CVE-2026-16317 and CVE-2026-16318: Issues with s2n-tls: an open-source implementation of the TLS/SSL protocols

Bulletin ID: 2026-062-AWS Scope: AWS Content Type: Important (requires attention) Publication Date: 07/21/2026 13:15 PM PDT Description: s2n-tls is an open source C99 implementation of the TLS/SSL protocol. We have identified two distinct issues: - CVE-2026-16317: Silent Drop of TLS 1.3 Encrypted Records in s2n-tls Missing validation of the outer content_type byte on TLS 1.3 encrypted records in s2n-tls allows an active man-in-the-middle to silently drop individual application data records without either endpoint detecting the modification. RFC 8446 Section 5.2 requires that the outer content_type of all encrypted TLS 1.3 records must be application_data (0x17). The s2n-tls AEAD implementation hardcodes this value in the additional authenticated data rather than using the actual wire byte, so the outer content_type is not covered by the authentication tag. All TLS 1.3 connections are affected. Both TLS clients and servers are affected. TLS 1.2 and QUIC connections are not affected. - CVE-2026-16318: QUIC Transport Parameters Memory Leak During HelloRetryRequest in s2n-tls Incorrect use of s2n_alloc instead of s2n_realloc in the QUIC transport parameters extension handler in s2n-tls causes one memory allocation to be leaked each time a QUIC-enabled TLS 1.3 connection goes through a HelloRetryRequest. This can occur during normal handshakes when a client offers a key share group the server does not prefer, and can lead to increased memory consumption on long-running server processes. Only QUIC-enabled deployments are affected. Impacted versions:
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