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attack-surface.md

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Attack Surface Analysis for erusev/parsedown

Attack Surface: Regular Expression Denial of Service (ReDoS)

  • Description: Parsedown's reliance on regular expressions for parsing Markdown can be exploited by specially crafted input, causing excessive CPU consumption and potentially making the application unresponsive. This is a high-severity vulnerability because it can lead to denial of service.
  • How Parsedown Contributes: Parsedown's core parsing logic heavily utilizes regular expressions, making it inherently vulnerable to ReDoS attacks if those expressions are not carefully crafted. The complexity of Markdown syntax and the potential for nested structures can make crafting malicious input easier.
  • Example: Nested emphasis (e.g., *******************bold*******************) or deeply nested links/images can, in certain versions or with specific configurations, trigger exponential backtracking in the regular expression engine.
  • Impact: Denial of Service (DoS), application unavailability, potential server resource exhaustion.
  • Risk Severity: High
  • Mitigation Strategies:
    • Regular Expression Optimization: Ensure Parsedown's regular expressions are optimized for performance and resilience against ReDoS. This includes avoiding overly complex or nested patterns, using atomic grouping where appropriate, and avoiding catastrophic backtracking.
    • Input Sanitization and Validation: Strictly validate and sanitize all user-supplied Markdown input before it reaches Parsedown. Limit the length of input, restrict allowed characters, and consider whitelisting allowed Markdown elements instead of blacklisting dangerous ones. This is the most crucial mitigation.
    • Input Length Limits: Enforce reasonable limits on the length of Markdown input to prevent excessively long strings from triggering exponential backtracking.
    • Timeout Mechanisms: Implement timeouts for the regular expression engine to prevent it from running indefinitely on malicious input. This is a crucial defense-in-depth measure.
    • Regular Updates: Keep Parsedown updated to the latest version. Security patches and improvements, including ReDoS fixes, are often included in updates.
    • Web Application Firewall (WAF): A WAF can be configured to detect and block common ReDoS patterns.
    • Resource Monitoring: Monitor CPU and memory usage to detect potential ReDoS attacks in progress.

Attack Surface: Cross-Site Scripting (XSS) (if HTML is enabled)

  • Description: If raw HTML input is allowed (i.e., setSafeMode(false) or setMarkupEscaped(false) is used, or if custom extensions are poorly implemented), attackers can inject malicious JavaScript code that will be executed in the context of the user's browser.
  • How Parsedown Contributes: While Parsedown attempts to sanitize HTML when setSafeMode(true) (the default), vulnerabilities or bypasses can exist. If HTML is explicitly allowed, Parsedown becomes a direct conduit for XSS.
  • Example: <script>alert('XSS');</script>, or using event handlers like <img src=x onerror=alert(1)>. More complex attacks can use obfuscation to bypass simple filters.
  • Impact: Session hijacking, cookie theft, website defacement, phishing, malware distribution, redirection to malicious sites.
  • Risk Severity: Critical (if HTML is allowed), High (if setSafeMode(true) but a bypass is found).
  • Mitigation Strategies:
    • Enable setSafeMode(true) (Default and Recommended): This is the most important mitigation. Parsedown will escape HTML entities, significantly reducing the risk.
    • Use a Dedicated HTML Sanitizer: Even with setSafeMode(true), it's highly recommended to use a robust, well-maintained HTML sanitization library (like DOMPurify or HTML Purifier) after Parsedown processes the input. This provides a second layer of defense. Parsedown's built-in sanitization is good, but a dedicated library is generally more comprehensive and frequently updated.
    • Content Security Policy (CSP): Implement a strong CSP to restrict the sources from which scripts can be loaded, even if XSS is injected. This is a critical defense-in-depth measure.
    • Input Validation: Validate and sanitize input before it reaches Parsedown, even with setSafeMode(true). This can help prevent bypasses. Focus on whitelisting allowed characters and patterns.
    • Output Encoding: Ensure that the output of Parsedown is properly encoded for the context in which it's displayed (e.g., HTML encoding for web pages).
    • Avoid Custom Extensions (or Audit Carefully): If you must use custom extensions, audit them extremely carefully for XSS vulnerabilities.
    • Regular Updates: Keep Parsedown and any extensions updated to the latest versions to benefit from security patches.

