CVE-2026-10740 Overview
CVE-2026-10740 is an unbounded memory allocation vulnerability in the CRYPTO frame reassembler of s2n-quic, AWS's Rust implementation of the QUIC protocol. The flaw exists in versions before 1.8.2 and stems from improper resource management [CWE-770]. An unauthenticated remote attacker can send crafted QUIC Initial packets that force the reassembler to allocate memory without bound. Sustained exploitation degrades service availability on hosts running vulnerable s2n-quic listeners. AWS published advisory GHSA-9q54-f358-3fqf and released v1.82.0 (referenced in the upstream tag) to address the issue.
Critical Impact
Unauthenticated remote attackers can trigger memory exhaustion against s2n-quic endpoints by sending crafted QUIC Initial packets, resulting in denial of service.
Affected Products
- s2n-quic versions prior to 1.8.2
- AWS services and customer applications embedding vulnerable s2n-quic builds
- Rust-based QUIC servers and clients linking the affected crate
Discovery Timeline
- 2026-06-10 - CVE-2026-10740 published to NVD
- 2026-06-10 - Last updated in NVD database
Technical Details for CVE-2026-10740
Vulnerability Analysis
The vulnerability resides in the CRYPTO frame reassembly logic of s2n-quic. QUIC carries TLS handshake data inside CRYPTO frames, which can arrive out of order and across multiple Initial packets. The reassembler buffers these fragments until contiguous bytes are available for the TLS state machine. In affected versions, the reassembler does not enforce an upper bound on buffered handshake data before authentication completes.
Because QUIC Initial packets are processed prior to any client authentication, the attacker does not need credentials or an established session. Repeatedly sending Initial packets carrying small, non-contiguous CRYPTO fragments forces the server to retain attacker-controlled bytes in memory. Aggregated across many spoofed connections, this consumes available memory and degrades availability for legitimate users.
Root Cause
The root cause is missing capacity enforcement on the pre-handshake CRYPTO buffer, classified as Allocation of Resources Without Limits or Throttling [CWE-770]. The reassembler trusts peer-controlled offsets and lengths to drive buffer growth without bounding the maximum unauthenticated state per connection.
Attack Vector
Exploitation requires only network reachability to a UDP listener running vulnerable s2n-quic. An attacker crafts QUIC Initial packets with CRYPTO frames designed to create gaps in the reassembly window, forcing the receiver to retain fragments while awaiting missing offsets. No user interaction, privileges, or prior handshake completion is required. Refer to the GitHub Security Advisory GHSA-9q54-f358-3fqf for protocol-level detail.
Detection Methods for CVE-2026-10740
Indicators of Compromise
- Sustained high volumes of QUIC Initial packets (UDP, typically port 443) from a narrow set of source addresses or spoofed ranges.
- Memory growth on s2n-quic processes that is not correlated with completed handshakes or accepted streams.
- Elevated rates of incomplete QUIC handshakes and connection timeouts in application logs.
Detection Strategies
- Monitor process resident set size (RSS) for services linking s2n-quic and alert on anomalous growth without matching connection counts.
- Inspect UDP/443 traffic for high ratios of Initial packets to completed handshakes, a signature of CRYPTO reassembly abuse.
- Correlate network telemetry with host metrics to identify exhaustion patterns tied to unauthenticated QUIC traffic.
Monitoring Recommendations
- Ingest QUIC connection metrics and host memory counters into a centralized analytics platform for cross-source correlation.
- Baseline normal Initial packet rates per source ASN and alert on statistically significant deviations.
- Track the deployed version of s2n-quic across fleets and flag any host still running a release below 1.8.2.
How to Mitigate CVE-2026-10740
Immediate Actions Required
- Upgrade s2n-quic to 1.8.2 or later (release tag v1.82.0) and redeploy all dependent services.
- Inventory build manifests and container images for transitive dependencies on the vulnerable crate.
- Apply rate limiting on UDP/443 at network edges to constrain abusive Initial packet floods while patching.
Patch Information
The fix is delivered in s2n-quic1.8.2, published in the upstream GitHub s2n-quic Release v1.82.0. AWS describes the remediation and affected services in AWS Security Bulletin 2026-042. Rebuild any application that statically links the crate; runtime updates alone are insufficient.
Workarounds
- Restrict QUIC exposure to trusted networks or disable QUIC where HTTPS over TCP/TLS provides adequate service.
- Deploy UDP flood protections and connection rate limits at load balancers or firewalls in front of s2n-quic listeners.
- Set per-source connection caps to limit the volume of unauthenticated state any single peer can create.
# Update the s2n-quic dependency to the patched version
cargo update -p s2n-quic --precise 1.8.2
# Verify the resolved version
cargo tree -p s2n-quic | head -n 1
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
