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The exec() method of the java.lang.Runtime class and the related ProcessBuilder.start() method can be used to invoke external programs. While running, these programs are represented by a java.lang.Process object. This process contains an input stream, output stream, and error stream. Because the Process object allows a Java program to communicate with its external program, the process's input stream is an OutputStream object, accessible by the Process.getOutputStream() method. Likewise, the process's output stream and error streams are both represented by InputStream objects, accessible by the Process.getInputStream() and Process.getErrorStream() methods.

These processes may require input to be sent to their input stream, and they may also produce output on their output stream, their error stream, or both. Incorrect handling of such external programs can cause unexpected exceptions, denial of service (DoS), and other security problems.

A process that tries to read input on an empty input stream will block until input is supplied. Consequently, input must be supplied when invoking such a process.

Output from an external process can exhaust the available buffer reserved for its output or error stream. When this occurs, the Java program can block the external process as well, preventing any forward progress for both the Java program and the external process. Note that many platforms limit the buffer size available for output streams. Consequently, when invoking an external process, if the process sends any data to its output stream, the output stream must be emptied. Similarly, if the process sends any data to its error stream, the error stream must also be emptiedThis recommendation discusses several issues resulting from the improper use of the exec() method. Similarly the is also prone to misuse.

Noncompliant Code Example (exitValue())

This noncompliant code example invokes notemaker, a hypothetical cross-platform notepad application using the external command notemaker. The notemaker application does not read its input stream but sends output to both its output stream and error stream.

This noncompliant code example invokes notemaker , using the exec() method, which returns an object of a subclass of the abstract class java.lang.Process Process object. The exitValue() method returns the exit value for processes that have terminated; , but it throws an IllegalThreadStateException when invoked on an active process. Because this noncompliant example program fails to wait for the notemaker process to terminate, the call to exitValue() is likely to throw an {IllegalThreadStateException}}.

public class Exec {
  public static void main(String args[]) throws IOException {
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("notemaker");
    int exitVal = proc.exitValue();
  }
}

Noncompliant Code Example (waitFor())

In this noncompliant code example, the waitFor() method blocks the calling thread until the invoked notemaker process terminates. This approach prevents the IllegalThreadStateException seen in from the previous example. However, the example program may experience an arbitrary delay before termination. First, the invoked notemaker process could legitimately require lengthy execution before completion. Although this possibility can present difficulties in practice, it is irrelevant for the purposes of this guideline. Second, output Output from the notemaker process can exhaust the available buffer for the standard output or standard error stream . When this occursbecause neither stream is read while waiting for the process to complete. If either buffer becomes full, it can block the notemaker process as well, preventing all forward progress for both processes. Note that many platforms limit the buffer size available for the standard output streamsthe notemaker process and the Java program.

public class Exec {
  public static void main(String args[])
                          throws IOException, InterruptedException {
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("notemaker");
    int exitVal = proc.waitFor();
  }
}

Compliant Solution (1)

Noncompliant Code Example (Process Output Stream)

This noncompliant code example properly empties the process's output stream, thereby preventing the output stream buffer from becoming full and blocking. However, it ignores the process's error stream, which can also fill and cause the process to blockAn partial solution would be to exhaust the output and the stderr streams before waiting for the invoked process. A better approach is to empty both the stderr stream and the output stream, as shown in this compliant solution. The example code, however, fails to process any arguments passed to the external program (notemaker) and exits with an OS-specific non-zero exit code. These limitations must also be addressed in production code.

public class Exec {
  public static void main(String args[])
                     throws IOException, InterruptedException {
    Runtime rt = Runtime.getRuntime();
    Process proc = rt.exec("notemaker");
    InputStream is = proc.getInputStream();
    InputStreamReader isr = new InputStreamReader(is);
    BufferedReader br = new BufferedReader(isr)int c;
    
    String line;
    while ((linec = bris.readLineread()) != null-1) {  
      System.out.println(line);  // Prints the error linesprint((char) c);
    }

    int exitVal = proc.waitFor();   
  }
}

Compliant Solution (redirectErrorStream(

...

))

This compliant solution spawns a command interpreter that executes the user supplied command. It reads the output from the external process, and write it to a separate OutputStreamredirects the process's error stream to its output stream. Consequently, the program can empty the single output stream without fear of blockage.

public class Exec extends Thread  {
  InputStreampublic is;
static void main(String type;args[])
  OutputStream os;

  Exec(InputStream is, String type) {
    this(is, type, null);
  }

  Exec(InputStream is, String type, OutputStream redirect) {
 throws IOException, InterruptedException this.is{
 = is;
  ProcessBuilder pb this.type= = typenew ProcessBuilder("notemaker");
    this.ospb = redirectpb.redirectErrorStream(true);
  }

  Process publicproc void= runpb.start() {;
    tryInputStream {
is = proc.getInputStream();
    PrintWriter pw = nullint c;
    while ((c if= is.read(os)) != null-1) {
        pw = new PrintWriter(osSystem.out.print((char) c);
      }  
     
      InputStreamReader isr = new InputStreamReader(isint exitVal = proc.waitFor();
      BufferedReader br = new BufferedReader(isr);

      String line = null; 
      while ((line = br.readLine()) != null}
}

Compliant Solution (Process Output Stream and Error Stream)

This compliant solution spawns two threads to consume the process's output stream and error stream. Consequently, the process cannot block indefinitely on those streams.

