aws-kms-tls-auth vulnerable to memory overallocation

Summary

### Summary aws-kms-tls-auth is an optional utility for s2n-tls that enables customers to use AWS KMS keys as part of the PSK extension field in a TLS 1.3 handshake. An issue exists in this library that can lead to overallocation of memory potentially resulting in a denial of service.

### Impact The PSK extension field in TLS 1.3 uses length-prefixed lists to encode variable-length data. aws-kms-tls-auth interprets the length prefix incorrectly resulting in additional memory allocation.

s2n-tls limits ClientHello messages to 64 KB. Due to this issue, the server may allocate up to 10× the received size, meaning a single message could trigger an allocation of up to 640 KB. Repeated allocations of this size could exhaust server resources and lead to a denial-of-service.

No AWS services are affected. Applications should continue to follow best practices by limiting the number of in-flight handshakes and concurrent connections. Applications using the aws-kms-tls-auth crate should upgrade to version 0.0.3.

**Impacted versions:** < 0.0.3

### Patches This issue has been addressed in aws-kms-tls-auth v0.0.3 [1].

### Workarounds There is no workaround. Applications using aws-kms-tls-auth should upgrade to the most recent release.

### Acknowledgement s2n-tls would like to thank Joshua Rogers (https://joshua.hu/) of AISLE Research Team (https://aisle.com/) for collaborating on this issue through the coordinated disclosure process.

If there are any questions or comments about this advisory, contact AWS/Amazon Security via the vulnerability reporting page [2] or directly via email to [aws-security@amazon.com](mailto:aws-security@amazon.com). Please do not create a public GitHub issue.

[1] https://crates.io/crates/aws-kms-tls-auth/0.0.3 [2] Vulnerability reporting page: https://aws.amazon.com/security/vulnerability-reporting