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Findings

RUSTSEC-2026-0347

Buffered application data sent to outer server after ECH rejection

Published Oct 14, 2026 · updated Oct 14, 2026

When a server rejects Encrypted Client Hello (ECH), the client completes the handshake using the outer ClientHello and authenticates the server against the ECH config's . At the end of that handshake, rustls encrypted and queued any application data buffered via *before* queueing the alert. The application correctly receives , but the next call (needed to send the alert) also sends the buffered application data. Plaintext meant for the inner server therefore goes to the client-facing server named by . That server is authenticated, so this is not exploitable by an unauthenticated party, but it violates RFC 9849 section 6.1.6, which requires the client to abort with "before sending any application data to the server". This affects only clients that: - use (GREASE ECH is not affected), - write application data before the handshake completes, and - connect to a server that rejects their ECH config (for example, because it is stale). The underlying ordering bug is older, but before 0.23.44 the client checked the server certificate against the inner server name rather than , so certificate verification failed first and no data was sent.

Timeline

Detection, updates, and resolution for this record.

  1. Oct 14, 2026 · GitHub Advisory Database

    Detected

    When a server rejects Encrypted Client Hello (ECH), the client completes the handshake using the outer ClientHello and authenticates the server against the ECH config's . At the end of that handshake, rustls encrypted and queued any application data buffered via *before* queueing the alert. The application correctly receives , but the next call (needed to send the alert) also sends the buffered ap

  2. Oct 14, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46

  3. Oct 14, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46

  4. Oct 14, 2026 · GitHub Advisory Database

    Updated

    When a server rejects Encrypted Client Hello (ECH), the client completes the handshake using the outer ClientHello and authenticates the server against the ECH config's . At the end of that handshake, rustls encrypted and queued any application data buffered via *before* queueing the alert. The application correctly receives , but the next call (needed to send the alert) also sends the buffered ap

Affected

  • crates.io rustls0.23.44 → 0.23.46

Sources

References