Rustls CVE-2026-123609

Exploitation is possible ONLY when the deployment uses a custom ClientSessionStore implementation that logs or Debug-formats Tls12ClientSessionValue or Tls13ClientSessionValue - rustls's built-in connection and cache diagnostics remain redacted, so default configurations are not affected. The affected versions are rustls < 0.23.46 and the pre-release line >= 0.24.0-dev.1 to < 0.24.0-dev.2. Realistic limits on exploitation: the non-default logging code must exist, TLS session resumption/caching must be in use to populate the secret-bearing values, and the attacker needs read access to the logs (local, privileged); per the CVSS vector this is AV:L/AC:H/AT:P/PR:H/UI:P, not remote unauthenticated. The 'attacker' is effectively a log reader rather than a network actor.

All signals point to a genuinely low-to-moderate priority despite the information-disclosure of secret key material. The input CVSS 4.0 base score is 4.1 (medium) and the vector is heavily gated: AV:L (local), AC:H (conditions apply), AT:P (attack requirements present), PR:H (high privileges), UI:P (user/operator interaction), with impact limited to VC:H and VI/VA:N - confidentiality-only. This is NOT in CISA KEV (no active-exploitation signal), and no EPSS score was supplied, so exploitation probability cannot be quantified from this data - state explicitly: EPSS is MISSING. The vuln.today decision label is 'this_month', consistent with routine patching rather than emergency action. Crucially, the leak is not a flaw in default rustls behavior: the advisory states that ordinary connection and built-in cache diagnostics remain redacted, and the exposure occurs only when a developer-written custom ClientSessionStore logs or otherwise Debug-formats the session values - a path the session-store interface already warns is sensitive. The advisory's 'Proof of concept' is a conceptual description of how a custom store could trigger the Debug trait; it is not verified as working exploit code, and there is no KEV/EPSS to contradict the low-urgency reading. Net: a real but narrow issue whose practical risk depends entirely on whether a deployment actually logs these values - most do not.

An application built on a vulnerable rustls version uses a custom ClientSessionStore that logs (or Debug-formats) a Tls12ClientSessionValue or Tls13ClientSessionValue; the derived Debug formatter writes the TLS 1.2 master secret or TLS 1.3 resumption PSK into application logs. An insider or attacker with read access to those logs then extracts the key material to decrypt captured TLS traffic or forge session resumption. The advisory describes a conceptual PoC of this custom-store logging path, but no working exploit code is published, and exploitation only matters where such non-default logging exists.