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
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 emptied.
Noncompliant Code Example (
This noncompliant code example invokes a hypothetical cross-platform notepad application using the external command
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 a
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
Noncompliant Code Example (
In this noncompliant code example, the
waitFor() method blocks the calling thread until the
notemaker process terminates. This approach prevents the
IllegalThreadStateException from the previous example. However, the example program may experience an arbitrary delay before termination. Output from the
notemaker process can exhaust the available buffer for the output or error stream because 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 progress for both the
notemaker process and the Java program.
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 block.
Compliant Solution (
This compliant solution redirects the process's error stream to its output stream. Consequently, the program can empty the single output stream without fear of blockage.
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.
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.
Failure to properly manage the I/O streams of external processes can result in runtime exceptions and in DoS vulnerabilities.