CVE-2026-57303: Jenkins Assembla XXE Vulnerability

CVE-2026-57303 Overview

CVE-2026-57303 is an XML External Entity (XXE) vulnerability affecting Jenkins Assembla Plugin version 1.4 and earlier. The plugin does not configure its XML parser to disable external entity processing. Attackers who can control responses from the configured Assembla server can exploit this flaw to extract secrets from the Jenkins controller or perform server-side request forgery (SSRF) attacks. The vulnerability is tracked under [CWE-918] and requires low-privileged authenticated access to trigger. Successful exploitation impacts the confidentiality of the Jenkins controller and enables internal network reconnaissance.

Critical Impact
Attackers controlling Assembla server responses can read sensitive files from the Jenkins controller and pivot through internal networks via SSRF.

Affected Products

Jenkins Assembla Plugin 1.4
Jenkins Assembla Plugin versions earlier than 1.4
Jenkins controllers using the jenkins:assembla component

Discovery Timeline

2026-06-24 - Jenkins publishes security advisory SECURITY-3692
2026-06-24 - CVE-2026-57303 published to NVD
2026-06-25 - Last updated in NVD database

Technical Details for CVE-2026-57303

Vulnerability Analysis

The Jenkins Assembla Plugin parses XML responses received from the configured Assembla server. The XML parser used by the plugin is not configured to disable external entity resolution or document type definition (DTD) processing. An attacker who controls or man-in-the-middles the Assembla server response can inject crafted XML payloads containing external entity declarations.

The Jenkins controller then resolves these entities during parsing. This resolution can read local files accessible to the Jenkins process, including credential stores, configuration files, and secrets stored on disk. The same mechanism enables SSRF by forcing the controller to issue HTTP requests to attacker-specified internal endpoints. The vulnerability requires low-privileged authenticated access to configure the Assembla integration but does not require user interaction once configured.

Root Cause

The root cause is insecure default configuration of the underlying XML parser. Java XML parsers process external entities and DTDs unless explicitly disabled through features such as FEATURE_SECURE_PROCESSING or disallow-doctype-decl. The Assembla Plugin instantiates its parser without applying these hardening settings, leaving entity resolution enabled.

Attack Vector

An attacker first needs the ability to influence responses returned to the Jenkins controller from the configured Assembla endpoint. This can be achieved by compromising the Assembla server, performing a network-layer man-in-the-middle attack, or by an authenticated Jenkins user pointing the plugin at an attacker-controlled host. The crafted XML response includes external entity references pointing to local files such as /etc/passwd, Jenkins secrets under $JENKINS_HOME/secrets/, or internal URLs. The plugin parses the response, resolves the entities, and either exfiltrates file contents in subsequent processing or completes SSRF requests against internal infrastructure.

No verified public exploit code is available for this vulnerability. Refer to the Jenkins Security Advisory 2026-06-24 for technical details.

Detection Methods for CVE-2026-57303

Indicators of Compromise

Outbound HTTP requests from the Jenkins controller to unexpected internal IP ranges or cloud metadata endpoints such as 169.254.169.254.
Jenkins audit logs showing reconfiguration of the Assembla server URL to non-standard hosts.
DNS lookups from the Jenkins controller for domains associated with out-of-band exfiltration services.
Anomalous file read activity by the Jenkins Java process targeting $JENKINS_HOME/secrets/ or /etc/ paths.

Detection Strategies

Inspect HTTP traffic between Jenkins controllers and Assembla endpoints for XML responses containing <!ENTITY or SYSTEM declarations.
Audit installed plugin versions across all Jenkins controllers and flag any instance of assembla at version 1.4 or earlier.
Correlate plugin configuration changes with subsequent outbound network activity from the Jenkins controller.

Monitoring Recommendations

Enable verbose logging on the Jenkins controller and forward logs to a centralized SIEM for plugin configuration changes and parser errors.
Monitor egress traffic from Jenkins controllers and alert on connections to internal management interfaces or cloud metadata services.
Track file access patterns of the Jenkins process and alert on reads of secret material outside expected job execution windows.

How to Mitigate CVE-2026-57303

Immediate Actions Required

Identify all Jenkins controllers with the Assembla Plugin installed and inventory their versions.
Disable the Assembla Plugin on controllers where Assembla integration is not actively used.
Restrict permissions so only trusted administrators can modify the Assembla server URL configuration.
Rotate credentials and secrets stored on Jenkins controllers if exposure is suspected.

Patch Information

At the time of publication, the Jenkins Security Advisory 2026-06-24 lists no fixed version of the Assembla Plugin. Administrators should monitor the advisory for updates and apply a patched release as soon as it becomes available.

Workarounds

Uninstall the Assembla Plugin from Jenkins controllers that do not require Assembla integration.
Ensure the configured Assembla server URL points only to trusted, TLS-protected endpoints under organizational control.
Apply network segmentation so the Jenkins controller cannot reach sensitive internal endpoints or cloud metadata services.
Limit plugin configuration permissions through Jenkins role-based access control to reduce the set of users who can repoint the integration.

bash

# Inventory Assembla Plugin versions across controllers via Jenkins CLI
java -jar jenkins-cli.jar -s https://jenkins.example.com/ \
  groovy =  <<'EOF'
Jenkins.instance.pluginManager.plugins.findAll { it.shortName == 'assembla' }
  .each { println "${it.shortName} ${it.version}" }
EOF