Add initial draft of a security model.

git-svn-id: https://svn.apache.org/repos/asf/httpd/httpd/trunk@1934213 13f79535-47bb-0310-9956-ffa450edef68

diff --git a/SECURITY.md b/SECURITY.md
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--- a/SECURITY.md
+++ b/SECURITY.md
@@ -1,15 +1,126 @@
-# Security Policy
+
+# Apache httpd Security Policy
+
+This document is an overview of the security model for Apache
+httpd. Security vulnerabilities reported to the project need to
+demonstrate how an attacker can violate the security model.
 
 ## Supported Versions
 
 Currently the only supported version is the latest patch release of the
-2.4.x stable branch.
+`2.4.x` stable branch. Vulnerabilities which exist *only* in
+unreleased branches (such as `trunk`) may be treated as normal bug
+reports.
 
-## Security Updates
+## Reporting Vulnerabilities
 
-[Apache 2.4 Security Vulnerabilities](http://httpd.apache.org/security/vulnerabilities_24.html)
+For information on how to report a new security problem please see
+[here](http://httpd.apache.org/security_report.html). The process of
+reporting and handling vulnerabilities is out of scope for this
+document.
 
-## Reporting a Vulnerability
+For a list of already-addressed vulnerabilities, see [Apache 2.4 Security
+Vulnerabilities](http://httpd.apache.org/security/vulnerabilities_24.html)
 
-For information on how to report a new security problem please see
-[here](http://httpd.apache.org/security_report.html)
+## Model scope
+
+If an issue is reported against an aspect of the security model which
+is not documented here, it MUST be accompanied by a clear description
+of that aspect the model, showing why a trust boundary exists and how
+it is violated. It is helpful to use references to documentation
+and/or demonstrate common usage patterns.
+
+Any security vulnerability SHOULD be reproducible:
+
+1. under a reasonable, supported configuration.
+2. without using third-party modules, or modules explicitly designed
+  for debugging.
+3. under a standard build on a supported platform.
+
+Issues which are reproducible only using instrumented builds (such as
+ASAN, or under valgrind) should be clearly explained as such.
+
+## Basic model
+
+Processing of requests by remote untrusted users (HTTP clients) MUST
+NOT crash or prematurely terminate server processes, nor gain code
+execution privileges.  In the default configuration, timeouts are
+applied to most aspects of HTTP request handling such that a single
+client SHOULD NOT tie up a single processing thread or process
+indefinitely.
+
+It is the responsibility of the server administrator to tune and
+configure httpd appropriately to the operating environment, for
+example adjusting MPM limits (see
+https://httpd.apache.org/docs/trunk/misc/security_tips.html).
+
+Denial of service attacks are expected to be mitigated at firewall or
+network level. It is expected that an attacker who is able to
+establish multiple simultaneous connections to the server will, to
+some extent, deny service to other remote users. 
+
+Example vulnerabilities which violated the model: CVE-2026-23918,
+CVE-2004-0786.
+
+## Resource Consumption
+
+Handling requests entails resource consumption (CPU, memory, disk
+space for logs, etc). It is expected that resource consumption by the
+server is at worst proportional to the volume of network traffic.
+
+Memory consumption by a single request should be capped, with
+configurable limits; e.g. LimitRequestFields limits the RAM
+consumption by HTTP headers, LimitXMLRequestBody limits the RAM
+consumption by parsing XML request documents.
+
+Example vulnerabilities which violated the model: CVE-2004-0942
+
+## Privilege separation on Unix platforms
+
+On Unix platforms, when httpd is started as the root user, privilege
+separation is used between the parent process which retains root
+privileges, and child processes (and threads). Child processes/threads
+run as a less-privileged user and group which is configurable via the
+`mod_unixd` module, https://httpd.apache.org/docs/2.4/mod/mod_unixd.html
+
+The less-privileged user:
+
+* cannot obtain root privileges, 
+* cannot read or truncate log files,
+* retains access to e.g. any private TLS key data loaded in memory.
+
+Use of platform-specific sandboxing or security features (such as use
+of containers, chroot, SELinux) are out of scope for this security
+model.
+
+Example vulnerabilities which violated the model: CVE-2007-3304,
+CVE-2012-0031.
+
+## Delegated Configuration 
+
+Server configuration can be delegated to trusted local site authors by
+allowing use of .htaccess files in non-default configurations.  Local
+site authors are trusted to not attack the server with malformed or
+malicious .htaccess files (for example, files of excessive size).
+
+In configurations supporting in-process scripting language interpreters
+which are not sandboxed, such as `mod_lua` or `mod_php`, local site
+authors have equivalent privileges to the less-privileged server user.
+
+(### TODO something about AllowOverride)
+
+## Dependent Services
+
+Many configurations depend on backend servers or services which are
+trusted entities.
+
+* Backend servers accessed in a proxy configuration should not be able
+  to influence HTTP protocol framing logic in the frontend (client)
+  ("response splitting" attacks).
+
+* Services used for authentication or caching privileged/protected
+  data (Redis/Valkey caches, database or LDAP servers) are trusted not
+  to attack the web server.
+
+Example vulnerabilities which violated the model: CVE-2026-33523,
+CVE-2024-42516.