The purpose of creating a jail, the "virtual root directory" created with chroot-type functions, is to limit access to the file system by isolating
the process inside this jail. However, many chroot function implementations don’t modify the current working directory, thus the process has still
access to unauthorized resources outside of the "jail".
Ask Yourself Whether
- The application changes the working directory before or after running chroot.
- The application uses a path inside the jail directory as working directory.
There is a risk if you answered no to any of those questions.
Recommended Secure Coding Practices
Change the current working directory to the root directory after switching to a jail directory.
Sensitive Code Example
The current directory is not changed with the chdir function before or after the creation of a jail with the chroot
function:
const char* root_dir = "/jail/";
chroot(root_dir); // Sensitive: no chdir before or after chroot, and missing check of return value
The chroot or chdir operations could fail and the process still have access to unauthorized resources. The return code
should be checked:
const char* root_dir = "/jail/";
chroot(root_dir); // Sensitive: missing check of the return value
const char* any_dir = "/any/";
chdir(any_dir); // Sensitive: missing check of the return value
Compliant Solution
To correctly isolate the application into a jail, change the current directory with chdir before the chroot and check the
return code of both functions:
const char* root_dir = "/jail/";
if (chdir(root_dir) == -1) {
exit(-1);
}
if (chroot(root_dir) == -1) { // compliant: the current dir is changed to the jail and the results of both functions are checked
exit(-1);
}
See
