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.”

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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.

The acting head of the U.S. Department of Homeland Security said the agency was assessing the cyber risk of smart TVs sold by the Chinese electronics giant TCL, following reports last month in The Security Ledger and elsewhere that the devices may give the company “back door” access to deployed sets.

Speaking at The Heritage Foundation, a conservative think tank, Acting DHS Secretary Chad Wolf said that DHS is “reviewing entities such as the Chinese manufacturer TCL.”

“This year it was discovered that TCL incorporated backdoors into all of its TV sets exposing users to cyber breaches and data exfiltration. TCL also receives CCP state support to compete in the global electronics market, which has propelled it to the third largest television manufacturer in the world,” Wolf said, according to a version of prepared remarks published by DHS. His talk was entitled “Homeland Security and the China Challenge.”

As reported by The Security Ledger last month, independent researchers John Jackson, (@johnjhacking) -an application security engineer for Shutter Stock – and a researcher using the handle Sick Codes (@sickcodes) identified and described two serious software security holes affecting TCL brand television sets. The first, CVE-2020-27403, would allow an unprivileged remote attacker on the adjacent network to download most system files from the TV set up to and including images, personal data and security tokens for connected applications. The flaw could lead to serious critical information disclosure, the researchers warned.

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The second vulnerability, CVE-2020-28055, would have allowed a local unprivileged attacker to read from- and write to critical vendor resource directories within the TV’s Android file system, including the vendor upgrades folder.

Both flaws affect TCL Android Smart TV series V8-R851T02-LF1 V295 and below and V8-T658T01-LF1 V373 and below, according to the official CVE reports. In an interview with The Security Ledger, the researcher Sick Codes said that a TCL TV set he was monitoring was patched for the CVE-2020-27403 vulnerability without any notice from the company and no visible notification on the device itself.

In a statement to The Security Ledger, TCL disputed that account. By TCL’s account, the patched vulnerability was linked to a feature called “Magic Connect” and an Android APK by the name of T-Cast, which allows users to “stream user content from a mobile device.” T-Cast was never installed on televisions distributed in the USA or Canada, TCL said. For TCL smart TV sets outside of North America that did contain T-Cast, the APK was “updated to resolve this issue,” the company said. That application update may explain why the TCL TV set studied by the researchers suddenly stopped exhibiting the vulnerability.

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While TCL denied having a back door into its smart TVs, the company did acknowledge the existence of remote “maintenance” features that could give its employees or others control over deployed television sets, including onboard cameras and microphones. Owners must authorize the company to access cameras and microphones, however, according to a company statement.

The company did not address in its public statements the question of whether prior notification of the update was given to TCL owners or whether TV set owners were given the option to approve the update before it was installed.

Sick Codes, in a phone interview with The Security Ledger, said the company’s ability to push and update code to its deployed sets without owner approval amounted to a back door that could give TCL access to audio and video streams from deployed sets, regardless of the wishes of owners.

“They can update the application and make authorization happen through that. They have full control,” he said.

Such concerns obviously raised alarms within the Department of Homeland Security as well, which has taken steps to ban technology from other Chinese firms from use on federal networks.

In his address on Monday, Acting Secretary Wolf said the warning about TCL will be part of a a broader “business advisory” cautioning against using data services and equipment from firms linked to the People’s Republic of China (PRC).

This advisory will highlight “numerous examples of the PRC government leveraging PRC institutions like businesses, organizations, and citizens to covertly access and obtain the sensitive data of businesses to advance its economic and national security goals,” Wolf said.

“DHS flags instances where Chinese companies illicitly collect data on American consumers or steal intellectual property. CCP-aligned firms rake in tremendous profits as a result,” he said.

The statement is part of escalating tensions between Washington and Beijing. On Friday, Commerce Secretary Wilbur Ross announced export controls on 77 Chinese companies including the country’s biggest chipmaker, SMIC, and drone maker DJI that restrict those firms’ access to US technology. The order cites those firms alleged ties to China’s military.

TCL did not respond to an email request for comment prior to publication of this story. We will update this story as more information becomes available.


Editor’s note: this story was updated to add reference to John Jackson, who helped discover the TCL vulnerabilities. – PFR 12/22/2020