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FACEBOOK MaliciousChat

June 2016 by Check Point

Check Point disclosed details about a vulnerability found in Facebook Messenger, both in the online and mobile applications. Following Check Point’s responsible discloser, Facebook promptly fixed the vulnerability.

What is this vulnerability?

The vulnerability allows a malicious user to change conversation thread in the Facebook Online
Chat & Messenger App. By abusing this vulnerability, it is possible to modify or remove any sent
message, photo, file, link, and much more.
The Vulnerability was fully disclosed to the Facebook Security team earlier this month.
Facebook immediately responded, and after a joint effort the vulnerability was patched.

What is the potential damage of this vulnerability?

There are a few potential attack vectors abusing this vulnerability. These schemes could have a
severe impact on users due to Facebook’s vital role in everyday activities worldwide. Many
users rely on Facebook for personal and business related communications, which makes this
type of vulnerability very attractive to attackers.
 Malicious users can manipulate message history as part of fraud campaigns. A malicious
actor can change the history of a conversation to claim he had reached a falsified
agreement with the victim, or simply change its terms.
 Hackers can tamper, alter or hide important information in Facebook chat
communications which can have legal repercussions. These chats can be admitted as
evidence in legal investigations and this vulnerability opened the door for an attacker to
hide evidence of a crime or even incriminate an innocent person.
 The vulnerability can be used as a malware distribution vehicle. An attacker can change
a legitimate link or file into a malicious one, and easily persuade the user to open it. The
attacker can use this method later on to update the link to contain the latest C&C
address, and keep the phishing scheme up to date.

“By exploiting this vulnerability, cybercriminals could change a whole chat thread without the
victim realizing, What’s worse. The hacker could implement automation techniques to
continually outsmart security measures for long-term chat alterations,” said Oded Vanunu,
Head of Products Vulnerability Research at Check Point. “We applaud Facebook for such a rapid
response and putting security first for their users.”

Full technical analysis

Check Point Security Researcher Roman Zaikin discovered the vulnerability allows hackers to
control the Facebook chat and adjust the messages according to his needs, including deleting
them and replacing text, links, and files.
Each message in the Facebook chat applications, both online and mobile, has its own identifier
“message_id” parameter. An attacker can store this request, containing the identifier, via a
proxy while he launches his malicious attempt.

The image below shows a request sent to: www.facebook.com/ajax/mercury/send_message.php

Figure 1: Sending Message

An attacker can reveal the “message_id” by sending the request to:
www.facebook.com/ajax/mercury/thread_info.php

Figure 2: Revealing Message IDs

Once the attacker has found the message ID, he can alter the content of the message and send
it to the Facebook servers. The content is changed without a push message to the users’ PC or
mobile device.

POC – abusing the vulnerability for a ransomware campaign

By abusing Facebook chat or Messenger chat, attackers can alter conversations for various
purposes. In this section, we will demonstrate a possible attack flow which exploits this
vulnerability to distribute ransomware.

First, the attacker sends a legitimate message to a potential target:

Figure 3: Legitimate Contact

The attacker will then alter the message to contain an infected link or file. As you can see in the
image below, the message “Hi” changed to “RANSOMWARE COMMAND AND CONTROL
ROULETTE”.

Figure 4: Altering The Message

Next, the hacker can manipulate the same attack vector to overcome one of the biggest
challenges standing in the face of ransomware today: maintaining an active command & control
server. Usually, ransomware campaigns last only several days because the infected links and
the C&C addresses become known, and blocked by security vendors, forcing the attacker to
shut down his activity and begin again from scratch. However, with this vulnerability, the
hacker could implement automation techniques to continually outsmart security measures
when the command & control servers are replaced.

Check Point disclosed the vulnerability to Facebook and Facebook took immediate action to
patch the vulnerability.

Check Point continues to be on the lookout for vulnerabilities in common software and internet
platforms, disclosing issues as they are discovered, alerting, protecting consumers and
customers against tomorrow’s threats.


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