# GHSA-vp88-798v-h67f: rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert - **Severity:** Not scored - **Exploited in the wild:** Not known - **Fix available:** Yes - **Published:** 14 Oct 2026 - **Also known as:** RUSTSEC-2026-0347 ## What is GHSA-vp88-798v-h67f? GHSA-vp88-798v-h67f is a vulnerability in rustls, published on 14 Oct 2026. ### Official description 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 `public_name`. At the end of the handshake, `start_traffic()` encrypts and queues any plaintext buffered by `ClientConnection::writer()`. It does this *before* checking for `EchStatus::Rejected` and queueing the `ech_required` alert. The application then receives `Error::PeerIncompatible(ServerRejectedEncryptedClientHello)`, as expected. However, the client is still expected to call `write_tls()` 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 `public_name`, 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 `public_name`. That server is properly authenticated, so this is not an attack by an unauthenticated party. However this violates [RFC 9849](https://www.rfc-editor.org/info/rfc9849), 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 "ech_required" alert **before sending any application data to the server.**" - Section 6.1.7: the certificate is checked only against `public_name`, 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. ## Which versions are affected by GHSA-vp88-798v-h67f? 5 open source packages ship an affected version, including rustls. | Package | Ecosystem | Affected | Fixed in | | --- | --- | --- | --- | | `rustls` | crates.io | From 0.23.44 | 0.23.46 | | `rustls` | crates.io | From 0.23.44 | 0.23.46 | | `rustls` | crates.io | From 0.23.44 | 0.23.46 | | `rustls` | crates.io | From 0.24.0-dev.1 | 0.24.0-dev.2 | | `rustls` | crates.io | From 0.24.0-dev.1 | 0.24.0-dev.2 | ## How do I fix GHSA-vp88-798v-h67f? Upgrade rustls to a fixed version for your release line: 0.23.46 or 0.24.0-dev.2. ## Is GHSA-vp88-798v-h67f being exploited? There is no known exploitation of GHSA-vp88-798v-h67f in the wild and no public exploit code. ## Fix prompt for AI coding agents ```text Check this codebase for GHSA-vp88-798v-h67f and fix it if it is affected. What it is: GHSA-vp88-798v-h67f is a vulnerability in rustls, published on 14 Oct 2026. Exploitation: There is no known exploitation of GHSA-vp88-798v-h67f in the wild and no public exploit code. Affected packages and fixed versions: - rustls (crates.io): affected from 0.23.44; fixed in 0.23.46 - rustls (crates.io): affected from 0.23.44; fixed in 0.23.46 - rustls (crates.io): affected from 0.23.44; fixed in 0.23.46 - rustls (crates.io): affected from 0.24.0-dev.1; fixed in 0.24.0-dev.2 - rustls (crates.io): affected from 0.24.0-dev.1; fixed in 0.24.0-dev.2 Task: 1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies. 2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it. 3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead. 4. Run the tests and summarise what changed and why. 5. If nothing in the project is affected, say so and change nothing. Reference: https://insights.sbomify.com/GHSA-vp88-798v-h67f/ (Markdown: https://insights.sbomify.com/GHSA-vp88-798v-h67f/index.md) ``` ## Sources - OSV: https://osv.dev/vulnerability/GHSA-vp88-798v-h67f --- From sbomify Insights: https://insights.sbomify.com/GHSA-vp88-798v-h67f/ Summaries are generated and may contain mistakes; the official description above is the record.