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Copyright (C) Daniel Stenberg, <daniel@haxx.se>, et al.

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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 an 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](https://datatracker.ietf.org/doc/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 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
