Reading through a vulnerability report about ClamAV, I came across a phrase that filled me with dread: “The file name is not sanitized”. It’s a feature, VirusEvent, that can be enabled in the ClamnAV config. And that configuration includes a string formatting function, where the string includes %v and %s, which gets replaced with a detected virus name and the file name from the email. And now you see the problem, I hope: The filename is attacker supplied input.
Where this really gets out of hand is what ClamAV does with this string. execle("/bin/sh", "sh", "-c", buffer_cmd, NULL, env). So let’s talk defensive program design for a minute. When it comes to running a secondary command, there are two general options, system() and the exec*() family of system calls. system() is very simple to use. It pauses execution of the main process and asks the operating system to run a string, just as if the user had typed that command into the shell. While this is very convenient to use, there is a security problem if any of that command string is user-supplied. All it takes is a semicolon or ampersand to break assumptions and inject a command.
To the rescue comes exec(). It’s a bit more complicated to use, requiring the programmer to manually call fork() and wait(). But it’s not running the command via the shell. exec() executes a program directly, totally eliminating the potential for command injection! Except… oops.
Yeah, exec() and related calls don’t offer any security protections when you use them to execute /bin/sh. I suspect the code was written this way to allow running a script without specifyi ..
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