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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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|---|---|---|---|---|
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enum { array_max = 100 };
/*
* Program running with elevated privileges where argv[1]
* and argv[2] are supplied by the user
*/
char x[array_max];
FILE *fp = fopen(argv[1], "w");
strncpy(x, argv[2], array_max);
x[array_max - 1] = '\0';
/*
* Write operation to an unintended file like /etc/passwd
* gets executed
*/
if (fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), fp) <
sizeof(x)/sizeof(x[0])) {
/* Handle Error */
}
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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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/*
* Make sure that the chroot/jail directory exists within
* the current working directory. Also assign appropriate
* permissions to the directory to restrict access. Close
* all file system descriptors to outside resources lest
* they escape the jail.
*/
if (setuid(0) == -1) {
/* Handle error */
}
if (chroot("chroot/jail") == -1) {
/* Handle error */
}
if (chdir("/") == -1) {
/* Handle error */
}
/* Drop privileges permanently */
if (setgid(getgid()) == -1) {
/* Handle error */
}
if (setuid(getuid()) == -1) {
/* Handle error */
}
/* Perform unprivileged operations */
enum {array_max = 100};
FILE *fp = fopen(argv[1], "w");
char x[array_max];
strncpy(x, argv[2], array_max);
x[array_max - 1] = '\0';
/* Write operation safe is safe within jail */
if (fwrite(x, sizeof(x[0]), sizeof(x)/sizeof(x[0]), fp) <
sizeof(x)/sizeof(x[0])) {
/* Handle error */
}
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