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Threat Model Analysis for bigskysoftware/htmx

Threat: DOM Manipulation via hx-target and hx-swap

  • Threat: Unauthorized DOM Modification
  • Description: An attacker crafts a malicious server response or exploits a client-side vulnerability to modify the hx-target or hx-swap attributes. They could then inject arbitrary HTML, potentially including malicious scripts or alter existing content, into unintended parts of the page. For example, they might change hx-target to point to a sensitive area displaying user data or containing hidden form fields. They might change hx-swap from innerHTML to outerHTML to replace a larger portion of the DOM. This directly leverages htmx's core functionality for malicious purposes.
  • Impact:
    • Execution of arbitrary JavaScript (XSS).
    • Data exfiltration (reading sensitive data from the manipulated DOM).
    • Defacement of the application.
    • Phishing attacks (by injecting deceptive content).
    • Bypass of security controls (e.g., manipulating form fields or hidden tokens).
  • Affected htmx Component: hx-target and hx-swap attributes.
  • Risk Severity: Critical
  • Mitigation Strategies:
    • Strict Server-Side Validation: Never directly reflect user input into hx-target or hx-swap. Use a whitelist of allowed, pre-defined targets and swap methods.
    • Prefer Static Targets: Use static hx-target values whenever possible.
    • Use hx-select: Extract only the necessary portion of the server response using hx-select, limiting the impact of a compromised response.
    • Content Security Policy (CSP): Implement a strong CSP to limit the execution of injected scripts.

Threat: Unintended Script Execution via hx-trigger

  • Threat: Client-Side Code Injection
  • Description: An attacker manipulates the hx-trigger attribute, potentially combined with a crafted server response containing inline event handlers (e.g., onload, onclick). If the server reflects user input into hx-trigger or the response includes unexpected inline handlers, the attacker could trigger immediate execution of JavaScript upon the response being loaded. This bypasses htmx's built-in protection against <script> tag execution and directly exploits htmx's event handling mechanism.
  • Impact:
    • Execution of arbitrary JavaScript (XSS).
    • Data theft.
    • Session hijacking.
    • Malware distribution.
  • Affected htmx Component: hx-trigger attribute, and the processing of server responses containing inline event handlers.
  • Risk Severity: High
  • Mitigation Strategies:
    • Sanitize Server Responses: Thoroughly sanitize server responses to remove any unintended inline event handlers or JavaScript.
    • Controlled hx-trigger: Avoid reflecting user input into hx-trigger. Use a whitelist of allowed trigger events.
    • Avoid Inline Handlers: Use htmx.on or event delegation instead of inline event handlers in server responses.
    • Content Security Policy (CSP): A strong CSP can mitigate the impact of injected scripts.

Threat: Sensitive Data Leakage via hx-vals

  • Threat: Data Exposure
  • Description: An attacker observes network traffic or server logs to obtain sensitive data inadvertently included in htmx requests via hx-vals. This could happen if hx-vals is dynamically populated with sensitive information (e.g., session tokens, user IDs) or if it includes unnecessary data from the DOM. This is a direct consequence of how hx-vals is used to transmit data.
  • Impact:
    • Exposure of sensitive user data.
    • Session hijacking.
    • Account compromise.
  • Affected htmx Component: hx-vals attribute.
  • Risk Severity: High
  • Mitigation Strategies:
    • Explicit hx-vals: Carefully define the values included in hx-vals. Avoid wildcards or automatic inclusion of all form data.
    • Prefer POST Requests: Use POST requests for htmx interactions involving sensitive data.
    • Don't Store Secrets in DOM: Avoid storing sensitive data in the DOM.
    • Secure Server-Side Logging: Configure server logs to avoid recording sensitive request data.

Threat: Client-Side Validation Bypass

  • Threat: Bypass of Security Checks
  • Description: An attacker manipulates htmx attributes or server responses to disable or circumvent client-side validation implemented using htmx extensions or custom JavaScript that directly interacts with htmx attributes. For example, they might remove hx-trigger modifiers that enforce validation or craft a response that bypasses the validation logic that depends on htmx.
  • Impact:
    • Submission of invalid or malicious data.
    • Bypass of security controls.
    • Potential for other vulnerabilities (e.g., SQL injection, XSS) if server-side validation is weak.
  • Affected htmx Component: Any htmx attribute or extension used for client-side validation that relies on htmx functionality.
  • Risk Severity: High
  • Mitigation Strategies:
    • Server-Side Validation is Essential: Never rely solely on client-side validation. Always perform thorough validation on the server.
    • Defense in Depth: Consider techniques to make it harder to tamper with htmx attributes, but don't rely on them as the primary defense.