Potentially observable differences in RSA premaster secret handling

When receiving a message from a client that sent an invalid RSA premaster secret, an issue in s2n-tls results in the server performing additional processing when the premaster secret contains an incorrect client hello version. While no practical attack on s2n-tls has been demonstrated, this causes a small timing difference which could theoretically be used as described in the Marvin Attack [1]. We would like to thank Hubert Kario [2] for reporting this issue. ### Impact The extent of this issue is a timing difference. No practical attack on s2n-tls has been demonstrated. This issue affects server applications that permit RSA key exchange. Applications that use the default, built-in blinding feature or properly implement self-service blinding are not affected. Impacted versions: <= v1.4.15. ### Patches The patch is included in v1.4.16 [3]. ### Workarounds Applications can work around this issue by using an s2n-tls security policy that disallows RSA key exchange. If you have any questions or comments about this advisory we ask that you contact AWS/Amazon Security via our vulnerability reporting page [4] or directly via email to [aws-security@amazon.com](mailto:aws-security@amazon.com). Please do not create a public GitHub issue. [1] https://people.redhat.com/~hkario/marvin/ [2] https://github.com/tomato42 [3] https://github.com/aws/s2n-tls/releases/tag/v1.4.16 [4] https://aws.amazon.com/security/vulnerability-reporting

Severity from

GitHub (reviewed advisory)

• github.com/aws/s2n-tls/security/advisories/GHSA-52xf-5p2m-9wrv
• github.com/aws/s2n-tls