On November 10, 2017, a vulnerability called #AVGater was discovered affecting some antivirus products. The vulnerability requires a non-administrator-level account to perform a restore of a quarantined file.
Windows Defender Antivirus and other Microsoft antimalware products, including System Center Endpoint Protection (SCEP) and Microsoft Security Essentials (MSE), are not affected by this vulnerability.
This vulnerability can be exploited to restore files that have been detected and quarantined by an antivirus product. To exploit this, malicious applications, including those launched by user-level accounts without administrator privileges, create an NTFS junction from the %System% folder to folder where the quarantined file is located. This NTFS junction can trigger the antivirus product to attempt to restore the file into the %System% folder.
This is a relatively old attack vector. By design, Microsoft antimalware products, including Windows Defender Antivirus, have never been affected by this vulnerability because it does not permit applications launched by user-level accounts to restore files from quarantine. This is part of the built-in protections against this and other known user-account permissions vulnerabilities.
Read more about Windows Defender Antivirus and the rest of our Windows Defender protection products at the following links:
*Edited 11/17/2017 to include other Microsoft antimalware products