exposure from the public internet.

bash

# List Jenkins plugins and identify Assembla version via Jenkins CLI
java -jar jenkins-cli.jar -s https://jenkins.example.com/ \
  -auth admin:API_TOKEN list-plugins | grep -i assembla

# Disable the plugin pending remediation
java -jar jenkins-cli.jar -s https://jenkins.example.com/ \
  -auth admin:API_TOKEN disable-plugin assembla -restart