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    6: 
    7: # SSL problems
    8: 
    9:   First, let's establish that we often refer to TLS and SSL interchangeably as
   10:   SSL here. The current protocol is called TLS, it was called SSL a long time
   11:   ago.
   12: 
   13:   There are several known reasons why a connection that involves SSL might
   14:   fail. This is a document that attempts to details the most common ones and
   15:   how to mitigate them.
   16: 
   17: ## CA certs
   18: 
   19:   CA certs are used to digitally verify the server's certificate. You need a
   20:   "ca bundle" for this. See lots of more details on this in the SSLCERTS
   21:   document.
   22: 
   23: ## CA bundle missing intermediate certificates
   24: 
   25:   When using said CA bundle to verify a server cert, you will experience
   26:   problems if your CA cert does not have the certificates for the
   27:   intermediates in the whole trust chain.
   28: 
   29: ## Protocol version
   30: 
   31:   Some broken servers fail to support the protocol negotiation properly that
   32:   SSL servers are supposed to handle. This may cause the connection to fail
   33:   completely. Sometimes you may need to explicitly select a SSL version to use
   34:   when connecting to make the connection succeed.
   35: 
   36:   An additional complication can be that modern SSL libraries sometimes are
   37:   built with support for older SSL and TLS versions disabled!
   38: 
   39:   All versions of SSL are considered insecure and should be avoided. Use TLS.
   40: 
   41: ## Ciphers
   42: 
   43:   Clients give servers a list of ciphers to select from. If the list doesn't
   44:   include any ciphers the server wants/can use, the connection handshake
   45:   fails.
   46: 
   47:   curl has recently disabled the user of a whole bunch of seriously insecure
   48:   ciphers from its default set (slightly depending on SSL backend in use).
   49: 
   50:   You may have to explicitly provide an alternative list of ciphers for curl
   51:   to use to allow the server to use a WEAK cipher for you.
   52: 
   53:   Note that these weak ciphers are identified as flawed. For example, this
   54:   includes symmetric ciphers with less than 128 bit keys and RC4.
   55: 
   56:   Schannel in Windows XP is not able to connect to servers that no longer
   57:   support the legacy handshakes and algorithms used by those versions, so we
   58:   advice against building curl to use Schannel on really old Windows versions.
   59: 
   60:   References:
   61: 
   62:   https://tools.ietf.org/html/draft-popov-tls-prohibiting-rc4-01
   63: 
   64: ## Allow BEAST
   65: 
   66:   BEAST is the name of a TLS 1.0 attack that surfaced 2011. When adding means
   67:   to mitigate this attack, it turned out that some broken servers out there in
   68:   the wild didn't work properly with the BEAST mitigation in place.
   69: 
   70:   To make such broken servers work, the --ssl-allow-beast option was
   71:   introduced. Exactly as it sounds, it re-introduces the BEAST vulnerability
   72:   but on the other hand it allows curl to connect to that kind of strange
   73:   servers.
   74: 
   75: ## Disabling certificate revocation checks
   76: 
   77:   Some SSL backends may do certificate revocation checks (CRL, OCSP, etc)
   78:   depending on the OS or build configuration. The --ssl-no-revoke option was
   79:   introduced in 7.44.0 to disable revocation checking but currently is only
   80:   supported for Schannel (the native Windows SSL library), with an exception
   81:   in the case of Windows' Untrusted Publishers blacklist which it seems can't
   82:   be bypassed. This option may have broader support to accommodate other SSL
   83:   backends in the future.
   84: 
   85:   References:
   86: 
   87:   https://curl.haxx.se/docs/ssl-compared.html