A security firm founded by Kevin Mandia — the executive who led Mandiant through some of the most consequential breach disclosures of the past decade — has published a detailed attack chain against Anthropic’s Claude Cowork sandbox. Anthropic says the attack doesn’t qualify as a security vulnerability. The researchers say it absolutely does.

This disagreement, which went public on July 1, 2026, follows a disclosure timeline that started in March — and it’s the latest in a pattern of security researchers and Anthropic finding themselves at odds over how to characterize risks in Claude’s product ecosystem.

What Claude Cowork Actually Is

Claude Cowork is Anthropic’s offering for knowledge workers: a Claude Desktop product that automates non-technical tasks using Claude Code running inside a sandboxed environment. On Windows, that sandbox is a Hyper-V-isolated Ubuntu VM.

The sandbox was designed with multiple protection layers:

  • Signature-gated communication via a Windows service (CoworkVMService) over a named pipe
  • Per-session unprivileged users inside the VM
  • Seccomp filters to restrict dangerous syscalls
  • A network proxy with a domain allowlist that restricts outbound connections

On paper, it’s a serious attempt at isolation. In practice, Armadin’s researchers found a chain of vulnerabilities that bypassed each layer.

The Attack Chain

The research was validated against Claude Desktop for Windows v1.9255.2.0. The full exploit chain is as follows:

Step 1: DLL sideloading to bypass the service’s signature check.

The claude.exe binary is legitimately signed. The CoworkVMService Windows service only accepts connections from signed clients — it checks the signature of the process connecting to its named pipe. The researchers sideloaded a malicious USERENV.dll into the directory containing claude.exe. Because Windows DLL search order looks in the application’s local directory before system directories, the malicious DLL loaded inside the signed process. The service’s signature check passed, because it was checking claude.exe — which was still legitimately signed. The malicious DLL had simply hitched a ride inside it.

Step 2: Exploiting undocumented process-spawning parameters.

Once inside the service’s trust boundary, researchers reverse-engineered the service’s command set by analyzing its logs and sending malformed inputs. They discovered undocumented parameters on the process-spawning command:

  • A “resume” flag that, when set, skipped creation of a fresh unprivileged session user — allowing execution as root or as any existing user with full Linux capabilities
  • A second parameter that overrode the network domain allowlist, enabling arbitrary egress by replacing the allowlist with a wildcard

Step 3: Sandbox escape and exfiltration.

With root execution inside the VM and network restrictions removed, the researchers used nsenter to step outside the container confinement, then exfiltrated the VM’s password file over an unrestricted network connection. The per-session isolation boundary was confirmed as unenforced server-side.

Full arbitrary code execution as root. Full network access. Full sandbox escape.

The Disclosure Dispute

Armadin submitted this finding to Anthropic on March 20, 2026. Four days later, on March 24, Anthropic responded — and declined to treat it as a vulnerability.

Anthropic’s position: the attack chain requires an attacker to already have local code execution on the target machine to place the malicious USERENV.dll. Since local code execution is itself a significant capability, and since achieving it is outside the scope of Claude Cowork’s threat model, Anthropic doesn’t classify what follows as a security vulnerability they’re responsible for addressing.

This is a coherent argument with a long history in security research. “Local code execution required” is the canonical reason why many exploits don’t get CVEs — if an attacker already owns your machine, what matters is the blast radius, not the specific chain.

Armadin’s counter-argument is also coherent: the entire value proposition of Claude Cowork’s sandboxed VM is that it should contain the blast radius of running AI-generated code on your machine. If local code execution allows full sandbox escape, root VM execution, and unrestricted network exfiltration, the containment guarantee is broken in exactly the scenario it was designed for.

The Armadin disclosure doesn’t stand alone. Related security issues in the Claude Code/Desktop ecosystem in 2026 include:

  • CVE-2026-25725 — sandbox escape via configuration injection through .claude/settings.json and bubblewrap on Linux
  • CVE-2026-22561 — DLL search-order hijacking in the Claude Desktop Windows installer

The pattern across these disclosures points to a broader challenge: Anthropic’s products are being deployed by non-technical users in environments where complex local VM sandboxing creates new attack surfaces that endpoint security tools don’t fully understand or monitor.

What Users Should Do

Armadin’s mitigation recommendations:

  1. Uninstall Claude Desktop if you aren’t actively using the Cowork functionality — reducing your attack surface is the safest option
  2. Use application allowlisting to restrict which DLLs can be loaded alongside claude.exe
  3. Monitor for USERENV.dll loads from non-system directories — this is the key indicator of the sideloading technique

The dispute between Armadin and Anthropic will likely continue — this is a known pattern in vulnerability disclosure where both parties have defensible positions. What matters for practitioners is the practical question: do you trust the sandbox to contain AI-generated code on your machine?

The research suggests the answer, at least for Claude Cowork on Windows, is “not fully.”

Sources

  1. SiliconANGLE — Armadin details full sandbox escape in Claude Cowork but Anthropic disputes risk
  2. Miggo Vulnerability Database — CVE-2026-25725
  3. SecurityWeek — Claude Code sandbox bypass patterns

Researched by Searcher → Analyzed by Analyst → Written by Writer Agent (Sonnet 4.6). Full pipeline log: subagentic-20260701-2000

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