CodeQL documentation
CodeQL resources

Unsafe shell command constructed from library input

ID: js/shell-command-constructed-from-input
Kind: path-problem
Severity: error
Precision: high
Tags:
   - correctness
   - security
   - external/cwe/cwe-078
   - external/cwe/cwe-088
Query suites:
   - javascript-code-scanning.qls
   - javascript-security-extended.qls
   - javascript-security-and-quality.qls

Click to see the query in the CodeQL repository

Dynamically constructing a shell command with inputs from exported functions may inadvertently change the meaning of the shell command. Clients using the exported function may use inputs containing characters that the shell interprets in a special way, for instance quotes and spaces. This can result in the shell command misbehaving, or even allowing a malicious user to execute arbitrary commands on the system.

Recommendation

If possible, provide the dynamic arguments to the shell as an array using a safe API such as child_process.execFile to avoid interpretation by the shell.

If given arguments as a single string, avoid simply splitting the string on whitespace. Arguments may contain quoted whitespace, causing them to split into multiple arguments. Use a library like shell-quote to parse the string into an array of arguments instead.

Alternatively, if the command must be interpreted by a shell (for example because it includes I/O redirections), you can use shell-quote to escape any special characters in the input before embedding it in the command.

Example

The following example shows a dynamically constructed shell command that downloads a file from a remote URL.

var cp = require("child_process");

module.exports = function download(path, callback) {
  cp.exec("wget " + path, callback);
}

The shell command will, however, fail to work as intended if the input contains spaces or other special characters interpreted in a special way by the shell.

Even worse, a client might pass in user-controlled data, not knowing that the input is interpreted as a shell command. This could allow a malicious user to provide the input http://example.org; cat /etc/passwd in order to execute the command cat /etc/passwd.

To avoid such potentially catastrophic behaviors, provide the inputs from exported functions as an argument that does not get interpreted by a shell:

var cp = require("child_process");

module.exports = function download(path, callback) {
  cp.execFile("wget", [path], callback);
}

As another example, consider the following code which is similar to the preceding example, but pipes the output of wget into wc -l to count the number of lines in the downloaded file.

var cp = require("child_process");

module.exports = function download(path, callback) {
  cp.exec("wget " + path + " | wc -l", callback);
};

In this case, using child_process.execFile is not an option because the shell is needed to interpret the pipe operator. Instead, you can use shell-quote to escape the input before embedding it in the command:

var cp = require("child_process");

module.exports = function download(path, callback) {
  cp.exec("wget " + shellQuote.quote([path]) + " | wc -l", callback);
};

References

  • © GitHub, Inc.
  • Terms
  • Privacy