So, shell injection is a serious vulnerability, but how do we prevent it? Unfortunately, there’s no foolproof way to ensure it doesn’t happen. Defense-in-depth is especially important here because the stakes are high if an attacker finds and exploits a vulnerability.

Our first layer should be one of an inherently safer design. We can avoid executing shell commands in our code and invoke other programs using safer methods. We can also refrain from using dependencies that are closed-source or have a poor security reputation.

If we must execute shell commands, we can sanitize the inputs we give to them, blocking all characters except those that are strictly required, taking special care not to allow any characters that have special meaning in the system shell-like ;, , , |, or &, among others.

We can also take steps to limit the damage an attacker can cause if they exploit a shell injection vulnerability. Similarly to how we only allowed characters that were strictly required, we can restrict the system accounts used by our backend code and webserver to only those that are strictly required, preventing them from running any noncritical commands or accessing any noncritical files. We can also use sandboxing or virtualization to add another layer that an attacker must bypass.

Finally, we can use tools like endpoint security software and intrusion detection systems to monitor for signs that an attacker has managed to breach our systems. If we detect the attack early and slow the attacker down with multiple layers of additional security, we can prevent them from doing too much damage.

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