Privileged Programs in Java

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Example 127 Code from Example 121 rewritten to disable signals while executing with privileges
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int main(int argc, char** argv) { uid_t caller_uid = getuid(); uid_t owner_uid = geteuid(); int sigmask; sigset_t maskall, saved; sigfillset(&maskall); /* Drop privileges right up front, but we'll need them back in a little bit, so use effective id */ if (setresuid(-1, caller_uid, owner_uid) != 0){ exit(-1); } /* Privileges not necessary or desirable at this point */ processCommandLine(argc, argv); /* disable signal handling */ if (sigprocmask(SIG_SETMASK, &maskall, &saved) != 0) { exit(-1); } /* Regain privileges */ if (setresuid(-1, owner_uid, caller_uid) != 0) { sigprocmask(sigmask); exit(-1); } openSocket(88); /* requires root */ /* Drop privileges for good */ if (setresuid(caller_uid, caller_uid, caller_uid) != 0) { sigprocmask(sigmask); exit(-1); } /* re-enable signals */ if (sigprocmask(SIG_SETMASK, &saved, NULL) != 0) { exit(-1); } doWork(); }
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Sometimes an attack against a privileged program is carried out by a legitimate user on the system (consider shared servers at a university), but often an attacker uses a two-step process to gain privileges on a machine First, the attacker targets a weakness in a network service or a poorly protected user account and gains access to a low-privilege account on the machine Next, the attacker uses this low-privilege access to exploit a vulnerability in a privileged program and take control of the machine Privilege escalation attacks can target any variety of software vulnerability, but in this section, we discuss classes of vulnerabilities that are primarily a risk in privileged programs This section covers the following vulnerabilities: File access race conditions Weak file permissions Insecure temporary files Command injection Misuse of standard file descriptors
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Attacks against some of these vulnerabilities require the capability to execute the privileged program directly and threaten only setuid programs; others are equally dangerous for programs run by root or another privileged user [Bishop, 1987] All the vulnerabilities share the common thread that they rely on an attacker s capability to interact with a privileged program through its local environment As you consider the vulnerabilities in this section, remember that the difference in the rate of success between local attacks and network-based attacks is significant The year 2006 saw two contests in which participants raced to gain root access to a machine running Apple OS X In the first contest, would-be attackers were given local accounts on the machine, and the winner gained root privileges in less than 30 minutes [Kotadia, 2006] In the second, attackers were not given accounts and only SSH and HTTP traffic was permitted to the machine No one had root access when the contest ended after 36 hours [Evers, 2006]
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File Access Race Conditions File access race conditions fall under a larger group of vulnerabilities known as time-of-check, time-of-use (TOCTOU) race conditions TOCTOU vulnerabilities are a classic time and state problem that occurs when a program performs a check to verify some property and later bases a decision on the assumption that the property still holds true On the filesystem, TOCTOU vulnerabilities are possible because a single filename can refer to different files at different points in time [Bishop, 1997] Attacks on filesystem TOCTOU vulnerabilities generally follow this sequence: 1 A program checks some property of a file, referencing the file by its filename rather than through a handle to the underlying file system object 2 An attacker alters the meaning of the filename the program checked so that it refers to different filesystem object 3 The program later performs a filesystem operation using the same filename and assumes that the previously checked property still holds The code in Example 128 contains a TOCTOU vulnerability discovered in the lpr printing utility installed with Versions 6 and earlier of Red Hat Linux [Twillman, 1999] The lpr utility is installed setuid root to enable users to communicate with printers on the system The program accepts filenames on its command line, attempts to open the corresponding files, and prints their contents Because it runs as root and, therefore, can read any file on the filesystem, lpr must check to make sure that the user has permission to print the requested file It performs the permission check by calling access(), which checks to see if the user has permission to access the file But because both access() and open() take a string parameter instead of a file handle, the attacker has a window of opportunity to change the file2 If an attacker changes the filename from referring to a file that passes the check to a file that the attacker cannot access between the call to access() and the call to open(), the program will use its root privileges to open and print the contents of the otherwise inaccessible file Figure 124 shows the way the operations in lpr could interleave with the execution of an attacker s code in a successful attack An attacker invokes lpr with the argument /tmp/attack and redirects the file to point to the system file /etc/shadow between the time it is checked and when it is used
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2 All programs that rely on access() for access control have this same problem Don't use access()
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