Telegram Desktop Bug Deletes Up to 800 GB of User Data on Windows Due to Spellcheck Library Error
A dangerous configuration error in Telegram Desktop for Windows led to the recursive deletion of nearly 800 GB of user data stored in the C:\custom directory. Home users reported that executable files began disappearing while folder structures remained, gradually emptying entire project and application directories.
The root cause was traced to the lib_spellcheck library responsible for handling user-added dictionary words in a file named custom. Following changes in version 7.1.0, the Windows code passed an empty path string to the spellchecker; Qt then appended /custom, resolving it to the absolute path C:\custom. When the library encountered a directory instead of the expected dictionary file, it executed removeRecursively(), systematically deleting all accessible contents.
Analysis with Process Monitor confirmed that Telegram Desktop was opening the path, traversing subdirectories, and requesting deletion rights without any malicious intent or updater malfunction. The flaw was limited to Windows builds containing the integrated spellchecker and existed in versions 7.1.0 and 7.1.1 for approximately 66 hours.
The researcher virtual-anvil submitted detailed logs and opened ticket #31170. Developers acknowledged the issue, implemented fixes that ensure the path is validated before the function is called and that the library is blocked from operating on an empty address, and released the corrected build in Telegram Desktop 7.1.2. Affected Windows users are strongly recommended to update without delay.
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