Attack Surface: Unsafe URL Handling (if not properly sanitized)

  • Description: Parsedown can process URLs within Markdown (e.g., links and images). If not handled carefully, these URLs could be used for phishing attacks, cross-site scripting (XSS) via javascript: URLs, or other malicious purposes.
  • How Parsedown Contributes: Parsedown parses and renders URLs. If the application doesn't properly validate or sanitize these URLs before displaying them, it creates a vulnerability.
  • Example: [Click here](javascript:alert('XSS')), [link](data:text/html;base64,PHNjcmlwdD5hbGVydCgxKTwvc2NyaXB0Pg==), or a link to a phishing site.
  • Impact: Phishing, XSS, redirection to malicious websites, potential for malware distribution.
  • Risk Severity: High
  • Mitigation Strategies:
    • URL Validation: Implement strict URL validation to ensure that only allowed schemes (e.g., http, https, mailto) are permitted. Reject any URLs that contain suspicious characters or patterns.
    • Whitelist Allowed Domains (if possible): If you only expect links to specific domains, whitelist those domains and reject all others.
    • Use a URL Sanitization Library: Employ a library specifically designed for URL sanitization to handle edge cases and ensure proper encoding.
    • Content Security Policy (CSP): CSP can help restrict the protocols and domains that can be loaded, mitigating some risks.
    • Rel="noopener noreferrer": When generating links, always include rel="noopener noreferrer" to prevent the linked page from accessing the opener window.

Attack Surface: Uncontrolled Resource Consumption (less common, but possible)

  • Description: Extremely large or deeply nested Markdown input could potentially consume excessive memory or CPU resources, leading to a denial-of-service (DoS) condition. This is less likely than ReDoS, but still a consideration.
  • How Parsedown Contributes: The parsing process itself, especially with complex or deeply nested Markdown, could consume significant resources if not properly limited.
  • Example: Extremely long lines, deeply nested lists, or a large number of inline code spans.
  • Impact: Denial of Service (DoS), application unavailability.
  • Risk Severity: Medium (Potentially High depending on server resources and configuration)
  • Mitigation Strategies:
    • Input Size Limits: Enforce strict limits on the size of Markdown input.
    • Recursion Limits: If possible, configure Parsedown (or the PHP environment) to limit the depth of recursion to prevent stack overflow errors.
    • Resource Monitoring: Monitor server resources (CPU, memory) to detect potential resource exhaustion attacks.
    • Rate Limiting: Limit the rate at which users can submit Markdown for processing.

Attack Surface: Potential for Information Disclosure through Error Messages (if not handled properly)

  • Description: If Parsedown encounters an error during parsing and the application doesn't handle the error gracefully, it might reveal sensitive information about the server's configuration, file paths, or internal code structure.
    • How Parsedown Contributes: Parsedown itself might throw exceptions or generate error messages that contain details about the parsing process.
    • Example: An unhandled exception might reveal the file path of the Parsedown library or other internal details.
    • Impact: Leakage of sensitive information that could be used by attackers to further compromise the system.
    • Risk Severity: Medium
    • Mitigation Strategies:
      • Proper Error Handling: Implement robust error handling to catch any exceptions thrown by Parsedown and display generic error messages to the user. Do not expose internal error details.
      • Logging: Log detailed error information (including stack traces) to a secure location for debugging purposes, but never expose this information to the user.
      • Production Mode: Ensure that your application is running in a production environment with appropriate error reporting settings (e.g., display_errors = Off in PHP).