1 _ _ ____ _ 2 ___| | | | _ \| | 3 / __| | | | |_) | | 4 | (__| |_| | _ <| |___ 5 \___|\___/|_| \_\_____| 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 WinSSL 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 WinSSL 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 WinSSL (the native Windows SSL library), with an exception in 81 the case of Windows' Untrusted Publishers blacklist which it seems can't be 82 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
