Invalidating the cache

Posted by / 09-Jul-2017 15:35

Invalidating the cache

There are multiple mechanisms available in HTTP to maintain session state within web applications, such as cookies (standard HTTP header), URL parameters (URL rewriting – RFC 2396), URL arguments on GET requests, body arguments on POST requests, such as hidden form fields (HTML forms), or proprietary HTTP headers.

The preferred session ID exchange mechanism should allow defining advanced token properties, such as the token expiration date and time, or granular usage constraints.

The session ID must be unpredictable (random enough) to prevent guessing attacks, where an attacker is able to guess or predict the ID of a valid session through statistical analysis techniques.

For this purpose, a good PRNG (Pseudo Random Number Generator) must be used.

The session ID value must provide at least 64 bits of entropy (if a good PRNG is used, this value is estimated to be half the length of the session ID).

NOTE: The session ID entropy is really affected by other external and difficult to measure factors, such as the number of concurrent active sessions the web application commonly has, the absolute session expiration timeout, the amount of session ID guesses per second the attacker can make and the target web application can support, etc [2].

This is one of the reasons why cookies (RFCs 2109 & 2965 & 6265 [1]) are one of the most extensively used session ID exchange mechanisms, offering advanced capabilities not available in other methods.

This session protection mechanism is mandatory to prevent the disclosure of the session ID through Mit M (Man-in-the-Middle) attacks.

The session ID must be long enough to prevent brute force attacks, where an attacker can go through the whole range of ID values and verify the existence of valid sessions.

The session ID length must be at least 128 bits (16 bytes).

It is recommended to create cryptographically strong session IDs through the usage of cryptographic hash functions such as SHA1 (160 bits).

The session management implementation defines the exchange mechanism that will be used between the user and the web application to share and continuously exchange the session ID.

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In order to protect the session ID exchange from active eavesdropping and passive disclosure in the network traffic, it is mandatory to use an encrypted HTTPS (SSL/TLS) connection for the entire web session, not only for the authentication process where the user credentials are exchanged.