CVE-2019-1215

What is ws2ifsl.sys?

ws2ifsl.sys is the Winsock Kernel Interprocess Communication (IFS) driver that Windows uses to bridge the user-mode Winsock networking API to kernel-mode socket handling. As a kernel driver reachable through everyday networking calls that any user-mode application can issue, it presents a broad, low-friction attack surface for local privilege escalation.

Overview

CVE-2019-1215 is a local privilege escalation vulnerability in the ws2ifsl.sys Winsock kernel driver caused by improper handling of objects in memory. A locally-authenticated, low-privileged attacker can trigger the flaw through crafted socket-related IOCTL requests to escalate to SYSTEM-level code execution. Public proof-of-concept code for this bug circulated before Microsoft's official patch, making it a genuine zero-day at the time of disclosure.

Technical Details

The vulnerability arises from the driver's IOCTL handling failing to properly validate or synchronize access to kernel objects, a pattern consistent with a null-pointer dereference or use-after-free triggered by sending a specific sequence of crafted IOCTL calls to the driver from an unprivileged process. Because the driver runs in kernel mode and processes are able to invoke it directly via standard networking APIs, an attacker who can run any code locally — even with low privileges — can reach the vulnerable code path without needing administrative access or user interaction, and with low attack complexity turn the resulting memory corruption into SYSTEM-level code execution.

Discovery

A working proof-of-concept exploit was released publicly by the researcher known as SandboxEscaper, who between 2018 and 2019 published a series of unpatched Windows local privilege escalation zero-days on GitHub without prior vendor coordination — this bug was one of several such "drop and disclose" 0-days from that period, colloquially referred to by researchers as "AngryPolarBearBug2." Microsoft patched the underlying flaw in the September 2019 cumulative update.

Exploitation Context

Because a working exploit was public before Microsoft shipped a fix, this vulnerability had a real window of active zero-day exposure, and its CISA KEV listing along with the ransomwareUse: true flag confirms it has since been folded into ransomware-affiliated intrusion toolkits as a local privilege-escalation step used after an initial foothold is established on a Windows host.

Remediation

1. Apply the September 2019 (or later) Windows cumulative update, which fixes the ws2ifsl.sys memory-handling flaw.
2. Prioritize patching given this bug's history as a publicly-disclosed, pre-patch zero-day with working exploit code freely available.
3. Restrict local logon rights and monitor for unusual process behavior invoking low-level Winsock IOCTL calls where feasible.
4. Deploy EDR tooling capable of detecting anomalous privilege escalation from standard user to SYSTEM context.
5. Treat local LPE bugs disclosed via public zero-day PoC releases (rather than coordinated disclosure) as especially high priority for expedited patch rollout.