CVE-2026-46621: Yamcs Vulnerable to Authenticated Remote Code Execution (RCE) via Jython Algorithm Code Injection

Should you act?

Critical, preconditions apply

Severe if exploited, but exploitation needs conditions that may not apply to you. This signal is computed from CISA KEV, EPSS and CVSS data; it is not an editorial verdict.

• Not in CISA KEV, but CISA’s SSVC assessment records a public proof-of-concept, so attack code exists.
• EPSS 1.1%, percentile 66: FIRST’s model estimates a 1.1% chance of exploitation activity in the next 30 days, higher than 66% of all scored CVEs (moderate). EPSS is a probability, not a verdict.
• Needs an account (high (admin)). Exposure depends on who can log in.
• Only applies if your dependency tree includes org.yamcs:yamcs-core (Maven) < 5.12.7, fixed in 5.12.7. Check your lockfile, including transitive dependencies.

What it is

Yamcs is a mission control framework. Prior to 5.12.7, the Yamcs script evaluation engine for Python algorithms dynamically compiled and evaluated user-controlled algorithm text using Jython through the JSR-223 ScriptEngine API without enforcing a secure sandbox, so an authenticated user with the ChangeMissionDatabase privilege could override an existing Python algorithm's logic through the mission database REST API and import and execute arbitrary Java classes such as java.lang.Runtime to achieve remote code execution on the underlying host operating system. This issue is fixed in versions 5.12.7 and 5.13.0, which disable algorithm editing by default.

From the GitHub advisory

Summary A Server-Side Code Injection vulnerability exists in the Yamcs script evaluation engine for Python algorithms. The application dynamically compiles and evaluates user-controlled algorithm text using Jython (via the JSR-223 ScriptEngine API) without enforcing a secure sandbox. An authenticated user with the ChangeMissionDatabase privilege can exploit this by overriding the algorithm logic through the REST API, achieving Remote Code Execution (RCE) on the underlying host operating system.

Details The vulnerability lies in how Yamcs handles dynamic script evaluation. When a user updates an algorithm via the MDB (Mission Database) API (/api/mdb/{instance}/realtime/algorithms/{name}), the AlgorithmManager uses the ScriptAlgorithmExecutorFactory to instantiate a JSR-223 ScriptEngine (in this case, Jython/Python).

Because Jython allows seamless interoperability with native Java classes, an attacker can import and execute arbitrary Java classes such as java.lang.Runtime. Any valid Python algorithm can be overwritten with a malicious payload that executes OS-level commands.

PoC

Prerequisites: 1. A running Yamcs instance with the Jython engine available in its classpath (e.g., jython-standalone dependency included). 2. An active authentication token for a user with the SystemPrivilege.ChangeMissionDatabase privilege. 3. An existing algorithm defined in the Mission Database (MDB) with its language explicitly set to python (e.g., a custom poc algorithm). Note: Yamcs prevents changing the underlying language engine of an algorithm via the API, so an existing Python algorithm must be targeted.

Weakness type: CWE-94.

Product: org.yamcs:yamcs-core (Maven) · Assigned by: GitHub_M · Published: 2026-07-16

Affected and fixed versions

Severity scores

• Attack vector: network (remotely reachable)
• Attack complexity: low
• Privileges required: high (admin)
• User interaction: none
• Scope: changed (can affect other components)

CVSS measures how bad a flaw is if exploited, not how likely exploitation is. Read it next to the exploitation data below.

Exploitation and urgency

• CISA KEV: not listed (no confirmed in-the-wild exploitation known to CISA).
• CISA SSVC assessment: exploitation poc, automatable no, technical impact total.
• FIRST EPSS (model of 2026-10-21): 1.12% probability of exploitation activity in the next 30 days; percentile 66.

Timeline

• 2026-05-27 — GitHub-reviewed advisory GHSA-2g95-6x5q-xjwj published (critical)
• 2026-07-16 — CVE record last updated
• 2026-07-16 — CVE record published by GitHub_M

Sources and references

Original sources first. Version and fix details should always be confirmed against the vendor or project advisory.

Vulnerability records

1. GitHub Advisory GHSA-2g95-6x5q-xjwj github.com
2. CVE.org record CVE-2026-46621 cve.org
3. NVD entry CVE-2026-46621 nvd.nist.gov

Other references from the CVE record and advisory

1. github.com github.com
2. github.com github.com
3. github.com github.com
4. github.com github.com
5. github.com github.com

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