Improper preservation of permissions in the Avast sandbox minifilter driver (aswSnx.sys) on Windows allows a local, low-privileged attacker executing inside the sandbox to escape file isolation and escalate to SYSTEM.

When the sandbox virtualizes a file it copies the original security descriptor, but the driver opened the virtualization target object with GENERICWRITE and FILEWRITEATTRIBUTES only, omitting WRITEDAC. Every attempt to apply the original DACL therefore failed, and the failure was discarded silently, leaving virtualized copies of sensitive files with permissive permissions. Because the IRPMJCREATE callback additionally did not strip WRITEDAC for sensitive directories, a sandboxed process could rewrite the security descriptor of a virtualized object, read the virtualized copy of the SAM database, extract local NTLM password hashes and execute code as SYSTEM.

The absence of an IRPMJSETSECURITY callback in the driver's operation registration table is a related defense-in-depth gap, but it is not the control that prevents this attack.