Erodenn/fetch-guard

Fetch Guard

An MCP server and CLI tool that fetches URLs and returns clean, LLM-ready markdown. A purpose-built extraction pipeline sanitizes HTML, pulls structured metadata, detects prompt injection attempts, and handles the edge cases that break naive fetchers: bot blocks, paywalls, login walls, non-HTML content types, and pages that require JavaScript to render.

The core problem is straightforward: LLMs need web content, but raw HTML is noisy and potentially hostile. Fetched pages can contain hidden text, invisible Unicode, off-screen elements, and outright prompt injection attempts embedded in the content itself. This pipeline strips all of that before the content reaches the model.

Three layers handle the injection defense specifically:

1. Pre-extraction sanitization removes hidden elements (display:none, visibility:hidden, opacity:0, font-size:0, transform:scale(0), clip:rect(0,0,0,0), zero-height overflow containers, and elements with matching foreground and background colors), elements hidden via CSS class/ID rules in <style> tags, off-screen positioned content, aria-hidden elements, <noscript> and <template> tags, and 26 categories of non-printing Unicode characters including bidi isolates and Unicode Tags. This happens before content extraction, so trafilatura never sees the attack vectors.
2. Pattern scanning runs a four-phase scan against the extracted text and metadata fields. Phase one applies 50 compiled regex patterns covering system prompt overrides, ignore-previous instructions, role injection, fake conversation tags, and hidden instruction markers, in English, Spanish, French, German, Japanese, Simplified Chinese, and Portuguese. Phase two normalizes the text via NFKC and confusable-character mapping, then rescans to catch homoglyph bypasses (Cyrillic or mathematical Unicode characters substituted for Latin, etc.). Phase three finds base64, hex-encoded, and URL percent-encoded blocks, decodes them, and scans against high-severity patterns. Phase four decodes the full document with ROT13 and scans against high-severity patterns. Metadata fields (title, description, og:title, etc.) are scanned independently with matches namespaced to their source field.
3. Session-salted output wrapping generates a random 8-character hex salt per invocation and wraps the body in <fetch-content-{salt}> tags. Since the salt is unpredictable, injected content cannot spoof the wrapper boundaries.