A vulnerability in Windows Defender has been patched by Microsoft, rendering it harmless to users. The issue was related to the improper authorization of sensitive information from a Global Files search, allowing an authorized attacker to disclose private files over a network. Although theoretically possible, no instances of the exploit have been reported. To fix this issue, Microsoft had previously recommended that users take action, but now confirms they can simply ignore it and do nothing.
Source: https://mashable.com/article/microsoft-windows-defender-vulnerability-action-needed