Schannel regression: CURLOPT_SSL_CIPHER_LIST has no effect on Windows 10 >= 10.0.17763

[zhihaoy]

I did this

Set CURLOPT_SSL_CIPHER_LIST on a connection that uses Schannel, TLS 1.2, and make a connection to an HTTPS server.

I expected the following

Investigate its Hello message, the cipher suites should match my settings.

curl/libcurl version

libcurl 7.85.0 to the latest

operating system

Fails (CURLOPT_SSL_CIPHER_LIST has no effect) on Windows 10 build 17763 and above, such as 10 Enterprise 1809 and 20H1.
Works (CURLOPT_SSL_CIPHER_LIST is in effect) on Windows 10 build 14393, such as Windows Server 2016

Cause analysis

The problem was introduced by 8beff43. The change moved this chunk of code:

curl/lib/vtls/schannel.c

Lines 503 to 510 in 801bd51

	if(SSL_CONN_CONFIG(cipher_list)) {
	result = set_ssl_ciphers(&schannel_cred, SSL_CONN_CONFIG(cipher_list),
	backend->algIds);
	if(CURLE_OK != result) {
	failf(data, "Unable to set ciphers to passed via SSL_CONN_CONFIG");
	return result;
	}
	}

to

curl/lib/vtls/schannel.c

Lines 1006 to 1021 in cd95ee9

	else {
	/* Pre-Windows 10 1809 */
	ALG_ID algIds[NUM_CIPHERS];
	char *ciphers = SSL_CONN_CONFIG(cipher_list);
	SCHANNEL_CRED schannel_cred = { 0 };
	schannel_cred.dwVersion = SCHANNEL_CRED_VERSION;
	schannel_cred.dwFlags = flags;
	schannel_cred.grbitEnabledProtocols = enabled_protocols;
	if(ciphers) {
	result = set_ssl_ciphers(&schannel_cred, ciphers, algIds);
	if(CURLE_OK != result) {
	failf(data, "Unable to set ciphers to passed via SSL_CONN_CONFIG");
	return result;
	}
	}

However, the new block was an else block started way above

curl/lib/vtls/schannel.c

Lines 790 to 792 in cd95ee9

	/* Windows 10, 1809 (a.k.a. Windows 10 build 17763) */
	if(curlx_verify_windows_version(10, 0, 17763, PLATFORM_WINNT,
	VERSION_GREATER_THAN_EQUAL)) {

Then the logic becomes: Since only Windows 10 build 17763 supports TLS 1.3, we configure only TLS 1.3 ciphers on those systems; if we are on a lower version of Windows, we go ahead and configure TLS 1.2 (or lower) ciphers. So the combination of "on a newer version of Windows 10" and "using TLS 1.2" falls into neither branch. Support for CURLOPT_SSL_CIPHER_LIST under that combined condition was lost.

[jay]

I've submitted a fix in PR #10746 which should allow TLS < 1.3 ciphers to be set as long as TLS 1.3 ciphers are not set. I'm not sure if it's possible to combine the two because they use different credential structs and we are only allowed to pass a single credential struct to schannel. The PR is marked as draft for now. If anyone can investigate further that would be helpful.

/cc @wyattoday

[vszakats]

Does this bug mean that when running on Windows 10 1809+, curl (with Schannel) is forcing TLS 1.3 with no option to use TLS 1.2? Or, requiring --tlsv1.2 option to access servers offering no TLS 1.3? Or, not even with an option?

[wyattoday]

Does this bug mean that when running on Windows 10 1809+, curl (with Schannel) is forcing TLS 1.3 with no option to use TLS 1.2? Or, requiring --tlsv1.2 option to access servers offering no TLS 1.3? Or, not even with an option?

Nope, it doesn’t mean that. This is about cipher lists.

See my reply to @jay PR (here: #10746 (comment) ) for my view on this (not a bug) and possible remedies if the consensus is that this is a bug (remedy 3 is probably the most secure of the insecure options).