RUSTSEC-2026-0347: Buffered application data sent to outer server after ECH rejection
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Assessment
- Difficulty
- 2/5
- Estimated time
- 1-3 hours
- Newbie friendliness
- 55/100
- Issue type
- Bug
- Clarity
- Mostly clear
- Activity status
- Active
- Tech stack
- rust
- Domain
- networking, security
Research direction
The fix is a dependency bump: the rustls version in Cargo.toml and Cargo.lock must resolve to 0.23.46 or later. Start with the rustls entries in Cargo.toml and Cargo.lock, then run cargo update -p rustls and cargo test. Done when the lockfile pins a patched rustls and the build and tests pass. Check the advisory page for any API changes that affect ECH handling in this crate.
Written by the indexing model from the issue text.
Description
Buffered application data sent to outer server after ECH rejection
| Details | |
|---|---|
| Package | rustls |
| Version | 0.23.45 |
| URL | https://github.com/rustls/rustls/pull/3300 |
| Date | 2026-10-14 |
| Patched versions | >=0.23.46 |
| Unaffected versions | <0.23.44 |
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 public_name. At the end of that handshake, rustls encrypted and
queued any application data buffered via ClientConnection::writer() before
queueing the ech_required alert.
The application correctly receives
Error::PeerIncompatible(ServerRejectedEncryptedClientHello), but the next
write_tls() 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 public_name.
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 ech_required "before sending any application data to the server".
This affects only clients that:
- use
EchMode::Enable(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 public_name,
so certificate verification failed first and no data was sent.
See advisory page for additional details.
- Dominant language
- Rust
- Stars
- 1
- Forks
- 0
- Avg merge
- 1d 15h
- Merged PRs (30d)
- 3
Getting set up
- No Dockerfile or Docker Compose file
- Has a pull request template
- Read the contributing guide
First steps
- Read the whole issue, then the project's contributing guide.
- Comment on the issue to say you are picking it up — it saves two people doing the same work.
- Fork the repository and make your change on a branch.
- Open a pull request that references the issue number.
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