GHSA-vp88-798v-h67f: Infoleak
In rustls 0.23.44 and 0.23.45, application data that a client buffers before the handshake completes is sent to the server even when the server rejects Encrypted Client Hello (ECH).
When the server rejects ECH, the client completes the handshake with the outer ClientHello and authenticates the server against the ECH config's publicname. At the end of the handshake, starttraffic() encrypts and queues any plaintext buffered by ClientConnection::writer(). It does this before checking for EchStatus::Rejected and queueing the echrequired alert.
The application then receives Error::PeerIncompatible(ServerRejectedEncryptedClientHello), as expected. However, the client is still expected to call writetls() to send the fatal alert, and that call also sends the encrypted application data. The server decrypts the data and only then receives the alert.
The ordering bug is older than 0.23.44. Before 0.23.44, the client checked the server certificate against the inner server name rather than publicname, so a correctly configured server failed certificate verification first and no data was sent. 0.23.44 fixed that check, which makes the ordinary rejection path reachable.
Impact
Plaintext meant for the inner server, the one named in the encrypted ClientHello, goes to the client-facing server identified by publicname. That server is properly authenticated, so this is not an attack by an unauthenticated party. However this violates RFC 9849, which says the following:
- Section 6.1.6: if the handshake and authentication succeed after ECH rejection, the client "MUST ... abort the connection with an "echrequired" alert before sending any application data to the server." - Section 6.1.7: the certificate is checked only against publicname, and the connection "MUST NOT" be reported as successful to the application. rustls does report failure correctly. The problem is only that data is sent before the alert.
This affects only clients that meet all of these conditions:
- They use EchMode::Enable. GREASE ECH is not affected. - They write application data before the handshake completes. - They connect to a server that rejects their ECH config, for example because the config is stale. This is the same case where the server returns retry configs.
The usual client response to rejection is to retry with the server's retry configs. In that case, the data has already gone to the outer server.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade
rust/rustlsto a version that resolves this vulnerability.Fixed in 0.24.0-dev.2 - Upgrade
Upgrade
rust/rustlsto a version that resolves this vulnerability.Fixed in 0.23.46
Event History
Frequently Asked Questions
Which deployments are exposed to this behavior?
Clients using rustls 0.23.44 or 0.23.45 are affected when they buffer application data through ClientConnection::writer() before the handshake completes and the server rejects ECH. The issue is reached on the ECH rejection path after the client authenticates the server using the ECH configuration's public_name.
What must happen for buffered application data to reach the server?
After receiving Error::PeerIncompatible(ServerRejectedEncryptedClientHello), the application must call write_tls() to send the required fatal alert. That call can send the already encrypted, queued application data before the alert, allowing the server to decrypt the data first.
How can I assess whether an application may have exposed data?
Check whether it uses rustls 0.23.44 or 0.23.45, writes application data before handshake completion, and continues by calling write_tls() after an ECH rejection. Those conditions indicate that pre-handshake buffered plaintext may have been transmitted to the rejecting server.