# blockchain-security

Tests Solidity contracts and blockchain CTF challenges for common EVM flaws. Use it when reviewing storage, delegatecall paths, or access checks.

- skill · `type=skill` · Claude Code, Codex, Cursor, GitHub Copilot, Gemini CLI, OpenCode · 548 installs · ★ 595
- Author: transilienceai
- License: MIT · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/transilienceai/communitytools/tree/HEAD/skills/blockchain-security/SKILL.md
- Updated: 2026-07-29
- HTML: https://affpapa.org/en/nevibecode/skills/blockchain-security

## What it does

The skill takes a contract review from source or bytecode through storage mapping and privileged operations. It covers reentrancy, access control, `tx.origin`, integer overflow, weak randomness, empty-array validation bypasses, and ECDSA signature malleability.

For `delegatecall` cases, it calls for matching the exploit contract's storage slot order to the target. An exact storage layout match is mandatory here, or writes land in the wrong slots. It also covers `CREATE` address prediction, reading private values with `eth_getStorageAt`, and checking an `isSolved()` condition.

Its examples use `web3.py`, `cast`, or `forge`; compilation uses `solcx` with Solidity `0.8.13`. The skill suggests scripting a full CTF flow because challenge instances can be temporary.

## Install

### Claude Code (native platform)

1. **Skills CLI (Vercel)**
```bash
npx skills add transilienceai/communitytools --skill blockchain-security -a claude-code
```

2. **Manually: put the files in place**
- File: `~/.claude/skills/blockchain-security/SKILL.md` — Copy the whole skill folder; for one project use .claude/skills/blockchain-security/

### Codex

1. **Skills CLI (Vercel)**
```bash
npx skills add transilienceai/communitytools --skill blockchain-security -a codex
```

2. **Manually: put the files in place**
- File: `.agents/skills/blockchain-security/SKILL.md` — For all projects use ~/.agents/skills/

### Cursor

1. **Skills CLI (Vercel)**
```bash
npx skills add transilienceai/communitytools --skill blockchain-security -a cursor
```

2. **Manually: put the files in place**
- File: `.cursor/skills/blockchain-security/SKILL.md` — Cursor also reads .agents/skills/ and .claude/skills/

### GitHub Copilot

1. **GitHub CLI**
```bash
gh skill install transilienceai/communitytools blockchain-security
```

2. **Manually: put the files in place**
- File: `.github/skills/blockchain-security/SKILL.md`

### Gemini CLI

1. **Skills CLI (Vercel)**
```bash
npx skills add transilienceai/communitytools --skill blockchain-security -a gemini-cli
```

2. **Platform CLI**
```bash
gemini skills install https://github.com/transilienceai/communitytools --path skills/blockchain-security
```

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/skills/blockchain-security/SKILL.md` — The legacy .windsurf/skills/ path still works

### OpenCode

**Skills CLI (Vercel)**
```bash
npx skills add transilienceai/communitytools --skill blockchain-security -a opencode
```

### Claude.ai

**Claude.ai**
- Zip the folder and upload it in Settings → Capabilities → Skills

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/transilienceai/communitytools/tree/HEAD/skills/blockchain-security/SKILL.md

## When to use it
- check a Solidity contract for reentrancy
- find missing access checks
- predict a `CREATE` contract address
- read a storage slot with `eth_getStorageAt`
- inspect an empty-array validation bypass

## Example prompts
- Review this Solidity contract for `delegatecall`, reentrancy, and access-control issues.
- Map the storage layout and inspect private values with `eth_getStorageAt`.
- Help solve this blockchain CTF and verify `isSolved()`.

## Source text · SKILL.md · MIT

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 7b3acff6cb404342.

BEGIN THIRD-PARTY FILE 7b3acff6cb404342
````text
---
name: blockchain-security
description: Smart contract security testing and blockchain CTF exploitation. Covers Solidity vulnerability analysis, EVM storage manipulation, delegatecall attacks, CREATE/CREATE2 address prediction, and common DeFi exploit patterns. Use when analyzing Solidity contracts, solving blockchain challenges, or testing smart contract security.
---

# Blockchain Security

## Quick Start
1. Download and decompile contracts (source or bytecode)
2. Map storage layout and identify privileged operations
3. Check for delegatecall, CREATE address prediction, reentrancy, access control
4. Deploy exploit contracts via web3.py or cast/forge
5. Verify win condition (isSolved/flag endpoint)

## Blockchain CTF Challenge Pattern
```bash
# Get connection info
curl http://$HOST:$PORT/connection_info  # -> PrivateKey, Address, TargetAddress, setupAddress
# RPC endpoint
RPC_URL="http://$HOST:$PORT/rpc"
# Win condition: Setup.isSolved() must return true
```

## Key Attack Vectors

### 1. Delegatecall Storage Manipulation
When contract A does `delegatecall` to contract B, B's code runs with A's storage.
- Deploy exploit contract that mirrors A's storage layout
- Exploit contract writes to A's storage slots via delegatecall
- **Critical**: Storage layout must match exactly (same slot ordering)
- See [reference/delegatecall-attacks.md](reference/delegatecall-attacks.md)

### 2. CREATE Address Prediction (Nonce Manipulation)
Contract addresses from CREATE are deterministic: `keccak256(rlp([sender, nonce]))[12:]`
- Brute-force nonce to find which nonce produces target address
- Send dummy transactions (self-transfers) to increment nonce
- Deploy exploit contract at the exact nonce that hits target address
- See [reference/create-address-prediction.md](reference/create-address-prediction.md)

### 3. Storage Layout & Slot Computation
- Mappings: `keccak256(h(key) || uint256(slot_number))`
  - Value types: `h(k) = abi.encode(k)` (left-padded to 32 bytes)
  - String/bytes: `h(k) = keccak256(k)`
- Read private variables via `eth_getStorageAt`
- See [reference/storage-layout.md](reference/storage-layout.md)

### 4. Empty Array / Zero-Length Input Bypass
When a function loops over a user-supplied array to validate items (signatures, approvals, votes), passing an **empty array** skips the loop entirely. If there's no minimum-length check, validation is bypassed.
- Check: `for (uint i = 0; i < arr.length; i++)` with no `require(arr.length >= N)`
- Exploit: Call the function with `[]` to skip all validation

### 5. ECDSA Signature Malleability
Raw `ecrecover` accepts both `(v, r, s)` and `(v', r, N-s)` (where N = secp256k1 order, v flipped 27↔28). If a contract deduplicates signatures by hash of raw bytes, the malleable form has a different hash but recovers to the same signer.
````
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