Microsoft Patch Tuesday fixes CVE-2020-17087 currently under active exploitation

Google Chrome bundles the open-source FreeType library for font rendering, and that library contains a heap buffer overflow (CWE-787, out-of-bounds write) in its Load_SBit_Png function. The flaw is triggered when the browser loads a crafted font containing a malicious PNG image embedded as embedded bitmap data, typically from a web page the victim visits, corrupting heap memory with attacker-controlled data. Successful exploitation can crash the browser or execute code in the renderer, and it was used in the wild as part of an exploit chain combined with CVE-2020-17087 (Windows kernel) and CVE-2020-16010 (Android) to escape the sandbox. Anyone running an affected Google Chrome release that ships the vulnerable FreeType code, across Windows, macOS, Linux, Chrome OS and Android, is affected, meaning effectively the entire Chrome install base at the time of disclosure. The vulnerability is confirmed exploited in the wild (listed in CISA's KEV catalog, added 2021-11-03; ransomware use unknown), Google patched it in Chrome 86.0.4240.111, no public proof-of-concept is known, and EPSS estimates a 44.3% probability of exploitation within 30 days (99th percentile).

Do: Update Google Chrome to 86.0.4240.111 or later (any current stable-channel release satisfies this), and where other software bundles FreeType directly, update to FreeType 2.10.4 or later per the upstream fix. Because the bug was chained with CVE-2020-17087 on Windows and CVE-2020-16010 on Android, also apply the corresponding Microsoft Windows and Android updates to close the sandbox-escape chain. Use endpoint management to inventory browser versions and confirm no endpoints remain below the fixed release, as required by the CISA KEV catalog.

CVE-2020-17087 is a local elevation-of-privilege flaw in the Windows kernel caused by an incorrect buffer size calculation (CWE-131), producing a kernel buffer overflow; public analyses from Microsoft and Google's disclosure place the vulnerable code in the kernel's cryptographic driver (cng.sys). A local attacker with low privileges can trigger the overflow without user interaction, gaining code execution in kernel context and effectively full control of the host (high impact on confidentiality, integrity, and availability; CVSS 7.8). Every Windows system on the affected builds is exposed: Windows 10 versions 1507 through 20H2, Windows 7, 8.1, RT 8.1, and Windows Server 2008, which at disclosure meant essentially the entire supported Windows install base. The flaw was exploited as a zero-day in the wild: Google disclosed its use in targeted attacks, reportedly chained with a Chrome zero-day, and CISA added it to the KEV catalog on 2021-11-03; EPSS currently estimates a 5.4% probability of exploitation within 30 days (92nd percentile), with ransomware association listed as unknown.

Do: Apply Microsoft's November 2020 Patch Tuesday security updates to all affected Windows 10, Windows 7, 8.1, RT 8.1, and Windows Server 2008 systems; this is CISA's required action for the KEV listing and no official workaround is known. Prioritize hosts where untrusted users can log on locally or via RDP, and ensure Chromium-based browsers are fully updated since this kernel bug was reportedly chained with a Chrome zero-day. After patching, verify the November 2020 update is installed; treat any ransomware association as currently unconfirmed.