Antivirus Engine Design Flaw Helps Malware Sink Its Teeth Into Your System by Catalin Cimpanu.
Cimpanu routs the chest beating of antivirus vendors with this report on a design flaw common to Windows antivirus products. Code named AVGater by its discoverer, Florian Bogner, who also created a colorful logo for the vulnerability:
(Source: #AVGater: Getting Local Admin by Abusing the Anti-Virus Quarantine by Florian Bogner)
Cimpanu gives a high level summary and Bogner more details to support further investigation of this design flaw. An incomplete list of impacted vendors: Trend Micro, Emsisoft, Kaspersky Lab, Malwarebytes, Ikarus, and Zone Alarm by Check Point.
So the answer is yes, antivirus engines do have design, and other, flaws.
Antivirus and other security software, increase the available attack surface for discovery of flaws and vulnerabilities.
If your antivirus or security software vendor denies increasing your attack surface, best you consider another vendor.