Add the old github keys as revoked

The patch to update the bundled ssh github host key did not
change anything for users who already had connected to github
one time before via ssh: if the attacker had access to the old
key, they'd be vulnerable to MITM attacks as their known_hosts
file would list the old github key. Only if they connected
again to github without attacker access, or if they saw the
announcement of the key rotation, they would update their key.

There is sadly no other way to distribute revocations of old host
keys to clients other than to bundle them with client software.

Author: est31

src/cargo/sources/git/known_hosts.rs

@@ -46,6 +46,20 @@ static BUNDLED_KEYS: &[(&str, &str, &str)] = &[
     ("github.com", "ssh-rsa", "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"),
 ];
 
+/// List of keys that public hosts have rotated away from.
+///
+/// We explicitly distrust these keys as users with the old key in their
+/// local configuration will otherwise be vulnerable to MITM attacks if the
+/// attacker has access to the old key. As there is no other way to distribute
+/// revocations of ssh host keys, we need to bundle them with the client.
+///
+/// Unlike [`BUNDLED_KEYS`], these revocations will not be ignored if the user
+/// has their own entries: we *know* that these keys are bad.
+static BUNDLED_REVOCATIONS: &[(&str, &str, &str)] = &[
+    // Used until March 24, 2023: https://github.blog/2023-03-23-we-updated-our-rsa-ssh-host-key/
+    ("github.com", "ssh-rsa", "AAAAB3NzaC1yc2EAAAABIwAAAQEAq2A7hRGmdnm9tUDbO9IDSwBK6TbQa+PXYPCPy6rbTrTtw7PHkccKrpp0yVhp5HdEIcKr6pLlVDBfOLX9QUsyCOV0wzfjIJNlGEYsdlLJizHhbn2mUjvSAHQqZETYP81eFzLQNnPHt4EVVUh7VfDESU84KezmD5QlWpXLmvU31/yMf+Se8xhHTvKSCZIFImWwoG6mbUoWf9nzpIoaSjB+weqqUUmpaaasXVal72J+UX2B+2RPW3RcT0eOzQgqlJL3RKrTJvdsjE3JEAvGq3lGHSZXy28G3skua2SmVi/w4yCE6gbODqnTWlg7+wC604ydGXA8VJiS5ap43JXiUFFAaQ=="),
+];
+
 enum KnownHostError {
     /// Some general error happened while validating the known hosts.
     CheckError(anyhow::Error),
@@ -357,6 +371,16 @@ fn check_ssh_known_hosts(
             });
         }
     }
+    for (patterns, key_type, key) in BUNDLED_REVOCATIONS {
+        let key = STANDARD.decode(key).unwrap();
+        known_hosts.push(KnownHost {
+            location: KnownHostLocation::Bundled,
+            patterns: patterns.to_string(),
+            key_type: key_type.to_string(),
+            key,
+            line_type: KnownHostLineType::Revoked,
+        });
+    }
     check_ssh_known_hosts_loaded(&known_hosts, host, remote_key_type, remote_host_key)
 }