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Exclusive: Flaws in Zoom’s Keybase App Kept Chat Images From Being Deleted

A serious flaw in Zoom’s Keybase secure chat application left copies of images contained in secure communications on Keybase users’ computers after they were supposedly deleted.

The flaw in the encrypted messaging application (CVE-2021-23827) does not expose Keybase users to remote compromise. However, it could put their security, privacy and safety at risk, especially for users living under authoritarian regimes in which apps like Keybase and Signal are increasingly relied on as a way to conduct conversations out of earshot of law enforcement or security services.

The flaw was discovered by researchers from the group Sakura Samurai as part of a bug bounty program offered by Zoom, which acquired Keybase in May, 2020. Zoom said it has fixed the flaw in the latest versions of its software for Windows, macOS and Linux.

Deleted…but not gone

According to researcher John Jackson of Sakura Samurai, the Keybase flaw manifested itself in two ways. First: Jackson discovered that images that were copy and pasted into Keybase chats were not reliably deleted from a temporary folder, /uploadtemps, associated with the client application. “In general, when you would copy and paste in a Keybase chat, the folder would appear in (the uploadtemps) folder and then immediately get deleted,” Jackson told Security Ledger in a phone interview. “But occasionally that wouldn’t happen. Clearly there was some kind of software error – a collision of sorts – where the images were not getting cleared.”

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Discovering that flaw put Sakura Samurai researchers on the hunt for more and they soon struck pay dirt again. Sakura Samurai members Aubrey Cottle (@kirtaner), Robert Willis (@rej_ex) and Jackson Henry (@JacksonHHax) discovered an unencrypted directory, /Cache, associated with the Keybase client that contained a comprehensive record of images from encrypted chat sessions. The application used a custom extension to name the files, but they were easily viewable directly or simply by changing the custom file extension to the PNG image format, Jackson said.

In a statement, a Zoom spokesman said that the company appreciates the work of the researchers and takes privacy and security “very seriously.”

“We addressed the issue identified by the Sakura Samurai researchers on our Keybase platform in version 5.6.0 for Windows and macOS and version 5.6.1 for Linux. Users can help keep themselves secure by applying current updates or downloading the latest Keybase software with all current security updates,” the spokesman said.

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In most cases, the failure to remove files from cache after they were deleted would count as a “low priority” security flaw. However, in the context of an end-to-end encrypted communications application like Keybase, the failure takes on added weight, Jackson wrote.

“An attacker that gains access to a victim machine can potentially obtain sensitive data through gathered photos, especially if the user utilizes Keybase frequently. A user, believing that they are sending photos that can be cleared later, may not realize that sent photos are not cleared from the cache and may send photos of PII or other sensitive data to friends or colleagues.”

Messaging app flaws take on new importance

The flaw takes on even more weight given the recent flight of millions of Internet users to end-to-end encrypted messaging applications like Keybase, Signal and Telegram. Those users were responding to onerous data sharing policies, such as those recently introduced on Facebook’s WhatsApp chat. In countries with oppressive, authoritarian governments, end to end encrypted messaging apps are a lifeline for political dissidents and human rights advocates.

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As a result of the flaw, however, adversaries who gained access to the laptop or desktop on which the Keybase application was installed could view any images contained in Keybase encrypted chats. The implications of that are clear enough. For example, recent reports say that North Korean state hackers have targeted security researchers via phishing attacks sent via Keybase, Signal and other encrypted applications.

The flaws in Keybase do not affect the Zoom application, Jackson said. Zoom acquired Keybase in May to strengthen the company’s video platform with end-to-end encryption. That acquisition followed reports about security flaws in the Zoom client, including in its in-meeting chat feature.

Jackson said that the Sakura Samurai researchers received a $1,000 bounty from Zoom for their research. He credited the company with being “very responsive” to the group’s vulnerability report.

The increased use of encrypted messaging applications has attracted the attention of security researchers, as well. Last week, for example, a researcher disclosed 13 vulnerabilities in the Telegram secure messaging application that could have allow a remote attacker to compromise any Telegram user. Those issues were patched in Telegram updates released in September and October, 2020.

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