CVE-2026-57303: Jenkins Assembla Plugin XXE Vulnerability – HIGH Severity

Technical summary

CVE-2026-57303 is an XML External Entity (XXE) injection vulnerability in Jenkins Assembla Plugin version 1.4 and earlier. The plugin fails to disable external entity resolution in its XML parser, allowing an attacker who controls or intercepts Assembla server responses to craft payloads that trigger arbitrary file reads or server-side request forgery (SSRF) attacks. The vulnerability is classified under CWE-918 (Server-Side Request Forgery), though the XXE mechanism is the root cause. Exploitation requires network access to Jenkins (PR:L in the CVSS vector indicates authenticated access is needed), but once inside, an attacker can exfiltrate secrets stored on the Jenkins controller or pivot to internal network resources.

Business impact

Compromise of a Jenkins instance via this vulnerability could expose stored credentials, API tokens, and build secrets that Jenkins uses to authenticate to downstream systems—potentially creating a pivotal stepping stone into your CI/CD pipeline and connected infrastructure. An attacker could also use SSRF to probe or attack internal services that the Jenkins controller can reach. The blast radius depends on the scope of secrets stored in Jenkins and what systems it is authorized to access.

Affected systems

Jenkins Assembla Plugin versions 1.4 and earlier are vulnerable. This plugin is used to integrate Jenkins with Assembla project management and version control services. Any Jenkins instance with this plugin installed and configured to communicate with an Assembla server is at risk.

Exploitability

Exploitation requires that an attacker can control or intercept responses from the Assembla server configured in the Jenkins plugin. This could occur if the Assembla account is compromised, if the network path between Jenkins and Assembla is compromised, or if the attacker operates the Assembla infrastructure. Once in a position to inject malicious XML, the attack is straightforward to execute. The CVSS score of 7.1 (HIGH) reflects high confidentiality impact (C:H), low integrity impact (I:L), and no availability impact (A:N), with the requirement for authenticated access to Jenkins and network access to the Assembla endpoint.

Remediation

Upgrade the Jenkins Assembla Plugin to a version newer than 1.4 that addresses XXE by disabling external entity resolution in the XML parser. Consult the Jenkins plugin repository or the plugin vendor's advisory to identify the patched version. Until patched, restrict network access to the configured Assembla server to trusted sources only, and ensure the Assembla account credentials are strongly protected and monitored.

Patch guidance

Upgrade Jenkins Assembla Plugin to the latest version available in the Jenkins plugin manager. Verify the patched version against the official Jenkins Assembla Plugin release notes or security advisory to confirm XXE mitigation is included. After upgrading, restart Jenkins and validate that the plugin still functions correctly with your Assembla configuration. No configuration changes are typically required post-patch.

Detection guidance

Monitor Jenkins logs and network traffic for unusual XML parsing errors or HTTP requests from the Jenkins controller to unexpected internal addresses. Intrusion detection systems should flag XXE payloads in network traffic (look for DOCTYPE declarations, ENTITY definitions, or file:// URIs in intercepted requests). Review Jenkins controller logs for file access patterns that coincide with Assembla plugin operations. Monitor outbound SSRF attempts to internal IP ranges or private services.

Why prioritize this

This vulnerability merits prompt patching because it allows authenticated attackers to extract secrets from the Jenkins controller itself—the central trust anchor of your CI/CD pipeline. Compromise of Jenkins can cascade into compromise of every system it deploys to or authenticates against. The HIGH severity reflects this, and the attack surface is non-trivial (requires Assembla server compromise or network interception), making it a targeted but high-impact risk.

Risk score, explained

CVSS 7.1 (HIGH) is driven by high confidentiality impact (C:H), reflecting the ability to exfiltrate secrets and configuration from the Jenkins controller. The integrity impact is low (I:L) because SSRF does not directly corrupt local files but could modify or delete resources on internal services. Availability is unaffected (A:N). The attack vector is network (AV:N) and complexity is low (AC:L), but exploitation requires authentication (PR:L) and affects only the confidentiality scope of the Jenkins controller (S:U). This scores as HIGH and justifies elevated priority in patch scheduling.

Frequently asked questions