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StrongSwan VPN client for Android leaks username to rouge server

From: "Alexander E. Patrakov" <patrakov () gmail com>
Date: Fri, 29 May 2015 22:48:04 +0500
Hello.
I found that, in the event of DNS spoofing, StrongSwan VPN client for Android can leak the username and the MSCHAPv2 authentication value to a rogue server if it has any valid X.509 certificate. Unless I misunderstand something about X.509 certificates and their use for confirming IKEv2 identities, and unless this is already known, this might use a CVE ID. 

The client that I am talking about is this Android application:

https://play.google.com/store/apps/details?id=org.strongswan.android
In the example below, the client was supposed to connect to vpn.xorp.ru using username "alice" and a password. The server identity is validated by a CA-issued certificate that ultimately chains to something in the default trust store. However, a hacker has spoofed the DNS (well, in the example, that's actually a deliberate misconfiguration by me) so that vpn.xorp.ru points to his server (185.48.56.74 in this example) instead. On that server, he (legitimately) has a valid certificate for vpn.armority.ru. 

The settings on the client are:

Profile Name: VPN
Gateway: vpn.xorp.ru
Type: IKEv2 EAP (Login/Password)
Login: alice
Password: <hidden>
CA Certificate: Choose automatically

And here is the log.


May 27 21:39:23 00[DMN] Starting IKE charon daemon (strongSwan 5.2.1dr1, Linux 3.4.5-CM-gb461bba, armv7l)
May 27 21:39:23 00[KNL] kernel-netlink plugin might require CAP_NET_ADMIN capability
May 27 21:39:23 00[LIB] loaded plugins: androidbridge charon android-log openssl fips-prf random nonce pubkey pkcs1 
pkcs8 pem xcbc hmac socket-default kernel-netlink eap-identity eap-mschapv2 eap-md5 eap-gtc eap-tls
May 27 21:39:23 00[LIB] unable to load 9 plugin features (9 due to unmet dependencies)
May 27 21:39:23 00[JOB] spawning 16 worker threads
May 27 21:39:23 07[IKE] initiating IKE_SA android[3] to 185.48.56.74
May 27 21:39:23 07[ENC] generating IKE_SA_INIT request 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) N(FRAG_SUP) ]
May 27 21:39:23 07[NET] sending packet: from 192.168.1.237[42224] to 185.48.56.74[500] (996 bytes)
May 27 21:39:23 11[NET] received packet: from 185.48.56.74[500] to 192.168.1.237[42224] (553 bytes)
May 27 21:39:23 11[ENC] parsed IKE_SA_INIT response 0 [ SA KE No N(NATD_S_IP) N(NATD_D_IP) CERTREQ N(FRAG_SUP) 
N(MULT_AUTH) ]
May 27 21:39:24 11[IKE] local host is behind NAT, sending keep alives
May 27 21:39:24 11[IKE] remote host is behind NAT
May 27 21:39:24 11[IKE] received cert request for "C=SE, O=AddTrust AB, OU=AddTrust External TTP Network, CN=AddTrust 
External CA Root"
May 27 21:39:24 11[IKE] received cert request for "C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO 
ECC Certification Authority"
May 27 21:39:24 11[IKE] received 3 cert requests for an unknown ca
May 27 21:39:24 11[IKE] sending cert request for "C=US, ST=UT, L=Salt Lake City, O=The USERTRUST Network, 
OU=http://www.usertrust.com, CN=UTN-USERFirst-Hardware"
May 27 21:39:24 11[IKE] sending cert request for "C=US, O=GeoTrust Inc., CN=GeoTrust Global CA"

