agents: Skill for Claude Code

Remote Code Execution (RCE) Detection

You are performing a focused security assessment to find Remote Code Execution vulnerabilities in a codebase. This skill uses a two-phase approach with subagents: sink discovery (find all places where OS commands are executed, code is dynamically evaluated, or untrusted data is deserialized) then taint analysis (confirm whether user-supplied input reaches those sinks).

Prerequisites: sast/architecture.md must exist. Run the analysis skill first if it doesn't.

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What is Remote Code Execution

Remote Code Execution (RCE) occurs when an attacker can cause the application to execute arbitrary OS commands or application-level code that they control. This is typically the highest-severity vulnerability class, often resulting in complete server compromise.

RCE arises from three primary root causes:

1. OS Command Injection: User input is embedded unsafely into an OS command string, allowing shell metacharacters to inject additional commands.
2. Code Injection (eval-like): User input is passed to functions that interpret it as executable code (eval, exec, Function(), etc.).
3. Unsafe Deserialization: User-supplied serialized data is deserialized using a gadget-prone deserializer, triggering arbitrary code execution via crafted payloads.

What RCE IS

• Passing user input directly or indirectly into OS command execution functions with shell interpretation enabled
• Using eval(), exec(), Function(), or equivalent constructs with user-controlled strings
• Deserializing user-supplied bytes/strings with inherently unsafe deserializers (pickle, PHP unserialize, Java native serialization, Ruby Marshal, etc.)
• Using yaml.load() without a safe loader on user-supplied content
• Dynamic require()/import() with user-controlled module paths
• PHP file inclusion (include/require) with user-controlled paths

What RCE is NOT

Do not flag these as RCE:

• SSRF: Making HTTP requests to attacker-controlled URLs — different vulnerability class (no code execution)
• Path Traversal: Reading/writing arbitrary files — separate class (unless the read file is then executed/deserialized)
• SSTI: Template injection via template engines — a separate though related class; flag as SSTI, not RCE
• XSS: JavaScript execution in a victim's browser — client-side only, not server-side RCE
• SQL Injection: Injecting into database queries — different class (even if xp_cmdshell can lead to OS commands, flag it as SQLi)
• Safe subprocess list-form calls: subprocess.run(["ls", user_arg]) with a list and no shell=True — arguments are passed directly to the OS without shell expansion; not vulnerable to command injection
• Safe deserialization: json.loads(), yaml.safe_load(), xml.etree.ElementTree.parse() — these formats have no code execution semantics