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Vulnerabilities

Vulnerability · GHSA-vp88-798v-h67f

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

Published Oct 27, 2026 · catalog updated Oct 27, 2026 · rustls: ECH rejection can

Last checked Oct 28, 2026, 11 days ago.

Open the advisory

Exploitation likelihood is not on this record for GHSA-vp88-798v-h67f. A missing EPSS score is not a low risk. Compare rustls: ECH rejection can with the linked advisory before you change production.

Gaps on this record

  • The stored product name is a package slug. The name on this page is taken from the title (rustls: ECH rejection can).
  • No vendor fix or required action is stored. Compare the linked advisory before you change production.
  • EPSS is missing. A missing score is not a low risk.
Severity
Low2.3
Known exploitation
Not in CISA/VulnCheck KEV
EPSS
Awaiting synchronization with FIRST
Remediation
Version ranges on file
Affected products
crates.io rustls · rust rustls

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 . At the end of the handshake, encrypts and queues any plaintext buffered by . It does this *before* checking for and queueing the alert. The application then receives , as expected. However, the client is still expected to call 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 , 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 . 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 , 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 . GREASE ECH is not affected. - They write application data before the handshake completes. - They connect to a server that rejects their ECH c

Are you affected?

Enter the version you run. The result uses only the ranges stored on this record.

The check returns affected, not affected, or incomplete. It does not guess a version that is missing from the record.

  • crates.io rustls0.23.44 → 0.23.46
  • crates.io rustls0.24.0-dev.1 → 0.24.0-dev.2
  • rust rustls>= 0.23.44, < 0.23.46
  • rust rustls>= 0.24.0-dev.1, < 0.24.0-dev.2

How to fix GHSA-vp88-798v-h67f

What to do

Three questions, each answered from this record only. Confirm the vendor advisory before you patch.

Should you patch?

Exploitation likelihood is not on this record for GHSA-vp88-798v-h67f. A missing EPSS score is not a low risk. Compare rustls: ECH rejection can with the linked advisory before you change production.

Which versions are affected?

Compare what you have installed with 0.23.44 → 0.23.46, 0.24.0-dev.1 → 0.24.0-dev.2, >= 0.23.44, < 0.23.46, >= 0.24.0-dev.1, < 0.24.0-dev.2. Compare the installed version with the linked vendor advisory before you change production.

What should an analyst verify?

CWE-200: Exposure of Sensitive Information. EPSS is not on this record yet. There is no CISA KEV listing. No Exploit-DB or Metasploit reference is linked. Use the source links for the original advisory.

Code

The project this record applies to. Exploit listings stay in References.

Timeline

Detection, updates, and resolution for this record.

  1. Oct 27, 2026 · GitHub Advisory Database

    Detected

    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 . At the end of the handshake, encrypts and queues any plaintext buffered

  2. Oct 27, 2026 · GitHub Advisory Database

    Updated

    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 . At the end of the handshake, encrypts and queues any plaintext buffered

  3. Oct 27, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46; rustls fixed in 0.24.0-dev.2

  4. Oct 27, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.23.46

  5. Oct 27, 2026 · GitHub Advisory Database

    Fix available

    rustls fixed in 0.24.0-dev.2

  6. Oct 27, 2026 · GitHub Advisory Database

    Updated

    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 . At the end of the handshake, encrypts and queues any plaintext buffered

Sources

References

Freshness

Last verified in this catalog Oct 27, 2026. Significant claims (KEV, EPSS, CVSS, affected ranges) come from the source rows above — open the original publisher for the authoritative advisory.

  • GitHub Advisory DatabaseOct 28, 2026
  • OpenSSF OSV / OSV ScannerOct 28, 2026