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Low, CVSS 2.3amended

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

GHSA-vp88-798v-h67f

What to do

Vendor fix available. Upgrade each affected package to the fixed version on its release line (rustls 0.24.0-dev.2, 0.23.46), redeploy, then verify what is actually installed.

This page has no inventory attached, so it cannot say whether you are affected. A vendor fix is not proof that your deployment is fixed.

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Affected packages

  • crates.io rustls

    Affected
    >= 0.23.44, < 0.23.46>= 0.24.0-dev.1, < 0.24.0-dev.2
    Fixed in
    0.24.0-dev.2, 0.23.46

Sources

Severity and exploitation
Exploitation
No confirmed exploitation recorded. Patch Next marks exploitation only from CISA KEV.
CVSS
2.3 · CVSS:3.1/AV:L/AC:L/PR:H/UI:N/S:U/C:L/I:N/A:N
EPSS
No dated EPSS signal available.
Source description

As published by the source; long descriptions are shortened.

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…

Dates and revisions
Published by the source
Last modified by the source
Source last checked
  1. Revision 3 · amended ·

    Amended from osv record GHSA-vp88-798v-h67f

  2. Revision 2 · amended ·

    Amended from ghsa record GHSA-vp88-798v-h67f

  3. Revision 1 · created ·

    Published from osv record RUSTSEC-2026-0347

Agent brief

Preview the exact text before you copy it. Commands are read-only suggestions; review them before running anything.

# Patch Next advisory brief: GHSA-vp88-798v-h67f

Advisory: rustls: ECH rejection can expose buffered application data to authenticated outer server before `ech_required` alert
IDs: GHSA-vp88-798v-h67f
Record: https://patchnext.com/advisories/rustsec-2026-0347 (revision 3, source checked 28 Oct 2026, 06:30 UTC)
Inventory: none attached. This brief comes from a public page and does not say whether any deployment is affected.
Severity: Low (CVSS 2.3)
Exploitation: no confirmed exploitation recorded. Patch Next confirms exploitation only from CISA KEV.
Vendor response: Vendor fix available; fixed versions by package: rustls 0.24.0-dev.2, 0.23.46. A vendor fix does not mean a deployment is fixed.

## Affected components
- crates.io rustls: affected >= 0.23.44, < 0.23.46; >= 0.24.0-dev.1, < 0.24.0-dev.2; fixed in 0.24.0-dev.2, 0.23.46

## Suggested inspection (read-only)
- crates.io rustls: check the installed version with your package manager.

## Supported action
Upgrade each affected component to a fixed version on its release line. For a transitive dependency, upgrade the parent package or add an override or constraint, then regenerate the lockfile. Redeploy every environment that ran an affected version.

## Required regression checks
- Run the test suite and a production build.
- Exercise the features that depend on rustls.
- Repeat the inspection and confirm that only fixed versions remain installed.

## Rollback considerations
- Keep the previous lockfile and deployment artifact. Rolling back reinstates the vulnerable version, so apply any documented mitigation while you fix forward.

## Unresolved questions
- Which deployments run an affected version?
- Exploitation status can change. Check again before lowering the priority.
- Mark the work resolved only after the fixed version is verified in production.

## Sources
- Verified: https://osv.dev/vulnerability/RUSTSEC-2026-0347 (OSV record RUSTSEC-2026-0347 lists affected ranges: crates.io rustls >=0.23.44 <0.23.46 (fixed))
- Verified: https://github.com/advisories/GHSA-vp88-798v-h67f (GitHub-reviewed advisory GHSA-vp88-798v-h67f lists affected ranges: crates.io rustls >=0.23.44 =0.24.0-dev.1 <0.24.0-dev.2)
- Verified: https://osv.dev/vulnerability/GHSA-vp88-798v-h67f (OSV record GHSA-vp88-798v-h67f lists affected ranges: crates.io rustls >=0.23.44 =0.24.0-dev.1 <0.24.0-dev.2 (fixed))

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