CVE-2026-16318 in s2n-tls
Summary
by MITRE • 07/22/2026
The QUIC transport parameters extension handler in s2n-tls incorrectly uses s2n_alloc instead of s2n_realloc to store the peer's transport parameters. When a TLS 1.3 connection goes through a HelloRetryRequest, the handler is called twice on the same connection. On the second call, s2n_alloc zeroes the existing pointer before allocating new memory, causing the first allocation to be leaked.
This can occur during normal QUIC traffic when a client offers a key share group the server does not prefer. An unauthenticated user can amplify the issue by deliberately forcing HelloRetryRequests, causing up to approximately 64 KB of unreachable memory per handshake. Over time, this can lead to increased memory consumption on long-running server processes. The unreachable memory is only reclaimed when the process is restarted.
Only server-side QUIC-enabled deployments are affected. Non-QUIC TLS connections are not affected.
We recommend you upgrade s2n-tls to version v1.7.6
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Analysis
by VulDB Data Team • 07/22/2026
The vulnerability identified in s2n-tls affects the QUIC transport parameters extension handler implementation where an improper memory management approach leads to resource leakage during specific TLS 1.3 handshake scenarios. This flaw manifests when a server processes HelloRetryRequest messages as part of the QUIC protocol negotiation, specifically when clients propose key share groups that servers do not prefer. The issue stems from the incorrect usage of s2n_alloc instead of s2n_realloc within the extension handler codepath, creating a classic memory management error pattern.
The technical flaw occurs because the QUIC transport parameters handler is invoked twice during a single connection lifecycle when HelloRetryRequests are processed. During the first invocation, memory is allocated to store peer transport parameters, but upon the second call, s2n_alloc performs an implicit zeroing operation on the existing pointer before proceeding with new allocation. This destructive behavior results in the original memory block becoming unreferenced and unreachable, creating a memory leak that persists throughout the server process lifecycle. The vulnerability operates at the protocol implementation level within the TLS 1.3 QUIC extension handling code.
The operational impact of this vulnerability extends beyond simple memory consumption issues to potentially affect server stability and performance over extended periods. When an unauthenticated attacker deliberately triggers HelloRetryRequests, they can amplify the memory leak to approximately 64 KB per handshake cycle, creating a significant resource exhaustion vector. This amplification factor makes the vulnerability particularly concerning for long-running server processes that handle substantial connection volumes. The unreachable memory blocks remain allocated until process restart, leading to progressive memory degradation and potential service disruption. The vulnerability aligns with CWE-401: Improper Release of Memory and demonstrates characteristics consistent with ATT&CK technique T1499.003: Endpoint Resource Exhaustion - Network Resources.
The attack surface is specifically limited to server-side QUIC-enabled deployments, as the vulnerability does not affect traditional TLS connections or client-side implementations. This targeted scope means that organizations using s2n-tls for standard TLS 1.3 connections remain unaffected by this particular memory management flaw. The vulnerability's occurrence is directly tied to QUIC protocol handling within the TLS 1.3 handshake process, making it specific to deployments that utilize QUIC features in their s2n-tls implementation.
Mitigation efforts should focus on upgrading s2n-tls to version v1.7.6 or later, where the memory management issue has been addressed through proper implementation of s2n_realloc instead of s2n_alloc. This upgrade resolves the fundamental allocation pattern that causes the memory leak during HelloRetryRequest processing. Organizations should also consider implementing monitoring for memory consumption patterns on QUIC-enabled servers to detect potential exploitation attempts, while maintaining awareness of the specific conditions required for the vulnerability to manifest. The fix represents a correction to the underlying protocol implementation rather than requiring architectural changes or operational workarounds.