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attack-surface.md

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Attack Surface Analysis for apache/httpd

Attack Surface: Unpatched Software Vulnerabilities (CVEs)

  • Description: Exploitation of known vulnerabilities (Common Vulnerabilities and Exposures) in the Apache httpd software or its modules.
  • httpd Contribution: The vulnerability exists within the httpd codebase or a loaded module, making httpd the direct target.
  • Example: An attacker exploits a known buffer overflow in an older version of mod_ssl to gain remote code execution on the httpd server.
  • Impact: Remote code execution, denial of service, privilege escalation (all directly impacting the httpd server).
  • Risk Severity: Critical to High (depending on the specific CVE).
  • Mitigation Strategies:
    • Patch Management: Implement a robust patch management process. Regularly check for and apply security updates from the Apache Software Foundation. Automate updates where possible. This is the primary defense.
    • Vulnerability Scanning: Use vulnerability scanners to identify outdated or vulnerable versions of httpd and its modules.
    • Minimal Installation: Install only the necessary modules to reduce the attack surface exposed by httpd.

Attack Surface: Misconfigured Modules (Specifically High-Impact Modules)

  • Description: Improperly configured Apache modules, leading to significant security weaknesses directly exploitable through httpd. Focus here is on modules with a high potential for severe impact.
  • httpd Contribution: The vulnerability arises from the configuration of a specific httpd module, and the exploitation occurs through httpd.
  • Example:
    • mod_proxy configured as an open proxy, allowing attackers to use the server for malicious purposes (SSRF, relaying attacks). This is a direct httpd configuration issue.
    • mod_rewrite rules that are too permissive, allowing attackers to access files outside the intended webroot (directory traversal) via crafted HTTP requests to httpd.
    • mod_security (if used) misconfigured to allow malicious requests that it should block.
  • Impact: Unauthorized access, server-side request forgery (SSRF), denial of service, potential for remote code execution (depending on the module and misconfiguration).
  • Risk Severity: High
  • Mitigation Strategies:
    • Principle of Least Privilege: Configure each module with the absolute minimum necessary permissions and features.
    • Configuration Review: Thoroughly review and understand the security implications of every configuration directive for high-impact modules. Focus on modules that handle requests, proxies, or authentication.
    • Testing: Extensively test module configurations, including negative testing (attempting to exploit potential misconfigurations).
    • Documentation: Refer to the official Apache documentation for each module to understand secure configuration practices. Pay close attention to security warnings.
    • Disable Unnecessary Modules: Disable all modules that are not strictly required. This is a crucial step to reduce httpd's attack surface.

Attack Surface: Denial-of-Service (DoS) Attacks (Targeting httpd Directly)

  • Description: Attacks that aim to make the httpd server itself unavailable to legitimate users by exhausting its resources.
  • httpd Contribution: httpd is the direct target of the DoS attack, and its configuration and resource limits determine its resilience.
  • Example: A Slowloris attack targeting httpd, which keeps many connections open with slow data transfer, exhausting httpd's connection pool. A flood of HTTP requests overwhelming httpd's processing capacity.
  • Impact: Service unavailability (of httpd), degraded performance.
  • Risk Severity: High
  • Mitigation Strategies:
    • Resource Limits: Configure appropriate limits within httpd on the number of concurrent connections, request sizes, and timeouts (e.g., MaxRequestWorkers, Timeout, KeepAliveTimeout). These are httpd-specific settings.
    • Rate Limiting: Use httpd modules like mod_qos, mod_evasive, or mod_security to limit the rate of requests from individual clients or IP addresses. These modules operate within httpd.
    • Connection Management: Use httpd modules like mod_reqtimeout to mitigate slow-request attacks at the httpd level.

Attack Surface: HTTP Request Smuggling/Splitting (Against httpd)

  • Description: Exploiting inconsistencies in how HTTP requests are parsed by front-end proxies and the back-end Apache httpd server to bypass security controls or inject malicious content.
  • httpd Contribution: httpd's request parsing logic is a key factor in the vulnerability; the attack is successful due to how httpd itself handles the crafted request.
  • Example: An attacker crafts a request that is interpreted as two separate requests by the back-end httpd server, allowing them to access restricted resources or inject malicious content into a subsequent request handled by httpd.
  • Impact: Bypassing security controls, unauthorized access, request forgery, cache poisoning (all directly affecting httpd's behavior).
  • Risk Severity: High
  • Mitigation Strategies:
    • Consistent Configuration: Ensure that front-end proxies and Apache httpd are configured to handle HTTP requests in a consistent manner, particularly regarding Transfer-Encoding and Content-Length headers. This includes httpd's configuration.
    • HTTP/2: Using HTTP/2 can help mitigate some request smuggling vulnerabilities, as it has stricter request parsing rules. This would involve configuring httpd for HTTP/2.
    • Keep httpd Updated: Ensure httpd is updated to the latest version, as patches often address these types of vulnerabilities within httpd's parsing logic.
    • Disable Unnecessary Features: If not using chunked encoding, consider disabling it within httpd's configuration.