schannel: Disable auto credentials; add an option to enable it

- Disable auto credentials by default. This is a breaking change
  for clients that are using it, wittingly or not.

- New libcurl ssl option value CURLSSLOPT_AUTO_CLIENT_CERT tells libcurl
  to automatically locate and use a client certificate for
  authentication, when requested by the server.

- New curl tool options --ssl-auto-client-cert and
  --proxy-ssl-auto-client-cert map to CURLSSLOPT_AUTO_CLIENT_CERT.

This option is only supported for Schannel (the native Windows SSL
library). Prior to this change Schannel would, with no notification to
the client, attempt to locate a client certificate and send it to the
server, when requested by the server. Since the server can request any
certificate that supports client authentication in the OS certificate
store it could be a privacy violation and unexpected.

Fixes https://github.com/curl/curl/issues/2262
Reported-by: Jeroen Ooms
Assisted-by: Wes Hinsley
Assisted-by: Rich FitzJohn

Ref: https://curl.se/mail/lib-2021-02/0066.html
Reported-by: Morten Minde Neergaard

Closes https://github.com/curl/curl/pull/6673

1 files changed, 0 insertions, 14 deletions

diff --git a/docs/TODO b/docs/TODO
index b2b082292..04511bb3b 100644
--- a/docs/TODO
+++ b/docs/TODO
@@ -126,7 +126,6 @@
  15. Schannel
  15.1 Extend support for client certificate authentication
  15.2 Extend support for the --ciphers option
- 15.3 Add option to disable client certificate auto-send
  15.4 Add option to allow abrupt server closure
 
  16. SASL
@@ -861,19 +860,6 @@
  - Specifying Schannel Ciphers and Cipher Strengths
    https://msdn.microsoft.com/en-us/library/windows/desktop/aa380161.aspx
 
-15.3 Add option to disable client certificate auto-send
-
- Microsoft says "By default, Schannel will, with no notification to the client,
- attempt to locate a client certificate and send it to the server." That could
- be considered a privacy violation and unexpected.
-
- Some Windows users have come to expect that default behavior and to change the
- default to make it consistent with other SSL backends would be a breaking
- change. An option should be added that can be used to disable the default
- Schannel auto-send behavior.
-
- https://github.com/curl/curl/issues/2262
-
 15.4 Add option to allow abrupt server closure
 
  libcurl w/schannel will error without a known termination point from the