CVE-2020-10199

What it actually does

Improper input validation permits unauthenticated or low-privileged attackers to supply expressions that are evaluated by the Java Expression Language engine, resulting in arbitrary code execution within the Nexus Repository process.

Why it matters to the business

Successful exploitation grants attackers full control over the repository server, enabling theft or tampering of proprietary software artifacts, supply-chain poisoning, and persistent access to internal development infrastructure; the vulnerability's presence on the CISA KEV list with an active in-the-wild exploit maturity rating means organizations face immediate operational and regulatory risk.

How an attack unfolds

An attacker locates a Nexus Repository instance and submits a specially crafted HTTP request containing a malicious Java EL expression in a parameter that is later interpreted by the server; because input is not sanitized, the expression executes, spawning a reverse shell or dropping a web shell that establishes ongoing remote code execution.

How to fix it

Upgrade every Sonatype Nexus Repository installation to version 3.21.2 or later; where immediate patching is impossible, restrict network access to administrative interfaces, enforce strong authentication, and monitor for anomalous EL evaluation behavior until the upgrade can be completed.

How to confirm the fix worked

Query the Nexus REST API or inspect the running container/image to confirm the installed version is at least 3.21.2; review change logs for the JavaEL injection fixes and retest any previously vulnerable endpoints with benign EL expressions to verify they are now rejected.

Typical attack path

1. Scan for publicly reachable Sonatype Nexus Repository instances
2. Identify endpoints accepting user-controlled input that reaches the EL evaluator
3. Craft and submit a Java EL payload that bypasses basic filters
4. Obtain a shell or execute commands to exfiltrate artifacts or install persistence
5. Pivot to connected CI/CD systems or artifact consumers

How to detect it

Search Nexus access and application logs for requests containing EL expression syntax such as ${...} or #{...} in unexpected parameters; correlate with process-creation events showing unexpected Java subprocesses or outbound connections from the Nexus process; review any post-exploitation artifacts such as newly created files in the Nexus work directory.

What your security team should do

• Immediately inventory all Sonatype Nexus instances and prioritize upgrade to 3.21.2+ given KEV listing and active exploitation
• Apply network segmentation so only trusted networks can reach Nexus administrative ports
• Enable and retain detailed application logs for at least 90 days to support forensic review
• Subscribe to vendor security advisories and automate version checks in the asset inventory