CVE-2026-94450 Overview
CVE-2026-94450 is a denial-of-service vulnerability in s2n-quic, the AWS-maintained Rust implementation of the QUIC transport protocol. Versions 1.88.0 and earlier improperly validate the Destination Connection ID (DCID) length in incoming QUIC packets. An unauthenticated remote attacker can shut down a vulnerable server endpoint using a single crafted UDP datagram. Only server endpoints explicitly configured to send Retry packets are affected. The issue is tracked as CWE-1284: Improper Validation of Specified Quantity in Input.
Critical Impact
A single crafted UDP datagram can crash a Retry-enabled s2n-quic server, terminating all active QUIC sessions with no authentication required.
Affected Products
- s2n-quic versions 1.88.0 and earlier
- QUIC server endpoints configured to send Retry packets
- Applications and services built on affected s2n-quic releases
Discovery Timeline
- 2026-09-22 - CVE-2026-94450 published to NVD
- 2026-09-23 - Last updated in NVD database
Technical Details for CVE-2026-94450
Vulnerability Analysis
The vulnerability resides in the QUIC packet-parsing path of s2n-quic. QUIC uses a Destination Connection ID field with a variable length between 0 and 20 bytes, as defined by RFC 9000. The affected code path fails to enforce this bound when processing incoming datagrams on server endpoints that emit Retry packets. A malformed length value propagates into downstream handling logic and causes the server endpoint to terminate. Because the flaw is reached before any handshake completes, no cryptographic material or session state is required.
Root Cause
The root cause is improper validation of a specified quantity in input, mapped to [CWE-1284]. The DCID length field taken from the wire is trusted without a preceding range check against the protocol-defined maximum. When a Retry-enabled server processes a datagram containing an out-of-spec DCID length, an internal invariant fails and the endpoint shuts down. The bug is isolated to the Retry-sending server configuration path.
Attack Vector
Exploitation requires only network reachability to the target UDP port serving QUIC. The attacker sends a single crafted Initial packet containing a manipulated DCID length. No authentication, prior session, or user interaction is required. The result is complete availability loss for the affected server endpoint, dropping all in-flight and future QUIC connections until the process is restarted. See the GitHub Security Advisory GHSA-5rw2-6x5m-v22x and the AWS Security Bulletin 2026-116 for vendor details.
No verified proof-of-concept code is available in public exploit repositories at this time.
Detection Methods for CVE-2026-94450
Indicators of Compromise
- Unexpected termination of s2n-quic server processes with no application-level cause
- QUIC Initial packets with Destination Connection ID length values outside the 0–20 byte range defined by RFC 9000
- Repeated UDP datagrams to QUIC listener ports followed by loss of the listening socket
- Restart loops in QUIC service supervisors correlated with inbound external traffic
Detection Strategies
- Inspect QUIC Initial packets at network sensors and flag datagrams whose DCID length byte violates RFC 9000 bounds
- Correlate s2n-quic process crashes with preceding UDP traffic bursts from single source addresses
- Enable verbose logging on Retry-enabled QUIC endpoints to capture parser errors before termination
Monitoring Recommendations
- Track process uptime and restart counts for services embedding s2n-quic
- Monitor UDP flow telemetry to QUIC ports for anomalous single-packet, single-source patterns preceding outages
- Alert on abnormal termination signals from QUIC worker processes
- Ingest s2n-quic and application logs into a centralized data lake for cross-source correlation
How to Mitigate CVE-2026-94450
Immediate Actions Required
- Upgrade s2n-quic to version v1.89.0 or later in all builds and container images
- Inventory services that embed s2n-quic and identify those configured to send Retry packets
- Restart dependent services after upgrading to ensure the patched library is loaded
- Review supervisor policies to ensure automatic restart of any crashed QUIC endpoint
Patch Information
AWS released the fix in s2n-quicv1.89.0. Consumers should update their Cargo.toml dependency and rebuild affected binaries. Release notes are available on the GitHub Release for s2n-quic v1.89.0. Downstream distributions and container base images that vendor s2n-quic must also be refreshed to pick up the fix.
Workarounds
- Disable the Retry packet configuration on server endpoints if operationally acceptable, since only Retry-enabled servers are affected
- Restrict inbound UDP access to the QUIC listener using firewall rules or security groups where feasible
- Place vulnerable endpoints behind a QUIC-aware load balancer that validates DCID length before forwarding
No vendor-supplied configuration snippet is published for this issue. Refer to the AWS Security Bulletin 2026-116 for authoritative remediation guidance.
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
