If you think that the media files you receive on your end-to-end encrypted secure messaging apps can not be tampered with, you need to think again.
Security researchers at Symantec yesterday demonstrated multiple interesting attack scenarios against WhatsApp and Telegram Android apps, which could allow malicious actors to spread fake news or scam users into sending payments to wrong accounts.
Dubbed “Media File Jacking,” the attack leverages an already known fact that any app installed on a device can access and rewrite files saved in the external storage, including files saved by other apps installed on the same device.
WhatsApp and Telegram allow users to choose if they want to save all incoming multimedia files on internal or external storage of their device.
However, WhatsApp for Android by default automatically stores media files in the external storage, while Telegram for Android uses internal storage to store users files that are not accessible to any other app.
But, many Telegram users manually change this setting to external storage, using “Save to Gallery” option in the settings, when they want to re-share received media files with their friends using other communication apps like Gmail, Facebook Messenger or WhatsApp.
It should be noted that the attack is not just limited to WhatsApp and Telegram, and affects the functionality and privacy of many other Android apps as well.
Media File Jacking – Technical Explanation
There are two types of storage location (internal and external storage) used by Android apps to store media files and data.
When users stored the files in internal storage can be accessed only by the respective app which means that other apps can’t access those files.
But, the files saved to an external storage public directory are world-readable/writeable can be accessed by other apps or users which actually beyond the app control.
In WhatsApp, files are stored in external storage by default (/storage/emulated/0/WhatsApp/Media/) similarly Telegram stored in same external storage (/storage/emulated/0/Telegram/ ).
So both apps load the received files from the public directories for users to see in the chat interface which leads to a critical security risk for the integrity of the media files.
According to Symantec Research, Write-to-external storage (WRITE_EXTERNAL_STORAGE) is a common permission requested by Android apps, with over a million apps in Google Play having this access. In fact, based on our internal app data, we found nearly 50% of a given device’s apps have this permission.
During their research, the attacker performs the exploitation with the help of malware that can instantaneously analyze and manipulate the files and gain access.
The manipulation process takes place between, when files are first received on a device and written to the disk ( Image 1) and when they are loaded for users to consume via the apps (Image 3)
Just like man-in-the-disk attacks, a malicious app installed on a recipient’s device can intercept and manipulate media files, such as private photos, documents, or videos, sent between users through the device’s external storage – all without the recipients’ knowledge and in real-time.
“The fact that files are stored in, and loaded from, external storage without proper security mechanisms, allows other apps with write-to-external storage permission to risk the integrity of the media files,” researchers said in a blog post.
“Attackers could take advantage of the relations of trust between a sender and a receiver when using these IM apps for personal gain or wreaking havoc.”
Researchers illustrated and demonstrated four attack scenarios, as explained below, where a malware app can instantaneously analyze and manipulate incoming files, leading to:
1.) Image manipulation
In this attack scenario, a seemingly innocent-looking, but actually malicious, app downloaded by a user can run in the background to perform a Media File Jacking attack while the victim uses WhatsApp and “manipulate personal photos in near-real-time and without the victim knowing.”
2.) Payment manipulation
In this scenario, which researchers call “one of the most damaging Media File Jacking attacks,” a malicious actor can manipulate an invoice sent by a vendor to customers to trick them into making a payment to an account controlled by the attacker.
3.) Audio message spoofing
In this attack scenario, attackers can exploit the relations of trust between employees in an organization. They can use voice reconstruction via deep learning technology to alter an original audio message for their personal gain or to wreak havoc.
4.) Spread fake news
In Telegram, admins use the concept of “channels” in order to broadcast messages to an unlimited number of subscribers who consume the published content. Using Media File Jacking attacks, an attacker can change the media files that appear in a trusted channel feed in real-time to spread fake news.
How to Prevent Hackers from Hijacking Your Android Files
Symantec already notified Telegram and Facebook/WhatsApp about the Media File Jacking attacks, but it believes the issue will be addressed by Google with its upcoming Android Q update.
Android Q includes a new privacy feature called Scoped Storage that changes the way apps access files on a device’s external storage.
Scoped Storage gives each app an isolated storage sandbox into the device external storage where no other app can directly access data saved by other apps on your device.
Until then, users can mitigate the risk of such attacks by disabling the feature responsible for saving media files to the device’s external storage. To do so, Android users can head on to:
- WhatsApp: Settings → Chats → Turn the toggle off for ‘Media Visibility’
- Telegram: Settings → Chat Settings → Disable the toggle for ‘Save to Gallery’