When the output and error streams are handled separately, they must be emptied independently. Failure to do so can cause the program to block indefinitely.

class StreamGobbler implements Runnable {
  private final InputStream is;
  private final PrintStream os;

  StreamGobbler(InputStream is, PrintStream os) {
        if (pw != null) {this.is = is;
    this.os      pw.println(line)= os;
  }

  public      pw.flushvoid run(); {
    try    }{
 
        System.out.println(type + ">" + line)int c;
      }
 
      if (pwwhile ((c = is.read()) != null-1)
  {
        pwos.flushprint((char) c);
      }
    } catch (IOException ioex) {
 /* Forward to handler */     // Handle error
    }
  }
}
	
public class ExecMeExec {
  public static void main(String[] args) {
    // ... perform command argument check  ...
	
    try {
      FileOutputStream fos = new FileOutputStream("c:\\output.txt");throws IOException, InterruptedException {

      Runtime rt = Runtime.getRuntime();
      Process proc = rt.exec("notemaker");

      // Any error message?
    Thread errorGobbler
 Exec    errorGobbler = new ExecThread(new StreamGobbler(proc.getErrorStream(), "ERROR"System.err));
	 
      // Any output?
    Thread outputGobbler
 Exec    outputGobbler = new Thread(new ExecStreamGobbler(proc.getInputStream(), "OUTPUT", fosSystem.out));
	
      errorGobbler.start();
      outputGobbler.start();
	
      // Any error?
      int exitVal = proc.waitFor();
      errorGobbler.join();     // Handle condition where the
      outputGobbler.join();    // process ends before the threads finish 

      fos.flush();
      fos.close();
    } catch (Throwable t) { /* forward to handler */ }
  }
}

When the output and error streams are handled separately, they must be drained concurrently. Failure to do so can cause the program to block indefinitely.

Compliant Solution (3) (Windows)

This compliant solution (based on the Sun forums query Runtime.exec hangs even If I drain output), uses a ProcessBuilder to simplify the handling mechanism by merging the error and output streams. The readToPrompt() method reads the output of the invoked process; details of its implementation necessarily depend on the exact formatting of the command prompt used by the platform's command interpreter.

public class Cmd {
  public static void main(String[] args) throws IOException {
    ProcessBuilder pb = new ProcessBuilder("cmd");
    pb = pb.redirectErrorStream(true);
    Process p = pb.start();
    InputStream is = p.getInputStream();
    OutputStream os = p.getOutputStream();

    PrintWriter pw = new PrintWriter(os, true);
    readToPrompt(is);
    
    pw.println("dir");
    readToPrompt(is);    
  }
 
  private static void readToPrompt(InputStream is) throws IOException {
    String s = "";
    for (;;) {
      int i = is.read();
      
      if (i < 0) {
        System.out.println();
        System.out.println("EOF");
        System.exit(0);
      }
 
      char c = (char)i; // Safe
      s += c;
  
      if (s.endsWith("\r\n") {
        System.out.print(s);
        s = "";
      }
      
      // Detects prompt, to break out
      if (c == '>' && s.length() > 2 && s.charAt(1) == ':') {
        System.out.print(s);
        break;
      }
    }
  }
}

Risk Assessment

Misuse of the exec() method can result in runtime exceptions and in denial of service vulnerabilities.

Exceptions

FIO07-J-EX0: Failure to supply input to a process that never reads input from its input stream is harmless and can be beneficial. Failure to empty the output or error streams of a process that never sends output to its output or error streams is similarly harmless or even beneficial. Consequently, programs are permitted to ignore the input, output, or error streams of processes that are guaranteed not to use those streams.

Risk Assessment

Failure to properly manage the I/O streams of external processes can result in runtime exceptions and in DoS vulnerabilities.

Automated Detection

Related Vulnerabilities

GROOVY-3275

Bibliography

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Image Added Image Added Image Added[API 06|AA. Bibliography#API 06]\] method [exec()|http://java.sun.com/javase/6/docs/api/java/lang/Runtime.html#exec(java.lang.String)] \[[Daconta 00|AA. Bibliography#Daconta 00]\] \[[Daconta 03|AA. Bibliography#Daconta 03]\] Pitfall 1FIO09-J. Exclude user input from format strings      12. Input Output (FIO)      FIO11-J. Do not attempt to read raw binary data as character data