CVE-2026-16318 QUIC Transport Parameters Memory Leak During HelloRetryRequest in s2n-tls
An unauthenticated attacker can cause memory exhaustion in s2n-tls servers by forcing HelloRetryRequests, leading to denial of service.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-16318?
CVE-2026-16318 is a medium-severity denial of service vulnerability in Amazon s2n-tls, published on 21 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
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
Weakness: CWE-401
Which versions are affected by CVE-2026-16318?
How do I fix CVE-2026-16318?
Upgrade to a fixed version.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-16318 being exploited?
There is no known exploitation of CVE-2026-16318 in the wild and no public exploit code.
Sources
Also known as GHSA-cr7x-863j-xrc7.