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On April 14-15, 2021, Kaspersky technologies detected a wave of highly targeted attacks against multiple companies. Closer analysis revealed that all these attacks exploited a chain of Google Chrome and Microsoft Windows zero-day exploits. While we were not able to retrieve the exploit used for remote code execution (RCE) in the Chrome web browser, we were able to find and analyze an elevation of privilege (EoP) exploit that was used to escape the sandbox and obtain system privileges.

The elevation of privilege exploit was fine-tuned to work against the latest and most prominent builds of Windows 10 (17763 – RS5, 18362 – 19H1, 18363 – 19H2, 19041 – 20H1, 19042 – 20H2) and it exploits two distinct vulnerabilities in the Microsoft Windows OS kernel. On April 20, 2021, we reported these vulnerabilities to Microsoft and they assigned CVE-2021-31955 to the information disclosure vulnerability and CVE-2021-31956 to the elevation of privilege vulnerability. Both vulnerabilities were patched on June 8, 2021, as a part of the June Patch Tuesday.

Remote code execution exploitAll of the observed attacks were conducted through Chrome browser. Unfortunately, we were unable to retrieve the JavaScript with full exploit code, but the timeframe of attacks and events preceding it led us to suspect one particular vulnerability.

On April 6-8, 2021 the Pwn2Own competition took place. This is a computer hacking contest where the Google Chrome web browser was one of the targets.

According to the ZDI (Zero Day Initiative, the organizer of Pwn2Own) website, one participating team was able to demonstrate a successful exploitation of the Chrome renderer process using a Typer Mismatch bug.

On April 12, 2021, the developers of Chromium committed two (issue 1196683, issue 1195777) Typer-related bug fixes to the open-source repository of V8 – a JavaScript engine used by Chrome and Chromium web browsers. One of these bug fixes (issue 1196683) was intended to patch a vulnerability that was used during Pwn2Own, and both bug fixes were committed together with regression tests – JavaScript files to trigger these vulnerabilities. Later on the same day, a user with the Twitter handle @r4j0x00 published a working remote code execution exploit on GitHub, targeting an up-to-date version of Google Chrome. That exploit used a vulnerability from issue 1196683 to execute a shellcode in the context of the browser renderer process.

The published exploit didn’t contain a sandbox escape exploit and was therefore intended to work only when the browser was launched with the command line option –no-sandbox.

On April 13, 2021, Google released Chrome update 89.0.4389.128 for Windows, Mac and Linux with a fix for two vulnerabilities; CVE-2021-21220 (used during Pwn2Own) was one of them.

Some of our customers who were attacked on April 14-15, 2021, already had their Chrome browser updated to 89.0.4389.128, and that’s why we think the attackers didn’t use CVE-2021-21220 in their attacks.

On April 14, 2021, Google released Chrome update 90.0.4430.72 for Windows, Mac and Linux with a fix for 37 vulnerabilities. On the same day, a new Chrome exploit was presented to the public.

This newly published exploit used a vulnerability from issue 1195777, worked on the newly released Chrome 90.0.4430.72, and was fixed as CVE-2021-21224 only a few days later, on April 20, 2021.

We suspect the attackers were also able to use this JavaScript file with regression test to develop the exploit (or acquire it from someone else) and were probably using CVE-2021-21224 in their attacks.

Elevation of privilege exploit

CVE-2021-31955 is an information disclosure vulnerability in ntoskrnl.exe. The vulnerability is affiliated with a Windows OS feature called SuperFetch. It was introduced in Windows Vista and is aimed to reduce software loading times by pre-loading commonly used applications into memory. For SuperFetch purposes the function NtQuerySystemInformation implements a special system information class SystemSuperfetchInformation. This system information class incorporates more than a dozen of different SuperFetch information classes. The vulnerability lies in the fact that data returned by the NtQuerySystemInformation function for the SuperFetch information class SuperfetchPrivSourceQuery contains EPROCESS kernel addresses for currently executed processes.

It’s noteworthy that this vulnerability can be observed in code that was available on GitHub for a few years before we caught it in the wild and Microsoft patched it.

The other vulnerability, CVE-2021-31956, is a heap-based buffer overflow in ntfs.sys. The function NtfsQueryEaUserEaList processes a list of extended attributes for the file and stores the retrieved values to buffer. This function is accessible via ntoskrnl syscall and among other things it’s possible to control the size of the output buffer. If the size of the extended attribute is not aligned, the function will calculate a padding and the next extended attribute will be stored 32-bit aligned. The code checks if the output buffer is long enough to fit the extended attribute with padding, but it doesn’t check for possible integer-underflow. As a result, a heap-based buffer overflow can happen.
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