Executive brief
Avast's sandbox security feature uses a minifilter driver to isolate applications and files. The driver failed to properly preserve file permissions when creating virtualized copies of sensitive files, leaving them accessible with excessive privileges. An attacker running a sandboxed application could exploit this to escape the sandbox isolation, modify system files, steal local passwords, and gain complete control of the computer (SYSTEM-level access).
Technical details
The vulnerability is a privilege escalation in the Avast sandbox minifilter driver (aswSnx.sys) caused by improper preservation of file permissions (DACL). When the driver virtualizes files during sandbox initialization, it copies the original security descriptor but opens the virtualization target with insufficient permissions (GENERIC_WRITE and FILE_WRITE_ATTRIBUTES), omitting WRITE_DAC. This causes attempts to restore the original DACL to fail silently, leaving virtualized copies with permissive default permissions. Additionally, the driver's IRP_MJ_CREATE callback fails to strip WRITE_DAC for sensitive directories. A sandboxed process can therefore modify the security descriptor of virtualized objects, read the virtualized SAM database containing NTLM password hashes, and execute arbitrary code with SYSTEM privileges. The root cause is compounded by the absence of an IRP_MJ_SET_SECURITY callback in the driver's operation registration, a defense-in-depth gap.
Affected products
- Avast Antivirus <UNKNOWN>
Timeline
- 2026-09-16: disclosed