Add a trust boundary section (#4716)

Signed-off-by: Josh Bressers <josh@bress.net>

SECURITY.md

@@ -14,3 +14,23 @@ affected versions, and, if known, mitigations for the issue.
 All support will be made on a best effort basis, so please indicate the "urgency level" of the vulnerability as Critical, High, Medium or Low.
 
 For more details, see our [security policy documentation](https://oss.anchore.com/docs/contributing/security/).
+
+## Trust Boundary
+
+Syft is a tool to scan content and product an SBOM. Syft is not a tool designed to scan malicious content. Detecting and properly reporting on purposely malicious artifacts is outside the scope of Syft's expected operating environment.
+
+There are many possible ways for malicious content to cause Syft to become confused or fail to include results in an SBOM. We do not consider this to be a security vulnerability.
+
+**Examples**
+- Removing or altering a package lock file
+- Removing or altering an RPM or DEB database
+- A malicious archive that Syft will skip but the runtime may not
+- Self modifying systems that change state when running
+
+We consider the security trust boundary for Syft to be anything that causes problems for the overall system running Syft, or Syft operating in a way that is dangerous to itself, the system, or the operator.
+
+**Examples**
+- Filling up temp space permanently
+- Syft executing arbitrary code when scanning an artifact
+- Syft leaking secrets from the environment or configuration files into logs or SBOMs
+- Syft operating outside of the expected artifact or directory (directory traversal)