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mcp-security-scanner

by badchars

rt_check_ansi_injection

Scans for ANSI escape sequences in tool descriptions and schema fields that hide malicious text from LLM while visible in terminal. Prevents ANSI injection attacks.

Instructions

Scan all tool descriptions and schema field descriptions for ANSI escape sequences (CSI codes, cursor movement, color codes) used to hide malicious text in terminal display while LLM still reads it.

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
commandYesServer command to execute (e.g. 'node', 'bun', 'npx')
argsNoCommand arguments (e.g. ['run', 'server.js'])
envNoAdditional environment variables
timeout_msNoConnection timeout in milliseconds (default: 30000)
Behavior2/5

Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

No annotations provided, so the description must carry the burden. It describes what is scanned but fails to disclose behavioral traits such as whether the tool is read-only, requires network access, or has side effects. This is insufficient for a security scanning tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness5/5

Is the description appropriately sized, front-loaded, and free of redundancy?

The description is a single sentence that efficiently conveys the purpose, scope, and threat model. No extraneous words.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness2/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

The description lacks context about how the scanning works (e.g., whether it executes the command to retrieve descriptions) and does not mention output format. Given no output schema and the mismatch between parameters and purpose, the description is incomplete.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters2/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 100%, so baseline is 3. However, the description adds no meaning to the parameters, and there is a mismatch: the tool scans descriptions but takes parameters for executing a server command (command, args, env, timeout_ms). The description does not explain how these relate to the scanning purpose, reducing semantic clarity.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose5/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description clearly states the tool scans all tool descriptions and schema field descriptions for ANSI escape sequences, specifying the types (CSI codes, cursor movement, color codes) and the malicious intent (hide text from LLM). This distinguishes it from siblings like rt_check_unicode_steganography or rt_check_tool_poisoning.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines3/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description implies use when checking for ANSI injection vulnerabilities but does not explicitly state when to use this tool over other runtime checks (e.g., rt_check_auth, rt_check_tool_shadowing). No guidance on when not to use it or prerequisites.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.

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