Millions could have been exposed to malware bug in LinkedIn Messenger.

Flaws in LinkedIn’s own security restrictions could have allowed cyber-criminals to upload malware-laden attachments in the social network’s messenger service.

According to security researchers at Checkpoint, when a valid file is uploaded and sent, LinkedIn’s security protections scan the attachment for malicious activity. But it was discovered that attackers could bypass the security restrictions and attach a malicious file to the LinkedIn messaging service.

“To do this, an attacker could have uploaded a normal-looking file that passes LinkedIn’s security checks; however, the file is only masquerading as a legitimate file, in reality, it is a form of malware that contains malicious content, able to infect the recipient’s network,” said the researchers in a blog post.

The researchers found four exploits in the LinkedIn security systems. First, an attacker could create a malicious Power Shell script. The script is saved as a .pdf file and this is uploaded to LinkedIn’s CDN server. If downloaded it would remain undetected.

The second flaw allowed a hacker to create a Windows registry file which contains a malicious PowerShell script and disguise it as a .pdf file. When the victim opens the file received via LinkedIn, the crafted REG containing the malicious payload runs, giving attacker control over the user’s machine. From now on, the script will run each time the user logs in to his computer.

The third flaw sees a hacker creating a malicious XLSM file, embedded with Macro, disguised as an XLSX file. The Macro is a scrambled VB script shell code. The masqueraded file passes the anti-virus check and then it is uploaded successfully to LinkedIn’s CDN and sent to the victim. When the victim opens the malicious XLSM file, Excel runs the VB scripts and the victim gets infected.

The last flaw is where a hacker creates a malicious DOCX file containing an external object. This object is linked to an HTA file on the attacker’s server. The DOCX file is then uploaded successfully to LinkedIn’s CDN, passing the virus check and sent to the victim. When the victim opens the malicious DOCX file, WINWORD automatically downloads the HTA file through the object link, and then runs it. Once the HTA file is executed, the victim is infected.

Check Point identified the four flaws and reported the discovery to LinkedIn on 14 June 2017. LinkedIn verified and acknowledged the security issues and deployed a fix effective 24 June 2017.

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