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I have noticed many guides saying you should disable the SSL v2 support while setting up SSL on a web server.

I can't understand the reason. Can somebody tell me why, please?

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Wikipedia:

SSL 2.0 is flawed in a variety of ways:1

  • Identical cryptographic keys are used for message authentication and encryption.
  • SSL 2.0 has a weak MAC construction that uses the MD5 hash function with a secret prefix, making it vulnerable to length extension attacks.
  • SSL 2.0 does not have any protection for the handshake, meaning a man-in-the-middle downgrade attack can go undetected.
  • SSL 2.0 uses the TCP connection close to indicate the end of data. This means that truncation attacks are possible: the attacker simply forges a TCP FIN, leaving the recipient unaware of an illegitimate end of data message (SSL 3.0 fixes this problem by having an explicit closure alert).
  • SSL 2.0 assumes a single service and a fixed domain certificate, which clashes with the standard feature of virtual hosting in Web servers. This means that most websites are practically impaired from using SSL. TLS/SNI fixes this but is not deployed in Web servers as yet.

1: http://en.wikipedia.org/wiki/Transport_Layer_Security#SSL_1.0.2C_2.0_and_3.0

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