aws-smithy-http-server CVE-2026-16756

Requires that the target Rust service is built on aws-smithy-http-server (< 0.66.5) and uses the default serve() code path, which ships without connection/header-read timeouts or a concurrent-connection cap, and that this path is directly reachable over the network. Given AV:N/AC:L/AT:N/PR:N/UI:N, no authentication, user interaction, or special attacker capability is needed against such a default deployment. Exploitation is meaningfully limited when the service sits behind a proxy or load balancer that enforces its own connection timeouts and rate limits, or when the operator has already replaced serve() with a custom server that sets timeouts and connection ceilings - in those cases the slowloris connections are dropped before exhausting server resources.

The signals are internally consistent for an availability-only DoS: the CVSS 4.0 vector (AV:N/AC:L/AT:N/PR:N/UI:N) confirms remote, low-complexity, unauthenticated reachability with VA:H and VC:N/VI:N - no data exposure or integrity loss, purely service exhaustion, yielding the 8.7 High score. There is no CISA KEV listing and no POC referenced, so exploitation is theoretical-but-plausible rather than observed; EPSS was not provided. The real-world priority is moderate-to-high for internet-exposed Smithy-based Rust services using the default serve() path, but lower where a fronting proxy, load balancer, or WAF already enforces connection timeouts and rate limits, which would blunt the slowloris pattern before it reaches the server. This is a genuine availability concern, not an inflated high-CVSS artifact, but its blast radius is bounded by how many production deployments actually expose the raw serve() path directly.

An attacker points a slowloris-style tool at an internet-exposed Smithy-based Rust API, opening thousands of TCP connections and sending each HTTP request one byte at a time without ever completing the headers. Because AV:N/AC:L/PR:N means no authentication or special conditions are needed, every connection ties up a socket and a Tokio task until the process runs out of file descriptors and legitimate clients can no longer connect. No public exploit code is referenced, but the technique requires only commodity tooling.