Security researchers at SAFA recently mapped out a set of four kernel‑heap overflows that sit inside Avast Antivirus’s aswSnx driver. These flaws, collectively catalogued as CVE‑2025‑13032, give a crafty attacker the ability to climb from a low‑privilege process straight into kernel mode, and ultimately seize full system control.

Why the aswSnx Driver Was a Magnet for Attackers

Avast ships several kernel drivers that bridge user‑space applications to the heart of the operating system through IOCTL (Input/Output Control) calls. Among them, aswSnx stands out because it exposes the most IOCTL endpoints to unprivileged users. In plain language, it gives attackers a wide playground to probe for weaknesses.

The Sandbox That Wasn’t

The driver was designed to sandbox untrusted processes, limiting their reach. The idea is similar to a secure lab: restrict what a researcher can do, but still let them run their experiments. Unfortunately, the sandbox’s own configuration file—%ProgramData%Avast SoftwareAvastsnx_lconfig.xml—became the key to opening the gates.

Breaking In Before Escalating

Most privilege escalation stories start with a “break out” from a restricted environment. Here, the attackers had to “break in” first. They reverse‑engineered the driver to discover that IOCTL 0x82AC0054 lets a process register itself in the sandbox with read‑only rights. By crafting a process name and setting the right flags, they slipped their malicious code into the sandbox and then triggered the vulnerable path.

Double Fetch: The Core of the Vulnerability

At the heart of the issue lies a “double fetch” bug: the kernel reads user‑controlled data twice without ensuring the data stays consistent between reads. Imagine a chef measuring flour, then mixing it, only to discover the bowl was emptied in the interim. The kernel, unaware of the change, allocates a buffer based on the first measurement, only to copy data that now exceeds that size.

Unicode Strings and Heap Overflows

One of the critical flaws occurs when handling Unicode strings. The driver first allocates a buffer sized to the string’s length, later copies the string into that buffer, but the length can be altered between those two steps. A malicious user can shorten the length after the allocation but before the copy, causing the kernel to write past the buffer’s boundary. The result is a heap overflow filled with attacker‑controlled data, a classic recipe for arbitrary code execution.

TOCTOU in the Kernel

Another layer of risk comes from the IOCTL handler 0x82AC0204, which fails to copy the user‑supplied structure into kernel space before processing it. This introduces a time‑of‑check to time‑of‑use (TOCTOU) race condition. The kernel checks the structure, then the attacker can modify it just before it’s used, flipping the logic to their advantage.

More Than One Flaw

Beyond the double fetch bug, researchers found three additional variants that affect different string encodings—ASCII, UTF‑8, and UTF‑16. Each variant follows the same pattern of inconsistent length calculations. They also uncovered two local denial‑of‑service issues caused by missing pointer validation checks, which could crash the driver but do not directly lead to privilege escalation.

Rapid Response and Patch Timeline

Avast’s reaction was notably swift. Within 12 days of the vulnerability’s formal acceptance, the company released patches that removed the redundant length calculations and added rigorous size checks before any memory operation. The formal CVE number, CVE‑2025‑13032, was assigned in November 2025, a testament to the vendor’s responsiveness.

What This Means for Security Software

Even mature, widely deployed security products can harbor critical flaws that a determined researcher can uncover with modest resources. The case of Avast’s sandbox demonstrates that the very mechanisms meant to protect users can, if not carefully audited, become the very conduit for attackers. It’s a reminder that security is a moving target and that rigorous kernel‑level code reviews are essential.

Looking Forward: Strengthening the Defenders

As operating systems evolve, the line between user space and kernel space continues to blur, especially with features like sandboxing and virtualization. Future security stacks must incorporate automated integrity checks, runtime verification, and more granular privilege boundaries to prevent the same double‑fetch patterns from slipping through. The Avast incident, while unsettling, also offers a blueprint: identify the most exposed driver interfaces, audit them for race conditions, and patch promptly. In the end, the goal is simple—keep the defenders strong enough that they never become the attackers’ playbook.