Creating a jail isolates a program from the rest of the file system. The idea is to create a sandbox, so entities that the program does not need to access under normal operation are made inaccessible. This makes it much harder to abuse any vulnerability that can otherwise lead to unconstrained system compromise and , consequently , functions as a defense-in-depth strategy. A jail may consist of world-viewable programs that require fewer resources to execute than those that exist on that system. Jails are useful only when there is no way to elevate privileges in the event of program failure.
Additionally, care must be taken to ensure that all the required resources (such as libraries, files, and so on) are replicated within the jail directory, and no reference is made to other parts of the file system from within this directory. It is also advisable to administer restrictive read/write permissions on the jail directories and resources based on the program's privilege requirements. Although creating jails is an effective security measure when used correctly, it is not a surrogate for compliance with the other rules and recommendations in this standard.
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A security flaw exists in this noncompliant code example resulting from the absence of proper canonicalization measures on the file path. This allows an attacker to traverse the file system and possibly write to a file of the attacker's choice, with the privileges of the vulnerable program. For example, it may be possible to overwrite the password file (such as the /etc/passwd, common to many POSIX-based systems) or a device file, such as the mouse, which in turn can aid further exploitation or cause a denial of service to occur.
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An attacker can control the value of argv[1] and , consequently , access any resource on the file system.
This noncompliant code example also violates recommendations FIO02-C. Canonicalize path names originating from untrusted sources and FIO03-C. Do not make assumptions about fopen() and file creation.
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Another essential step is to drop superuser privileges permanently after these calls. (See recommendation POS02-C. Follow the principle of least privilege.) The chroot() system call is not secure against the superuser changing the current root directory (if privileges are not dropped). Successful jail creation prevents unintentional file system access even if an attacker gives malicious input, such as through command-line arguments.
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An alternative sequence is to call chdir("chroot/jail") first and then chroot("."). However, calling chdir("/some/path"), then chroot("/some/path"), should be avoided because this sequence may be susceptible to a race condition: an attacker with sufficient privileges can arrange for /some/path to refer to different directories in the two system calls. Consequently, the program will not have its current working directory set to the new root directory. Using either chdir("/") after chroot(), or chroot(".") after chdir(), guarantees that the current working directory will be the same directory as the new root.
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[Wheeler 2003] Section 7.4, "Minimize Privilegesprivileges"
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