A critical-severity vulnerability in Sogou Input Method has been exploited by a Chinese threat actor to deploy a backdoor, Gen Threat Labs reports.
Developed by Tencent, Sogou Input Method is one of the most popular Chinese-language input method editors (IMEs) for Windows and is used by hundreds of millions of users.
It is a collection of executables that communicate using a custom protocol scheme named sgbiz. When a URL is opened, the protocol handler (biz_helper.exe) parses the URL and dispatches it to the appropriate component.
The critical flaw, tracked as CVE-2026-51990, chained three security weaknesses in a one-click exploit: unvalidated command-line argument injection, unrestricted URL navigation, and an outdated, un-sandboxed Chromium browser engine.
According to Gen Threat Labs, the first issue existed because, during URL parsing, the protocol handler did not sanitize or validate the ‘param’ parameter, which controls the command-line arguments passed to the executable.
This allowed an attacker to inject command-line arguments in the URL to declare a ‘skincenter’ page, which another function would simply copy and navigate the browser to.
The next security hole goes deeper: the browser in Sogou Input Method is based on a Chromium 80 iteration released in March 2020 that is missing roughly six years of security patches, has the sandbox completely disabled, strips additional protections (including same-origin policy), and allows URLs to read other local files.
The China-linked threat actor UNC3569 used this exploit chain to send crafted sgbiz URLs to unsuspecting victims. Once clicked, the exploit provided the attackers with system-level code execution.
“We observed this vulnerability actively exploited in the wild by the UNC3569 threat group to deploy the GrayRabbit backdoor through a crafted link,” Gen Threat Labs says.
Potentially linked to Chinese private contractor company i-SOON, UNC3569 is known for exploiting vulnerabilities in popular software to attack government, education, technology, and finance organizations globally.
The GrayRabbit backdoor, which has been consistently observed across the threat actor’s intrusions since at least 2021, provides attackers with a reverse shell and can execute processes, load plugins, write data to the interactive shell, upload files to its command-and-control (C&C) server, collect system information, and terminate itself.
Gen Threat Labs reported CVE-2026-51990 to Tencent on April 9. The security defect was addressed in Sogou Input Method version 16.3.0.3498, which was rolled out to all users via the automatic update mechanism.
The fix added a check for URL-bearing switches in the protocol handler, but left the underlying Chromium configuration unchanged. According to Gen Threat Labs, as of September 10, the configuration and version have not been updated.
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