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Security: Cookie name encoding removed

  • Article

The HTTP cookie standard allows only specific characters in cookie names and values. To support disallowed characters, ASP.NET Core:

  • Encodes when creating a response cookie.
  • Decodes when reading a request cookie.

In ASP.NET Core 5.0, this encoding behavior changed in response to a security concern.

For discussion, see GitHub issue dotnet/aspnetcore#23578.

Version introduced

5.0 Preview 8

Old behavior

Response cookie names are encoded. Request cookie names are decoded.

New behavior

Encoding and decoding of cookie names was removed. For prior supported versions of ASP.NET Core, the team plans to mitigate the decoding issue in-place. Additionally, calling IResponseCookies.Append with an invalid cookie name throws an exception of type ArgumentException. Encoding and decoding of cookie values remains unchanged.

Reason for change

An issue was discovered in multiple web frameworks. The encoding and decoding could allow an attacker to bypass a security feature called cookie prefixes by spoofing reserved prefixes like __Host- with encoded values like __%48ost-. The attack requires a secondary exploit to inject the spoofed cookies, such as a cross-site scripting (XSS) vulnerability, in the website. These prefixes aren't used by default in ASP.NET Core or Microsoft.Owin libraries or templates.

If you're moving projects to ASP.NET Core 5.0 or later, ensure that their cookie names conform to the token specification requirements: ASCII characters excluding controls and separators "(" | ")" | "<" | ">" | "@" | "," | ";" | ":" | "\" | <"> | "/" | "[" | "]" | "?" | "=" | "{" | "}" | SP | HT. The use of non-ASCII characters in cookie names or other HTTP headers may cause an exception from the server or be improperly round-tripped by the client.

Affected APIs