Rustls: Session Debug formatting exposes TLS master secrets and resumption PSKs

CISA KEV is observed exploitation evidence. FIRST EPSS is a dated prediction of exploitation in the next 30 days. Missing scores are unknown. This collector requests predictions only for explicit CVE aliases in the public NuGet archive; Microsoft release views retain their existing prediction scope.

No collected risk observation for these identifiers. This does not establish absence of exploitation or a zero probability.

### 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](https://github.com/rustls/rustls/blob/99f2358cae2954837dbb866faf6727de75489ab9/SECURITY.md#boundary-public-api). ### 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 [[email protected]](mailto:[email protected]) for the disclosure.

Source published

27 Oct 2026 16:08:47 UTC

Source modified

27 Oct 2026 16:15:05 UTC

Observed here

27 Oct 2026 16:45:46 UTC

Source terms

GitHub Advisory Database · CC-BY-4.0

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · GitHub Advisory Database · CC-BY-4.0. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N

Publisher rating · MODERATE

The `Debug` implementations of `Tls12ClientSessionValue` and `Tls13ClientSessionValue` were derived, and so included the TLS 1.2 master secret or TLS 1.3 resumption PSK in their output. Custom `ClientSessionStore` implementations that format these values with `Debug` (for example, when logging) could disclose these secrets to readers of the logs. This conflicts with rustls's [key-material redaction policy](https://github.com/rustls/rustls/blob/99f2358cae2954837dbb866faf6727de75489ab9/SECURITY.md#boundary-public-api). Ordinary connection and built-in session cache `Debug` output is redacted and is not affected.

Source published

14 Oct 2026 12:00:00 UTC

Source modified

14 Oct 2026 13:30:02 UTC

Observed here

24 Oct 2026 13:19:54 UTC

Source terms

OSV · upstream source license applies

Original source record · Cite this exact observation · Complete original publisher JSON

This view projects the publisher's source fields for reading; the original JSON retains additional fields and CVE enrichment containers. Attribution: osv · OSV · upstream source license applies. Source text is retained; field grouping and change labels are provided by ChangeIntel.

CVSS_V3 · CVSS:3.1/AV:L/AC:H/PR:H/UI:R/S:U/C:H/I:N/A:N