Muster found an unauthenticated relay vulnerability in MetaMask's transaction infrastructure
The Muster engine identified a critical misconfiguration in MetaMask's tx-sentinel relay infrastructure that allowed any website to submit sponsored transactions on Polygon and Monad, with no authentication and fully permissive CORS.
2
Chains affected
100%
Autonomous discovery
✓
Responsibly disclosed
The target
MetaMask is the world's most widely used self-custodial crypto wallet, serving over 30 million monthly active users across browser and mobile.
As part of its transaction infrastructure, MetaMask operates tx-sentinel, a public relay service that processes sponsored (gasless) transactions on behalf of users across multiple chains. This infrastructure is a high-value target: any vulnerability in its relay layer could be exploited to drain sponsor funds or abuse MetaMask's gas subsidies at scale.
Powered by the Muster engine
This vulnerability was identified through autonomous reconnaissance of MetaMask's public API surface, mapping endpoints, testing authentication boundaries, and analyzing CORS policies, without manual researcher intervention.
Fully autonomous discoveryThe vulnerability
The eth_sendRelayTransaction endpoint on MetaMask's tx-sentinel infrastructure accepted requests from arbitrary browser origins with no authentication. The server responded with Access-Control-Allow-Origin: *, meaning any website could invoke sponsored relay transactions cross-origin.
# Response headers from the tx-sentinel endpoint Access-Control-Allow-Origin: * Access-Control-Allow-Methods: POST, OPTIONS # Any origin can call eth_sendRelayTransaction # No API key, no auth token, no origin allowlist
Confirmed on-chain impact
On both Polygon and Monad networks, this misconfiguration allowed real on-chain transactions to be triggered and paid for through MetaMask's sponsor infrastructure, meaning an attacker could drain gas subsidies from any arbitrary webpage.
Vulnerability reported to MetaMask Security
The finding was responsibly disclosed to MetaMask's security team with full reproduction steps, confirmed on-chain transaction evidence, and remediation recommendations.
What this means
Gasless relay infrastructure is becoming a core primitive across Web3, but without proper authentication and origin controls, it becomes a financial liability. This finding demonstrates that the Muster engine can identify infrastructure-level vulnerabilities that extend beyond smart contract logic into the API and middleware layers that power modern blockchain applications.
“A wildcard CORS policy on a sponsored relay is essentially an open checkbook. Our system flagged it immediately: no auth, no origin restriction, live on two chains. This is exactly the kind of infrastructure-level issue that manual smart contract audits miss entirely.”
Why Muster
✓Beyond smart contracts
Muster audits the full stack: relay infrastructure, APIs, CORS policies, and authentication boundaries.
✓Autonomous reconnaissance
Maps public attack surfaces and tests security boundaries without manual guidance.
✓On-chain validation
Confirms real-world exploitability, not just theoretical risk.
✓Responsible disclosure built in
Generates complete reports with evidence and remediation steps.
Know your full attack surface.
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reported April 2026 · findings via public bug-bounty programs · not affiliated with these companies