Microsoft (& Apple) Patch Tuesday, April 2023 Edition

The Windows Common Log File System (CLFS) driver contains an input-validation flaw (CWE-20) that leads to an out-of-bounds write (CWE-787), allowing an attacker who already has a foothold or local access on a system to escalate privileges. It is triggered when the driver processes malformed or specially crafted common log file data, rather than via a remote network request on its own. Successful exploitation yields elevated (typically SYSTEM-level) privileges, giving the attacker full control of the host and making the bug a common post-exploitation link in ransomware chains. CISA lists the affected product broadly as 'Microsoft Windows', so essentially all Windows installations current at the time of disclosure were in scope. The flaw is confirmed exploited in the wild: CISA added it to the KEV catalog on 2022-09-14 with known ransomware use, EPSS puts 30-day exploitation probability at 28.3% (98th percentile), and no public PoC is known.

Do: Apply Microsoft's September 2022 (or later) cumulative updates to every Windows host per vendor instructions, prioritizing endpoints and servers exposed for user or remote access. Because this is a local privilege escalation, treat it as a post-exploitation risk: also harden and patch initial-access surfaces (RDP, VPN, other exposed services) and hunt for signs of compromise such as unexpected SYSTEM-spawned processes or anomalous activity involving clfs.sys. Ransomware operators are known to use this bug, so remediation should be treated as urgent.

CVE-2023-28205 is a use-after-free flaw (CWE-416) in the WebKit engine shipped with Apple iOS, iPadOS, macOS, and the Safari browser, where memory is freed and then incorrectly reused while processing HTML. It is triggered when a device processes maliciously crafted web content, meaning simply loading an attacker-controlled page in Safari or any WebKit-based HTML renderer can trigger the bug. Successful exploitation allows the attacker to achieve code execution in the context of the WebKit process on the victim device. All users of iOS, iPadOS, macOS, and Safari are potentially affected, as are non-Apple products that rely on WebKit for HTML processing. The flaw is being actively exploited in the wild — it was added to CISA's Known Exploited Vulnerabilities catalog on 2023-04-10, with the required action to apply updates per vendor instructions — and EPSS assigns a 27.1% probability of exploitation within 30 days (98th percentile).

Do: Apply Apple's current security updates for iOS, iPadOS, macOS, and Safari as soon as possible, per the vendor instructions cited in the CISA KEV listing. Because this is a browser/HTML-engine flaw exploited in the wild, prioritize patching internet-facing and high-risk user fleets; users of non-Apple WebKit-based HTML parsers should check with their software vendors for updated WebKit components. Until patched, exercise caution with untrusted web content.

Apple's IOSurfaceAccelerator component in iOS, iPadOS, and macOS contains an out-of-bounds write flaw (CWE-787). The bug is triggered by an application running locally on the device, which can corrupt memory in the component during a write past a buffer boundary. A successful exploit allows the app to execute arbitrary code with kernel privileges, giving it full control of the device beyond the normal app sandbox. Any user of an Apple iOS, iPadOS, or macOS device running an affected, unpatched version is exposed. The flaw was added to CISA KEV on 2023-04-10, confirming known in-the-wild exploitation; ransomware use is unknown, no public PoC is available, and EPSS assigns a 24.5% probability of exploitation within 30 days (98th percentile).

Do: Update all iPhones, iPads, and Macs to the latest iOS/iPadOS/macOS versions available as of April 2023, per CISA's required action and Apple's security advisories. Use MDM or patch-reporting tooling to inventory endpoints and confirm no devices remain on pre-patch builds. Because exploitation is confirmed in the wild and any local app can act as the trigger, patching is the primary mitigation and there is no dependable configuration workaround.

Layer 2 Tunneling Protocol Remote Code Execution Vulnerability

DHCP Server Service Remote Code Execution Vulnerability

CVE-2023-28252 is a heap-based buffer overflow (CWE-122) in the Microsoft Windows Common Log File System (CLFS) driver that allows privilege escalation. The flaw is triggered when the kernel's CLFS driver processes malformed or maliciously crafted log file data, corrupting heap memory; an attacker who can already run code on a target system (e.g., a low-privileged user or an attacker chained with another flaw such as a remote code execution bug) can leverage it to gain SYSTEM-level privileges. Because the CLFS driver ships with supported Windows client and server releases, essentially the entire Windows installed base is potentially exposed. The vulnerability is being actively exploited: CISA added it to the KEV catalog on 2023-04-11 with known ransomware use, and EPSS estimates a 49.0% probability of exploitation within 30 days (99th percentile). It was addressed in Microsoft's April 2023 security updates, and the required remediation action is to apply the vendor updates.

Do: Apply Microsoft's April 2023 (or later) cumulative security updates to all Windows clients and servers per vendor instructions, prioritizing high-value and domain infrastructure systems given the known ransomware use; there is no public PoC or known workaround, so patching is the primary mitigation. Note that this is a local elevation-of-privilege flaw, so also hunt for prior low-privilege access or exploitation chains on hosts, and confirm remediation by checking installed update levels across the estate.