An issue was discovered in Bloop Airmail 3 3.5.9 for macOS. Its primary WebView instance implements "webView:decidePolicyForNavigationAction:request:frame:decisionListener:" such that requests from HTMLIFrameElements are blacklisted. However, other sub-classes of HTMLFrameOwnerElements are not forbidden by the policy. An attacker may abuse HTML plug-in elements within an email to trigger frame navigation requests that bypass this filter.