<many more "sending cert request" messages go here>

May 27 21:39:24 11[IKE] sending cert request for "C=EE, O=AS Sertifitseerimiskeskus, CN=EE Certification Centre Root CA, 
E=pki () sk ee"
May 27 21:39:24 11[IKE] establishing CHILD_SA android
May 27 21:39:24 11[ENC] generating IKE_AUTH request 1 [ IDi N(INIT_CONTACT) CERTREQ CPRQ(ADDR ADDR6 DNS DNS6) 
N(ESP_TFC_PAD_N) SA TSi TSr N(MOBIKE_SUP) N(ADD_6_ADDR) N(ADD_6_ADDR) N(ADD_6_ADDR) N(ADD_6_ADDR) N(ADD_6_ADDR) 
N(ADD_6_ADDR) N(MULT_AUTH) N(EAP_ONLY) ]
May 27 21:39:24 11[ENC] splitting IKE message with length of 3660 bytes into 3 fragments
May 27 21:39:24 11[ENC] generating IKE_AUTH request 1 [ EF ]
May 27 21:39:24 11[ENC] generating IKE_AUTH request 1 [ EF ]
May 27 21:39:24 11[ENC] generating IKE_AUTH request 1 [ EF ]
May 27 21:39:24 11[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (1360 bytes)
May 27 21:39:24 11[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (1360 bytes)
May 27 21:39:24 11[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (1072 bytes)
May 27 21:39:24 12[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (544 bytes)
May 27 21:39:24 12[ENC] parsed IKE_AUTH response 1 [ EF ]
May 27 21:39:24 12[ENC] received fragment #1 of 5, waiting for complete IKE message
May 27 21:39:24 13[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (544 bytes)
May 27 21:39:24 13[ENC] parsed IKE_AUTH response 1 [ EF ]
May 27 21:39:24 13[ENC] received fragment #2 of 5, waiting for complete IKE message
May 27 21:39:24 14[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (544 bytes)
May 27 21:39:24 14[ENC] parsed IKE_AUTH response 1 [ EF ]
May 27 21:39:24 14[ENC] received fragment #3 of 5, waiting for complete IKE message
May 27 21:39:24 16[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (544 bytes)
May 27 21:39:24 16[ENC] parsed IKE_AUTH response 1 [ EF ]
May 27 21:39:24 16[ENC] received fragment #4 of 5, waiting for complete IKE message
May 27 21:39:24 08[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (176 bytes)
May 27 21:39:24 08[ENC] parsed IKE_AUTH response 1 [ EF ]
May 27 21:39:24 08[ENC] received fragment #5 of 5, reassembling fragmented IKE message
May 27 21:39:24 08[ENC] parsed IKE_AUTH response 1 [ IDr CERT CERT AUTH EAP/REQ/ID ]
May 27 21:39:24 08[IKE] received end entity cert "OU=Domain Control Validated, OU=PositiveSSL, CN=vpn.armority.ru"
May 27 21:39:24 08[IKE] received issuer cert "C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, CN=COMODO ECC 
Domain Validation Secure Server CA"
May 27 21:39:24 08[CFG]   using certificate "OU=Domain Control Validated, OU=PositiveSSL, CN=vpn.armority.ru"
May 27 21:39:24 08[CFG]   using untrusted intermediate certificate "C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA 
Limited, CN=COMODO ECC Domain Validation Secure Server CA"
May 27 21:39:24 08[CFG]   using trusted ca certificate "C=GB, ST=Greater Manchester, L=Salford, O=COMODO CA Limited, 
CN=COMODO ECC Certification Authority"
May 27 21:39:24 08[CFG]   reached self-signed root ca with a path length of 1
May 27 21:39:24 08[IKE] authentication of 'vpn.armority.ru' with ECDSA-256 signature successful


Wait... we are supposed to connect to vpn.xorp.ru!


May 27 21:39:24 08[IKE] server requested EAP_IDENTITY (id 0x00), sending 'alice'


Oops... the server admin now knows a valid login at vpn.xorp.ru.


May 27 21:39:24 08[ENC] generating IKE_AUTH request 2 [ EAP/RES/ID ]
May 27 21:39:24 08[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (76 bytes)
May 27 21:39:24 09[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (108 bytes)
May 27 21:39:24 09[ENC] parsed IKE_AUTH response 2 [ EAP/REQ/MSCHAPV2 ]
May 27 21:39:24 09[IKE] server requested EAP_MSCHAPV2 authentication (id 0xAD)
May 27 21:39:24 09[ENC] generating IKE_AUTH request 3 [ EAP/RES/MSCHAPV2 ]
May 27 21:39:24 09[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (140 bytes)
Now he has an authentication value and can mount an offline dictionary attack. I don't know if he could offer something worse than EAP_MSCHAPv2 here for easier password cracking, or maybe convince the client to reveal a plaintext password. 


May 27 21:39:25 10[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (140 bytes)
May 27 21:39:25 10[ENC] parsed IKE_AUTH response 3 [ EAP/REQ/MSCHAPV2 ]
May 27 21:39:25 10[IKE] EAP-MS-CHAPv2 succeeded: 'Welcome2strongSwan'
May 27 21:39:25 10[ENC] generating IKE_AUTH request 4 [ EAP/RES/MSCHAPV2 ]
May 27 21:39:25 10[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (76 bytes)
May 27 21:39:25 07[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (76 bytes)
May 27 21:39:25 07[ENC] parsed IKE_AUTH response 4 [ EAP/SUCC ]
May 27 21:39:25 07[IKE] EAP method EAP_MSCHAPV2 succeeded, MSK established
May 27 21:39:25 07[IKE] authentication of 'alice' (myself) with EAP
May 27 21:39:25 07[ENC] generating IKE_AUTH request 5 [ AUTH ]
May 27 21:39:25 07[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (92 bytes)
May 27 21:39:25 11[NET] received packet: from 185.48.56.74[4500] to 192.168.1.237[54739] (236 bytes)
May 27 21:39:25 11[ENC] parsed IKE_AUTH response 5 [ AUTH CPRP(ADDR DNS) SA TSi TSr N(AUTH_LFT) N(MOBIKE_SUP) 
N(NO_ADD_ADDR) ]
May 27 21:39:25 11[IKE] authentication of 'vpn.armority.ru' with EAP successful
May 27 21:39:25 11[CFG] constraint check failed: identity 'vpn.xorp.ru' required
Dear StrongSwan VPN client, you were supposed to notice this hostname mismatch earlier. 


May 27 21:39:25 11[CFG] selected peer config 'android' inacceptable: constraint checking failed
May 27 21:39:25 11[CFG] no alternative config found
May 27 21:39:25 11[ENC] generating INFORMATIONAL request 6 [ N(AUTH_FAILED) ]
May 27 21:39:25 11[NET] sending packet: from 192.168.1.237[54739] to 185.48.56.74[4500] (76 bytes)


--
Alexander E. Patrakov
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