GHSA-c8m5-c952-6cm4: Medium severity rust/rustls vulnerability

Q: Are applications using rustls's built-in client session cache exposed?

Ordinary connections and built-in cache diagnostics remain redacted. The described exposure applies to custom ClientSessionStore implementations that handle and log session values.

Q: What must happen for secret material to be disclosed?

A custom session store must use a Tls12ClientSessionValue or Tls13ClientSessionValue in a way that invokes its Debug implementation, such as debug logging or derived formatting. Readers of the resulting logs could obtain a TLS 1.2 master secret or TLS 1.3 resumption PSK.

Q: How can teams identify whether they are affected before patching?

Review custom ClientSessionStore implementations and their logging paths for Debug formatting of session-value objects. Avoid formatting or logging these sensitive values, as the session-store interface already warns that they contain sensitive data.

Published Oct 27, 2026

Updated

Summary

Formatting a client session value with Debug exposes its TLS 1.2 master secret or TLS 1.3 resumption PSK. A custom session store that logs these values can disclose keys to readers of the logs. Ordinary connection and built-in cache diagnostics remain redacted, and the session-store interface already warns that its values are sensitive.

Details

Derived formatters traverse the session's secret-bearing fields. This conflicts with the project's explicit key-material redaction policy.

Proof of concept

Custom ClientSessionStore implementations could leak secret data by using the provided Tls12ClientSessionValue or Tls13ClientSessionValue in a way that triggers its implementation of the Debug trait.

Impact

Such store implementations could leak the master secret for the TLS connection.

Acknowledgement Thanks to OpenAI OutboundDisclosures@openai.com for the disclosure.

Affected Software

2 affected componentsFixes available

rust/rustls>=0.24.0-dev.1<0.24.0-dev.2

0.24.0-dev.2

rust/rustls<0.23.46

0.23.46

Remediation

Recommended actions to resolve this vulnerability, in priority order.

Upgrade
Upgrade rust/rustls to a version that resolves this vulnerability.
Fixed in 0.24.0-dev.2
Upgrade
Upgrade rust/rustls to a version that resolves this vulnerability.
Fixed in 0.23.46

Event History

Oct 27, 2026

Advisory Published

via GitHub·04:08 PM

Data Sourced

via GitHub·04:08 PM

DescriptionSeverityWeaknessAffected Software

Frequently Asked Questions

Are applications using rustls's built-in client session cache exposed?

Ordinary connections and built-in cache diagnostics remain redacted. The described exposure applies to custom ClientSessionStore implementations that handle and log session values.

What must happen for secret material to be disclosed?

A custom session store must use a Tls12ClientSessionValue or Tls13ClientSessionValue in a way that invokes its Debug implementation, such as debug logging or derived formatting. Readers of the resulting logs could obtain a TLS 1.2 master secret or TLS 1.3 resumption PSK.

How can teams identify whether they are affected before patching?

Review custom ClientSessionStore implementations and their logging paths for Debug formatting of session-value objects. Avoid formatting or logging these sensitive values, as the session-store interface already warns that they contain sensitive data.

GHSA-c8m5-c952-6cm4: Medium severity rust/rustls vulnerability

Affected Software

Remediation

Event History

Frequently Asked Questions

Are applications using rustls's built-in client session cache exposed?

What must happen for secret material to be disclosed?

How can teams identify whether they are affected before patching?

Company

Resources

Contact

Frequently Asked Questions

Are applications using rustls's built-in client session cache exposed?

What must happen for secret material to be disclosed?

How can teams identify whether they are affected before patching?