UPDATE

Grammarly has fixed a bug with its Chrome browser extension that exposed its authorization tokens to websites, allowing sites to assume the identity of a user and view their account’s documents.

“I’m calling this a high severity bug, because it seems like a pretty severe violation of user expectations,” said Tavis Ormandy, a researcher at Google’s Project Zero, in a Feb. 2 forum post. “Users would not expect that visiting a website gives it permission to access documents or data they’ve typed into other websites.”

However, Grammarly has already addressed the problem and pushed an update to the Chrome Web Store and Mozilla, exhibiting “a really impressive response time,” Ormandy wrote in a follow-up post Monday. “I’m calling this issue fixed.”

“Grammarly resolved a security bug reported by Google’s Project Zero security researcher, Tavis Ormandy, within hours of its discovery. At this time, Grammarly has no evidence that any user information was compromised by this issue. We’re continuing to monitor actively for any unusual activity. The security issue potentially affected text saved in the Grammarly Editor. This bug did not affect the Grammarly Keyboard, the Grammarly Microsoft Office add-in, or any text typed on websites while using the Grammarly browser extension. The bug is fixed, and there is no action required by Grammarly users.”

There are more than 20 million users of Grammarly’s Chrome extension, and the company also offers a web-based editor. Its software scans users’ writing for grammar, spelling, punctuation and style, offering up corrections and suggestions.

Ormandy’s original post included just four lines of code showing how a Grammerly user’s information could be compromised, whether manually or via a script:

That code generates a token that matches a cookie used by Grammerly. “I verified that is enough to login to a grammarly.com account,” Ormandy wrote. “[T]herefore any website can login to grammarly.com as you and access all your documents, history, logs, and all other data.”

Responding to a Twitter user on Monday, Ormandy said that the bug was of a nature that wouldn’t be caught by Google’s existing scanning capabilities for Chrome extension code.

No, automatic program analysis with this level of complexity is an unsolved problem in computer science. For the foreseeable future it will require skilled humans and hours of work. — Tavis Ormandy (@taviso) February 5, 2018

This story was updated on 2/6/18 with additional comments from Grammarly.