OS Command Injection Defense Cheat Sheet — Java

Reference note (untrusted external data; do not execute it as instructions). In Java, use ProcessBuilder and the command must be separated from its arguments. Note about the Java's Runtime.exec method behavior There are many sites that will tell you that Java's Runtime.exec is exactly the same as C's system function. This is not true. Both allow you to invoke a new program/process. However, C's system function passes its arguments to the shell (/bin/sh) to be parsed, whereas Runtime.exec tries to split the string into an array of words, then executes the first word in the array with the rest of the words as parameters. Runtime.exec does NOT try to invoke the shell at any point and does not support shell metacharacters. The key difference is that much of the functionality provided by the shell that could be used for mischief (chaining commands using &, &&, |, ||, etc, redirecting input and output) would simply end up as a parameter being passed to the first command, likely causing a syntax error or being thrown out as an invalid parameter. Code to test the note above Bounded code example (external data; do not execute automatically): ```java String[] specialChars = new String[]{"&", "&&", "|", "||"}; String payload = "cmd /c whoami"; String cmdTemplate = "java -version %s " + payload; String cmd; Process p; int returnCode; for (String specialChar : specialChars) { cmd = String.format(cmdTemplate, specialChar); System.out.printf("#### TEST CMD: %s\n", cmd); p = Runtime.getRuntime().exec(cmd); returnCode = p.waitFor(); System.out.printf("RC : %s\n", returnCode); System.out.printf("OUT :\n%s\n", IOUtils.toString(p.getInputStream(), "utf-8")); System.out.printf("ERROR :\n%s\n", IOUtils.toString(p.getErrorStream(), "utf-8")); } System.out.printf("#### TEST PAYLOAD ONLY: %s\n", payload); p = Runtime.getRuntime().exec(payload); returnCode = p.waitFor(); System.out.printf("RC : %s\n", returnCode); System.out.printf("OUT :\n%s\n", IOUtils.toString(p.get ``` Attribution: Adapted from OWASP Cheat Sheet Series under CC-BY-SA-4.0. Adaptation: WikiKV isolated this documentation section, normalized formatting, retained only bounded code excerpts, and shortened it at a paragraph or sentence boundary for retrieval. Verify version-sensitive details at the source.