set TLS option CURLSSLOPT_REVOKE_BEST_EFFORT #115
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The Windows native TLS backend (Schannel) makes synchronous certificate revocation checks against a CRL server. For users behind a firewall, this server may be unreachable, causing the TLS connection to fail. The CURLSSLOPT_REVOKE_BEST_EFFORT option addresses precisely this situation, configuring Schannel to make a best effort revocation check but allowing the connection if the CRL server cannot be reached, as long as the certificate isn't already known to be revoked. This behavior matches the default revocation checking behavior on macOS (asynchronous best effort) and is strictly more secure than Linux where no CRL checking is done. Since the typical advice in such situations is to disable TLS host verification entirely, this is an improvement in that with this option, so long as the client's system CA roots are configured correctly, host verification will work and at least local MITM attacks are prevented.
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I'm just going to merge this and hope that it improves things since it seems unlikely that anyone in this situation (Windows & strict firewall) will test this change. 🤞🏻 |
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The Windows native TLS backend (Schannel) makes synchronous certificate revocation checks against a CRL server. For users behind a firewall, this server may be unreachable, causing the TLS connection to fail. The CURLSSLOPT_REVOKE_BEST_EFFORT option addresses precisely this situation, configuring Schannel to make a best effort revocation check but allowing the connection if the CRL server cannot be reached, as long as the certificate isn't already known to be revoked. This behavior matches the default revocation checking behavior on macOS (asynchronous best effort) and is strictly more secure than Linux where no CRL checking is done. Since the typical advice in such situations is to disable TLS host verification entirely, this is an improvement in that with this option, so long as the client's system CA roots are configured correctly, host verification will work and at least local MITM attacks are prevented. (cherry picked from commit 4e87640)
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The Windows native TLS backend (Schannel) makes synchronous certificate revocation checks against a CRL server. For users behind a firewall, this server may be unreachable, causing the TLS connection to fail. The
CURLSSLOPT_REVOKE_BEST_EFFORToption addresses precisely this situation, configuring Schannel to make a best effort revocation check but allowing the connection if the CRL server cannot be reached, as long as the certificate isn't already known to be revoked. This behavior matches the default revocation checking behavior on macOS (asynchronous best effort) and is strictly more secure than Linux where no CRL checking is done.Since the typical advice in such situations is to disable TLS host verification entirely, this is an improvement in that with this option, so long as the client's system CA roots are configured correctly, host verification will work and at least local MITM attacks are prevented.
See this discussion where several Windows uses behind firewalls reported this issue.