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Additionally, care must be taken to ensure that all the required resources (libraries, files, and so on) are replicated within the jail directory and that 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 on the basis of 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.
Noncompliant Code Example
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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This noncompliant code example also violates FIO02-C. Canonicalize path names originating from untrusted tainted sources and FIO03-C. Do not make assumptions about fopen() and file creation.
Compliant Solution (UNIX)
Some UNIX-based systems (such as OpenBSD) can restrict file system access by creating a chroot() jail. The chroot() jail requires care to implement securely [Wheeler 2003]. It is achieved by passing a predefined directory name as an argument to chroot(). The call to chroot() requires superuser privileges. However, this call does not leave the process inside the jail directory as might be expected. A subsequent chdir() is required to restrict access to the jail boundaries.
Another essential step is to drop superuser privileges permanently after these calls . (See see 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 first call chdir("chroot/jail") and then call 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.
Risk Assessment
Failing to follow this recommendation may lead to full-system compromise if a file system vulnerability is discovered and exploited.
Recommendation | Severity | Likelihood | Remediation Cost | Priority | Level |
|---|
POS05-C |
Medium |
Probable |
High | P4 | L3 |
Automated Detection
| Tool | Version | Checker | Description | ||||||
|---|---|---|---|---|---|---|---|---|---|
| CodeSonar |
| BADFUNC.CHROOT MISC.CHROOT.NOCHDIR | Use of chroot | ||||||
| Polyspace Bug Finder |
| CERT C: Rec. POS05-C | Checks for file manipulation after chroot() without chdir("/") (rec. fully covered) |
Related Vulnerabilities
Search for vulnerabilities resulting from the violation of this rule on the CERT website.
Related Guidelines
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Bibliography
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