AIKIDO-2026-409684
rustls is vulnerable to Observable Discrepancy
33
Low Risk
This Affects:
rustlsTL;DR
rustls provides Encrypted Client Hello (ECH) client support in rustls/src/client/ech.rs, which pads the inner ClientHello so its length does not reveal the protected inner server name. Before the fix, the inner-hello name padding was derived from the configured inner name even when the SNI extension was omitted, and the RFC-recommended padding scheme for SNI names was implemented incorrectly. As a result the encrypted inner ClientHello length could vary with the inner name, letting a passive network observer infer the length of the protected inner name and weakening the privacy ECH is meant to provide. The fix bases padding on the actual inner ClientHello server_name extension and corrects the padding scheme so the inner hello length is invariant with respect to the inner name.
Who does this affect?
You are affected if you are using a version that falls within the vulnerable range and your client uses Encrypted Client Hello (ECH).
Background info
rustls is vulnerable to Observable Discrepancy in versions 0.23.10 - 0.23.39.
How to fix this
Upgrade the rustls library to the patch version.
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SOC 2Compliant
ISO 27001Compliant