Is there a way to expose a single page (aspx) for anonymous access?

You can allow anonymous access to a specific page by overriding authentication settings in web.config .

First, check if the below lines are set in web.config to deny anonymous users globally:

<system.web>
  <authorization>
    <deny users="?" />
  </authorization>
</system.web>

Remove the above lines and use <location> tags in web.config to define authorization settings restrict access to protected pages and allow anonymous access to unprotected pages, as shown below:

<location path="Default.aspx">
  <system.web>
    <authorization>
      <deny users="?" />
    </authorization>
  </system.web>
</location>

  <location path="About.aspx">
  <system.web>
    <authorization>
      <allow users="*" />
    </authorization>
  </system.web>
</location>

Output: