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 OpenURL is the default URL handler. A navigation request is processed by the default URL handler only if the currentEvent is NX_LMOUSEUP or NX_OMOUSEUP. An attacker may abuse HTML elements with an EventHandler for a chance to validate navigation requests for URLs that are processed during the NX_LMOUSEUP event triggered by clicking an email.