Unlocking the Web3 Vault: Exposing Sensitive Data via Unprotected debug_traceBlockByNumber

Unlocking the Web3 Vault: Exposing Sensitive Data via Unprotected debug_traceBlockByNumber | by Hacker MD - Freedium Milestone: 20GB Reached We’ve reached 20GB of stored data — thank you for helping us grow! Patreon Ko-fi Liberapay Close < Go to the original Unlocking the Web3 Vault: Exposing Sensitive Data via Unprotected debug_traceBlockByNumber Hello fellow hackers and Web3 enthusiasts! Hacker MD Follow ~4 min read · March 24, 2026 (Updated: March 24, 2026) · Free: Yes In the world of blockchain bug bounty hunting, we often hear about smart contract vulnerabilities like reentrancy or logic flaws. However, the infrastructure supporting these blockchains — specifically the RPC (Remote Procedure Call) nodes — is just as critical and often overlooked. Today, I want to share a recent finding where I discovered a critical security misconfiguration on a major public RPC endpoint. By exploiting an exposed debug_ namespace, I was able to access highly sensitive internal execution traces and identify a massive Denial of Service (DoS) vector. Let's dive into how I found it, what the bug actually is, and the severe real-world impact it carries. The Reconnaissance: Filtering the Noise When hunting on Web3 infrastructure targets, you are often handed wildcard domains like *.target-network.com . After running standard subdomain enumeration (using tools like subfinder and amass ), I was staring at a massive list of over 5,000 subdomains. Testing each one manually is a quick path to burnout. Instead, I built an automated pipeline: Live Host Detection: I used httpx to filter out dead domains. RPC Discovery: Many subdomains returned standard 404 Not Found Nginx pages. However, in the Web3 world, a 404 on a GET request doesn't mean the server is dead; it often means it's waiting for a specific POST request. I used a custom nuclei template to send a basic {"method":"web3_clientVersion"} JSON-RPC payload to all live hosts. This smart filtering reduced my target list from 5,000+ down to just a handful of active EVM (Ethereum Virtual Machine) compatible RPC nodes. Now, the real hunt began. Understanding the Bug: What is debug_traceBlockByNumber ? To understand the vulnerability, we need to put on our developer hats for a moment. An RPC node is basically a bridge that allows outside applications (like your MetaMask wallet) to talk to the blockchain. Standard public nodes allow safe methods, like eth_getBlockByNumber (to see public block data) or eth_sendRawTransaction (to send money). However, developers also need tools to fix broken smart contracts. Enter the debug_ namespace. Methods like debug_traceTransaction and debug_traceBlockByNumber are incredibly powerful. They don't just look at the final result of a block; they re-execute it. Imagine watching a movie (the transaction) versus reading the script, looking at the camera angles, and analyzing the actor's heart rate frame-by-frame (the trace). A trace reveals the internal memory stack, opcodes, exact gas usage per instruction, and hidden DELEGATECALL inputs. Because this process is computationally exhausting, security best practices strictly dictate that the debug_ API must be disabled on public-facing nodes. On my target ( rpc.redacted-network.com ), the administrators had restricted several methods, but they missed one massive door: debug_traceBlockByNumber . The Exploitation: Proving the Concept To prove the vulnerability existed without causing harm to the infrastructure, I crafted a precise HTTP request using curl and intercepted it in Burp Suite to document the evidence. First, I identified the "latest" block on the network. Then, I fired the following payload: The Malicious Request: POST / HTTP/2 Host: rpc.redacted-network.com User-Agent: curl/8.15.0 Accept: */* Content-Type: application/json Content-Length: 106 {"jsonrpc":"2.0","method":"debug_traceBlockByNumber","params":["latest", {"tracer": "callTracer"}],"id":1} To my surprise, the server did not return an "Access Denied" or "Method Disabled" error. Instead, it paused to process the heavy request and returned a massive JSON object containing the complete internal traces of every single transaction within that block. The Server Response (Snippet): HTTP/2 200 OK Content-Type: application/json {"id":1,"jsonrpc":"2.0","result":[{"txHash":"0x892b91...","result":{"from":"0x...","gas":"0xf4240","gasUsed":"0xf9f5","input":"0x015d8eb9...","calls":[{"type":"DELEGATECALL", "to":"0xc0d3..."}]}}]} The Real-World Impact Why is this a big deal? Leaving this specific method open introduces two critical risks: 1. Sensitive Information Disclosure Blockchain ledgers are public, but internal contract execution states are not always transparent on standard block explorers. By using the callTracer , an attacker can monitor internal private transactions, reverse-engineer proprietary smart contract logic, and intercept parameters passed in internal DELEGATECALL s. This gives malicious actors a deep, x-ray vision into the network's operations, paving the way for more complex smart contract exploits. 2. Severe Denial of Service (DoS) Risk This is the most dangerous aspect. Tracing a single transaction is heavy; tracing an entire block (which could contain hundreds of transactions) requires massive CPU and RAM overhead from the node. If an attacker were to send a rapid loop of debug_traceBlockByNumber requests, it would instantly bottleneck the server's CPU. Legitimate users (like decentralized apps and crypto wallets) relying on that RPC node would experience timeouts, effectively taking the infrastructure offline. Note: As an ethical hacker, I only sent a single request to generate the Proof of Concept and did not test the DoS limits to protect the program's infrastructure. The Takeaway The report was submitted to the program's triage team. Interestingly, they noted that while the method was indeed exposed, they had aggressive rate-limiting layers in place that mitigated the risk of a full-scale DoS attack, classifying the issue as Informative/Low severity but rewarding reputation points for the discovery. Regardless of the payout, this was a fantastic learning experience. It reinforces a golden rule of Web3 infrastructure security: Always explicitly disable internal namespaces ( debug , admin , personal ) on public-facing RPC endpoints. For my fellow hunters: Don't just stop at a 404 page. Fuzz those RPC paths, check for enabled debugging methods, and always look for the side doors! Happy Hunting! 🛡️💻 #BugBounty #Web3Security #CyberSecurity #InfoSec #EthicalHacking #BlockchainSecurity #RPC #EVM #SmartContractSecurity #Vulnerability #Web3 #BugBountyTips #HackerOne #Bugcrowd #Hackenproof #bug-bounty #web3-security #cybersecurity #blockchain #infosec Reporting a Problem Sometimes we have problems displaying some Medium posts. If you have a problem that some images aren't loading - try using VPN. Probably you have problem with access to Medium CDN (or fucking Cloudflare's bot detection algorithms are blocking you).