facebook error ‘Error validating verification code’

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There are presently (as of March 2011) undocumented requirements regarding what makes a valid redirect_uri.

First, both redirect_uri paramaters to authorize and access_token must match.

Apparently Facebook (or rather OAuth2) is using the redirect_uri as a internal key to encode the code returned for the access_token request. It’s kinda clever since it verifies back to your site. It explains why the access_token request which wouldn’t otherwise need a redirect_uri parameter requires one.

Second, you cannot use many special characters in the redirect_uri.

A lot of discussion rages whether parameters can be passed at all. They can, you’re limited which characters are valid but no one has published a list that I know. Traditional methods like url/html encoding will fail because percent(%) is not valid. Slash (/) is not valid either so a nested redirection url will always fail. The ONLY way to overcome the special char limitation is to encode the value of the parameter to base64. If you’re using ASP.NET, look up Convert.ToBase64.

Lastly, and this is more of a side-note. There are a lot of programmers passing along misinformation that a simple solution is to pass type=client_cred. This may limit your access to some of the permissions you requested in your authorization. It is inadvisable.

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