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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
88