Security researcher Malcolm Stagg has disclosed a new attack class called NatJack that manipulates network address translation (NAT) connection state to hijack active TCP sessions, spoof DNS responses, disclose victim IP addresses and mapped ports, and exhaust NAT tables.

Presented at Black Hat USA 2026, Stagg said the techniques were demonstrated across network infrastructure devices, virtualization, containerization, and cloud infrastructure, including Windows, Linux, and macOS implementations.

Two implementation-specific flaws have been assigned CVEs: CVE-2026-56181 (CVSS score: 8.3) in Windows NAT used by Hyper-V, and CVE-2026-63913 (CVSS score: 8.2) in Linux Netfilter conntrack.

The attack model generally places an attacker-controlled system behind the same NAT infrastructure as the victim, while the exact capabilities and preconditions vary by technique and platform. The mitigation guidance therefore emphasizes separating untrusted workloads from trusted systems that share NAT infrastructure.

There is no single patch for the broader attack class. Organizations should apply available Windows and Linux updates and encrypt traffic even within internal networks. The research also recommends Internet Protocol (IP) Source Guard where applicable.

The NatJack research, conducted independently by Stagg through SODIUM-24, targets an assumption built into many NAT implementations: hosts behind the same NAT are generally assumed not to manipulate one another's connection state. An attacker controlling a system behind the same NAT can, depending on the implementation, manipulate connection-tracking entries belonging to another system.

The presentation groups NatJack into five attacks: UDP DNS hijacking, two TCP/IP hijacking variants, victim IP/port disclosure, and denial of service.

In the DNS attack, the attacker fills the NAT table with dummy entries to evict the victim's pending DNS entry, then creates replacement entries pointing to the attacker. The legitimate DNS response is sent to the attacker instead of the victim, after which the attacker sends a spoofed replacement response back to the victim.

For TCP, Stagg demonstrated downstream and upstream spoofing variants that remove and replace a victim's NAT entry to hijack the connection. In the upstream variant, the attacker and victim can be in different subnets and broadcast domains. The research also describes victim IP/port disclosure and denial-of-service attacks against NAT state.

Synack said Stagg demonstrated proof-of-concept exploitation in a controlled environment. His Black Hat presentation says 13 vendors were notified, 32 products and configurations were tested, 95 test reports were produced, and seven PoCs plus product-specific variants were developed. Stagg said all tested products were vulnerable.

The presentation names the 13 vendors and provides status information for selected products and services, but does not include a complete 32-item product-and-configuration matrix. The Hacker News found no public evidence that NatJack techniques have been exploited in the wild as of August 7, 2026.

Stagg also demonstrated a macOS virtualization attack. Apple told him it considers the behavior a known transport-layer limitation rather than a vulnerability and said it was considering additional hardening as a defense-in-depth measure.

For Linux, the kernel.org CNA record says a crafted SYN followed by a reset packet with an invalid sequence number can prematurely force an active Netfilter NAT entry into a closed state because the conntrack logic failed to validate its direction. Fixed stable releases include 5.10.259, 5.15.210, 6.1.176, 6.6.143, 6.12.93, 6.18.35, 7.0.12, and 7.1.

Stagg says the kernel change fixes the code flaw but only mitigates the broader downstream-spoofing technique, increasing attack complexity rather than fully addressing it.

Microsoft's CNA record describes the Windows issue as an origin-validation error that enables spoofing from an adjacent network. Affected releases include Windows 11 24H2 before 26100.8875, 25H2 before 26200.8875, 26H1 before 28000.2525, and Windows Server 2025 before 26100.33158.

NatJack also builds on earlier research into NAT-state manipulation. An NDSS 2024 study demonstrated TCP hijacking through NAT mapping manipulation and found 52 of 67 tested routers susceptible to its attack, work that produced ten CVEs.