Markdown image exfiltration check

Markdown injection occurs when user-controlled input is rendered by a downstream system that interprets Markdown syntax, such as documentation platforms, issue trackers, or internal dashboards. APIs that accept or propagate Markdown text can inadvertently enable exfiltration when rendered content includes embedded images or links pointing to external endpoints.

Attackers embed image syntax with a controlled URL to trigger outbound HTTP requests from the rendering context. Because the rendering engine often sends credentials or internal network information to the target, this behavior can disclose data paths, service names, or authentication tokens. Treat any field that later becomes Markdown as an injection surface, even if the field is stored as plain text.

Teams frequently underestimate the risk because Markdown is perceived as a formatting language rather than a data channel. A common mistake is assuming that rendering happens only in the client, while server-side previews, changelog generators, or CI documentation builders also evaluate embedded content.

Another mistake is allowing unrestricted Markdown in fields that map to integration messages, such as deployment notifications or incident reports. Without validation, these fields become a covert channel for probing internal addresses, testing egress rules, or enumerating accessible services through timing and error differences in image requests.

middleBrick scans API definitions and runtime behavior for data exfiltration vectors involving Markdown rendering contexts. It identifies parameters that accept Markdown-like payloads and maps them against sinks where the content may be rendered or forwarded to external systems.

The scanner includes specific probes for image and link patterns that would trigger outbound HTTP requests when processed by common Markdown libraries. Findings include the presence of unvalidated user input in fields later processed by rendering engines, highlighting the need for strict allowlists on URL schemes and destinations.

Implement a workflow where Markdown input is validated before rendering or storage. Define strict allowlists for URL protocols and hosts, and remove or encode constructs that trigger network requests, such as images with remote sources or inline links to private addresses.

Code example for sanitization in a Node.js environment using a library that limits protocols and domains:

import { remark } from 'remark';
import { visit } from 'unist-util-visit';

function sanitizeMarkdown(tree) {
  visit(tree, 'link', (node) => {
    const url = new URL(node.url, 'http://local');
    if (!['http:', 'https:'].includes(url.protocol)) {
      node.url = '#';
    }
    if (!url.hostname.endsWith('trusted.example.com')) {
      node.url = '#';
    }
  });
}

const processor = remark().data('settings', { sanitize: true });

Use schema validation to reject or transform fields that contain raw Markdown when the downstream consumer does not explicitly require it.

middleBrick maps relevant findings to OWASP API Top 10 (2023) and supports audit evidence for controls related to input validation and data exposure. The scanner checks for indicators of Markdown-based exfiltration patterns using read-only methods, ensuring no destructive payloads are sent.

This approach surfaces risky data flows but does not replace code review or threat modeling specific to your rendering pipeline. Business logic vulnerabilities around conditional rendering or authenticated image proxies require human expertise and contextual understanding of your infrastructure.