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authorDaniel Stenberg <daniel@haxx.se>2014-03-08 22:21:15 +0000
committerDaniel Stenberg <daniel@haxx.se>2014-03-08 22:21:41 +0000
commita55e7f0abdf85a88832ecb10485fcedc6a90bdef (patch)
tree0688d83b695e70faef1ccac7f15ead8f4f7b3e23 /docs/SSL-PROBLEMS
parent63e3e03dae4abc19d176b35bc39ddf6e7c23a546 (diff)
SSL-PROBLEMS: describes common curl+SSL problems
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+ _ _ ____ _
+ ___| | | | _ \| |
+ / __| | | | |_) | |
+ | (__| |_| | _ <| |___
+ \___|\___/|_| \_\_____|
+
+SSL problems
+
+ First, let's establish that we often refer to TLS and SSL interchangably as
+ SSL here. The current protocol is called TLS, it was called SSL a long time
+ ago.
+
+ There are several known reasons why a connection that involves SSL might
+ fail. This is a document that attempts to details the most common ones and
+ how to mitigate them.
+
+CA certs
+
+ CA certs are used to digitally verify the server's certificate. You need a
+ "ca bundle" for this. See lots of more details on this in the SSLCERTS
+ document.
+
+SSL version
+
+ Some broken servers fail to support the protocol negotiation properly that
+ SSL servers are supposed to handle. This may cause the connection to fail
+ completely. Sometimes you may need to explicity select a SSL version to use
+ when connecting to make the connection succeed.
+
+ An additional complication can be that modern SSL libraries sometimes are
+ built with support for older SSL and TLS versions disabled!
+
+SSL ciphers
+
+ Clients give servers a list of ciphers to select from. If the list doens't
+ include any ciphers the server wants/can use, the connection handshake
+ fails.
+
+ curl has recently disabled the user of a whole bunch of seriously insecure
+ ciphers from its default set (slightly depending on SSL backend in use).
+
+ You may have to explicitly provide an alternative list of ciphers for curl
+ to use to allow the server to use a WEAK cipher for you.
+
+ Note that these weak ciphers are identified as flawed. For example, this
+ includes symmetric ciphers with less than 128 bit keys and RC4.
+
+ References:
+
+ http://tools.ietf.org/html/draft-popov-tls-prohibiting-rc4-01
+
+Allow BEAST
+
+ BEAST is the name of a TLS 1.0 attack that surfaced 2011. When adding means
+ to mitigate this attack, it turned out that some broken servers out there in
+ the wild didn't work properly with the BEAST mitigation in place.
+
+ To make such broken servers work, the --ssl-allow-beast option was
+ introduced. Exactly as it sounds, it re-introduces the BEAST vulnerability
+ but on the other hand it allows curl to connect to that kind of strange
+ servers.