curl/docs/SSL-PROBLEMS.md

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SSL problems

First, let's establish that we often refer to TLS and SSL interchangeably 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 detail 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.

CA bundle missing intermediate certificates

When using said CA bundle to verify a server cert, you may experience problems if your CA store does not contain the certificates for the intermediates if the server does not provide them.

The TLS protocol mandates that the intermediate certificates are sent in the handshake, but as browsers have ways to survive or work around such omissions, missing intermediates in TLS handshakes still happen that browser users do not notice.

Browsers work around this problem in two ways: they cache intermediate certificates from previous transfers and some implement the TLS "AIA" extension that lets the client explicitly download such certificates on demand.

Protocol 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 explicitly 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!

All versions of SSL and the TLS versions before 1.2 are considered insecure and should be avoided. Use TLS 1.2 or later.

Ciphers

Clients give servers a list of ciphers to select from. If the list does not 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.

Schannel in Windows XP is not able to connect to servers that no longer support the legacy handshakes and algorithms used by those versions, so we advise against building curl to use Schannel on really old Windows versions.

Reference: Prohibiting RC4 Cipher Suites

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 did not 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.

Disabling certificate revocation checks

Some SSL backends may do certificate revocation checks (CRL, OCSP, etc) depending on the OS or build configuration. The --ssl-no-revoke option was introduced in 7.44.0 to disable revocation checking but currently is only supported for Schannel (the native Windows SSL library), with an exception in the case of Windows' Untrusted Publishers block list which it seems cannot be bypassed. This option may have broader support to accommodate other SSL backends in the future.

References:

https://curl.se/docs/ssl-compared.html