User-influenced Shell Commands Are Still Considered Harmful

It is pretty standard advice to avoid using user-input within code that executes operating system commands. However, most of that advice tends to revolve around how dangerous it is for a user to provide the command to execute and I have not seen (good) advice on whether other parts of a command (e.g. flags, flag parameters) are safe to be user-controlled.

Command injection vulnerabilities do not necessarily require special shell directives or user-controlled commands. This form of command injection is straightforward and has had plenty written about it so I will focus on less obvious examples.

Consider the following code snippet that I'm borrowing from an OWASP page on command injection :

Runtime rntime = Runtime.getRuntime();
Process proc = rntime.exec("find" + " " + args[0]);

The page claims “it is not possible to inject additional commands” so it must be secure! However, compiling the full Java file and running java Example1 "bad -exec cat {} +" on a Linux machine modifies the command being executed. A program that originally listed file names matching the user-supplied argument is now a program that prints the contents of the user-supplied file.

What about tar? Consider the following two code snippets:

String cmd = "tar tf " + userControlledFilename;
Process proc = Runtime.getRuntime().exec(cmd);
String[] cmdArray = new String[3];
cmdArray[0] = "tar";
cmdArray[1] = "tf";
cmdArray[2] = userControlledFilename;
Process proc = Runtime.getRuntime().exec(cmdArray);

Are they safe? In short the answer is “no” because user-controlled tar flags can lead to command injection. The following example will execute echo hello (the tar version may affect results):

tar tf file.tar --checkpoint=1 --checkpoint-action=exec="echo hello"

The other interesting property of Runtime.exec is its behavior depends on how its called and how its arguments are passed. The following Java program demonstrates this (I use gtar as a GNU tar alias on OS X):

// Full file here:

//Executes a local script.
//String cmd = "gtar tf file.tar --checkpoint=1";

//In some versions, GNU tar runs the checkpoint action with bash.
//String cmd = "gtar tf file.tar --checkpoint=1 --checkpoint-action=exec={echo,test0}";

//GNU tar seperates on spaces, tabs, and newlines and `exec` seperates on spaces.
//String cmd = "gtar tf file.tar --checkpoint=1 --checkpoint-action=exec=echo\ttest0";

//Test 1: Does not execute echo command
String cmd = "gtar tf file.tar --checkpoint=1 --checkpoint-action=exec=\"echo test1\"";
Process proc1 = Runtime.getRuntime().exec(cmd);

//Test 2: Executes echo command
String[] cmdArray = new String[5];
cmdArray[0] = "gtar";
cmdArray[1] = "tf";
cmdArray[2] = "file.tar";
cmdArray[3] = "--checkpoint=1";
cmdArray[4] = "--checkpoint-action=exec=\"echo test2\"";
Process proc2 = Runtime.getRuntime().exec(cmdArray);

//Test 3: Does not execute echo command
cmdArray = new String[2];
cmdArray[0] = "gtar";
cmdArray[1] = "tf file.tar --checkpoint=1 --checkpoint-action=exec=\"echo test3\"";
Process proc3 = Runtime.getRuntime().exec(cmdArray);

This produces interesting results I would like to point out. Suppose everything after /usr/local/bin/gtar tf was user-controlled. First, I find it interesting that Test 2 does executes echo and Test 3 does not. I have some suspicions on why this is but I need to poke through OpenJDK and figure that out. Second, I have included comments on interesting behaviors of Runtime.exec's method of parsing its parameters. Depending on the context, differences in input parsing could lead to input validation bypasses (and subsequently command injection).

In Conclusion…

Runtime.exec is (still) unsafe for user-controlled input! I imagine this is not specific to Java or Runtime.exec either. Code that constructs OS commands using user-input is scary and error-prone. It should be avoided!

If there are not high-level and secure libraries to perform a task without resorting to OS-level commands I recommend coming up with a solution that avoids using user-controlled input in that command all together. Shell escaping is too fragile and should be avoided as well.

This work is licensed under a Creative Commons Attribution-ShareAlike 4.0 International License.

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