98 lines
4.0 KiB
Markdown
98 lines
4.0 KiB
Markdown
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# SSL problems
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First, let's establish that we often refer to TLS and SSL interchangeably as
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SSL here. The current protocol is called TLS, it was called SSL a long time
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ago.
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There are several known reasons why a connection that involves SSL might
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fail. This is a document that attempts to detail the most common ones and
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how to mitigate them.
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## CA certs
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CA certs are used to digitally verify the server's certificate. You need a
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"ca bundle" for this. See lots of more details on this in the `SSLCERTS`
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document.
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## CA bundle missing intermediate certificates
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When using said CA bundle to verify a server cert, you may experience
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problems if your CA store does not contain the certificates for the
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intermediates if the server does not provide them.
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The TLS protocol mandates that the intermediate certificates are sent in the
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handshake, but as browsers have ways to survive or work around such
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omissions, missing intermediates in TLS handshakes still happen that browser
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users do not notice.
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Browsers work around this problem in two ways: they cache intermediate
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certificates from previous transfers and some implement the TLS "AIA"
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extension that lets the client explicitly download such certificates on
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demand.
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## Protocol version
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Some broken servers fail to support the protocol negotiation properly that
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SSL servers are supposed to handle. This may cause the connection to fail
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completely. Sometimes you may need to explicitly select a SSL version to use
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when connecting to make the connection succeed.
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An additional complication can be that modern SSL libraries sometimes are
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built with support for older SSL and TLS versions disabled!
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All versions of SSL and the TLS versions before 1.2 are considered insecure
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and should be avoided. Use TLS 1.2 or later.
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## Ciphers
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Clients give servers a list of ciphers to select from. If the list does not
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include any ciphers the server wants/can use, the connection handshake
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fails.
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curl has recently disabled the user of a whole bunch of seriously insecure
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ciphers from its default set (slightly depending on SSL backend in use).
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You may have to explicitly provide an alternative list of ciphers for curl
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to use to allow the server to use a weak cipher for you.
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Note that these weak ciphers are identified as flawed. For example, this
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includes symmetric ciphers with less than 128 bit keys and RC4.
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Schannel in Windows XP is not able to connect to servers that no longer
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support the legacy handshakes and algorithms used by those versions, so we
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advise against building curl to use Schannel on really old Windows versions.
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Reference: [Prohibiting RC4 Cipher
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Suites](https://datatracker.ietf.org/doc/html/draft-popov-tls-prohibiting-rc4-01)
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## Allow BEAST
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BEAST is the name of a TLS 1.0 attack that surfaced 2011. When adding means
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to mitigate this attack, it turned out that some broken servers out there in
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the wild did not work properly with the BEAST mitigation in place.
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To make such broken servers work, the --ssl-allow-beast option was
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introduced. Exactly as it sounds, it re-introduces the BEAST vulnerability
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but on the other hand it allows curl to connect to that kind of strange
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servers.
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## Disabling certificate revocation checks
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Some SSL backends may do certificate revocation checks (CRL, OCSP, etc)
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depending on the OS or build configuration. The --ssl-no-revoke option was
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introduced in 7.44.0 to disable revocation checking but currently is only
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supported for Schannel (the native Windows SSL library), with an exception
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in the case of Windows' Untrusted Publishers block list which it seems cannot
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be bypassed. This option may have broader support to accommodate other SSL
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backends in the future.
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References:
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https://curl.se/docs/ssl-compared.html
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