This blog post focuses on the privacy issues that Microsoft Teams & Skype desktop clients pose. The log database in both clients stores all the chats and images as plain non-encrypted data. The chats are encrypted via network as mentioned here https://docs.microsoft.com/en-us/microsoftteams/teams-security-guide but not encrypted at rest in local storage.
With Supervision policies, only the authorized person can monitor chats, but due to the non-encrypted storage, any account with administrative privileges can view the chats of the users from the target machine. This issue may pose a violation of regulatory compliance and/or risk management.
One can easily locate the chat database files and exfiltrate via any offensive medium.
~/Library/Application Support/Microsoft/Skype for Desktop/Cache/
You can find the files with no extension.
Read the file with any hex editors and identify the images with file headers.
Rename and open
Microsoft Teams – 1.3.00.30866 Skype for Desktop – 188.8.131.52
Microsoft Teams – 1.3.00.30874 Skype for Desktop - 184.108.40.206
Using the above steps an attacker can grab:
Normal chats and images
Deleted chats and images
Unseen chats and images which were deleted from other users
Skype’s Private chat’s, which are meant to be encrypted end-to-end
Skype’s deleted private chat’s, which are meant to be encrypted end-to-end
A seasoned log folder could have multiple backups of old chats:
An open risk to leakage of confidential information contained in chat
A low profile malware payload can grab and upload the files to a C2 server
High possibilities for an Insider attack
Failure of Data Risk Management and Regulatory compliance
Failure of GDPR Compliance
From the Threat Hunting perspective,
Monitor the above paths for any abnormal access like Username and Users parameters are different
Monitor for file behaviors like copy and delete for the above folders
A quick workaround for this issue is to deny read/write permissions for the standard user accounts to folders “IndexedDB” and “Cache” to prevent the creation of the logs as you can see in the screenshot below:
We reported this issue to Microsoft through our responsible disclosure program. They responded, "We determined that this behavior is considered to be by design."
Trustwave is exercising extra vigilance in monitoring this kind of traffic. We regularly monitor and improve our threat hunting plans for all global cyber events and all organizations can undertake similar operations within their environment. Contact Trustwave about engaging professional threat hunting.