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Response snapshot Oct 27, 2026, 17:26 UTC

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VulnerabilityGHSA-vp88-798v-h67f

rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert

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).

At a glance

Exploited
Not known
Severity
Not scored yet
Chance of exploitation
Not estimated
Sources
1 source

What to do

GitHub lists first patched versions: rustls: 0.23.46; rustls: 0.24.0-dev.2. Confirm the affected range and upgrade guidance in the exact advisory.

Read GitHub-reviewed advisory GHSA-vp88-798v-h67f (opens a new tab)

What happened

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.

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# 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.

Affected package ranges reported by GitHub: rustls: >= 0.23.44, < 0.23.46 rustls: >= 0.24.0-dev.1, < 0.24.0-dev.2

Related technology

Sources

  1. Primary source

    GitHub-reviewed advisory GHSA-vp88-798v-h67f (opens a new tab)

    The public advisory supplies the description, affected package/version ranges, and any listed CVSS metric. GitHub review is not local review.

    Published Checked

  2. Primary source

    GitHub public advisory API page (opens a new tab)

    The API page supplied GHSA-vp88-798v-h67f, its actual identifiers, source dates, and structured metrics.

    Published Date not suppliedChecked

Cite this record

For factual claims, cite the primary sources above; cite KEVWire for its compilation, dates, and classifications.

APA

KEVWire. (2026, October 27). rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert. https://kevwire.com/record/GHSA-vp88-798v-h67f

MLA

"rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert." KEVWire, 27 Oct. 2026, kevwire.com/record/GHSA-vp88-798v-h67f.

Chicago

KEVWire. "rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert." Last modified October 27, 2026. https://kevwire.com/record/GHSA-vp88-798v-h67f.

Evidence summary (plain text)

rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert (Oct 27, 2026).
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.
Sources:
GitHub-reviewed advisory GHSA-vp88-798v-h67f: https://github.com/advisories/GHSA-vp88-798v-h67f
GitHub public advisory API page: https://api.github.com/advisories?type=reviewed&per_page=100&sort=updated&direction=desc
Record ID: GHSA-vp88-798v-h67f.
Record origin: Ingested record.
AI evidence: Not assessed.
Known exploitation: Not assessed.
Zero-day status: Not assessed. KEV membership alone does not establish zero-day status.
About this data

This record was ingested from a data source. Retrieved: Oct 27, 2026, 17:25 UTC. Ingestion is not an independent editorial investigation.

A CISA KEV listing documents known exploitation, not necessarily a zero-day; absence from the catalog does not prove a vulnerability has never been exploited.

Technology tags indicate possible relevance, not verified exposure. Confirm the affected product, version, and configuration against your own inventory; KEVWire does not scan your systems.

A personal reading checklist on this device, not verified remediation.

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