An email agent router should treat the visible From value as a claim in the message—not proof of who sent it or permission to act. Resolve identity from evidence supplied by a trusted mail ingress, map that verified principal to permissions in application code, and treat the model’s interpretation of the email as a proposal that must pass those checks.
This separation matters because email can carry prompt-injection instructions as well as misleading headers. OWASP advises treating emails and other external content as untrusted; delimiters or a screening model can help, but neither can replace deterministic authorization and tool-argument validation. See the OWASP AI Agent Security Cheat Sheet and OWASP LLM Prompt Injection Prevention Cheat Sheet.
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Separate sender claims, authenticated identity, and routing
Keep four concepts distinct in the design. A message’s From, Reply-To, and display name are claimed sender data; parsing them tells you what the message says, not who is authorized. An authenticated principal is an identity established by the trusted mail ingress under a documented verification policy. Application authorization maps that principal to permitted tenants, workflows, data, and actions. Task classification is the model’s interpretation of the email and can suggest a route, but cannot grant access.
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A safe route therefore combines trusted principal context, a server-side allowlist or policy, and a constrained task classification. Do not let a sender-controlled value select an arbitrary tenant, agent, tool, or privilege level. This is an application-design consequence of OWASP’s guidance on external-data distrust, least privilege, and independent authorization; it is not a protocol-specific sender-verification rule. OWASP AI Agent Security Cheat Sheet · OWASP LLM Prompt Injection Prevention Cheat Sheet · OWASP Authentication Patterns Cheat Sheet.
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Build the router around a trusted ingress
- Define the ingress trust contract. Document which gateway or mail provider supplies identity evidence, how the router receives it, and which outcomes your application accepts. Verify those outcomes against that component’s documented contract before mapping them to an account. This guidance does not establish a particular SPF, DKIM, DMARC, ARC, SMTP, MIME, or gateway policy, and a visible sender field alone is not an authenticated principal.
- Protect identity context in transit. If a trusted gateway injects identity headers, ensure it removes or overwrites every inbound value of those headers and that callers cannot reach the application by bypassing the gateway. Alternatively, verify signed context with checks for issuer, intended audience, scope, freshness, replay, and applicability to the requested operation. A signature establishes neither account access nor permission to perform an action; the application must still authorize it. OWASP Authentication Patterns Cheat Sheet.
- Fail closed on uncertain identity. In application code, resolve the trusted principal to a tenant and permitted workflow. Reject or quarantine a message if identity evidence is absent, conflicting, stale, or unverifiable rather than falling back to
From,Reply-To, or a display name. - Keep classification inside policy. Ask the model to classify or summarize content only within the routes available to the authenticated principal. Treat its route as a proposal to validate, not a command to execute.
Parse addresses without confusing syntax for identity
Use a maintained email/address validation library that supports the formats your service accepts; avoid a home-grown strict regular expression. Apply message size and resource limits, reject malformed structures according to an explicit policy, and preserve the original value separately from any canonical comparison value. OWASP recommends a consistent normalization policy, including lowercasing the domain, while avoiding provider-specific transformations such as dot removal unless the system fully controls their behavior. OWASP Email Validation and Verification in Identity Systems Cheat Sheet · OWASP Input Validation Cheat Sheet.
- Make local-part handling an explicit application policy: SMTP technically permits case sensitivity, and provider behavior varies.
- Handle Unicode and internationalized domains carefully, including visually similar characters and IDN comparisons.
- Encode values appropriately when displaying or logging them; do not treat validation as proof of mailbox ownership.
Keep parsing, authentication, and authorization as separate stages: the parser reports what the message contains, while trusted ingress evidence and application policy determine identity and access.
Treat email content and retrieved material as data
Email bodies, attachments, quoted threads, and retrieved documents can contain instructions intended to manipulate an agent. OWASP specifically includes emails among external sources that may carry malicious instructions. Delimit untrusted content from system instructions and consider screening it, but do not assume delimiters or a guardrail model can reliably neutralize injection. A screening model can itself fail; it cannot replace least privilege, deterministic checks, or approval for destructive actions. OWASP AI Agent Security Cheat Sheet · OWASP LLM Prompt Injection Prevention Cheat Sheet.
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For workflows that need stronger isolation, use a quarantined parsing stage: a model with no tool access reads risky content and extracts facts; a separate privileged planner and constrained interpreter decide whether an action is allowed. This reduces the parser’s direct blast radius, but adds capability-tracking and policy complexity and is not a guarantee against manipulation. OWASP describes limitations to this approach and warns that its referenced research artifact is not a supported security component. Retrieved content also remains data rather than authority: check access at retrieval time, independently authorize tools, require confirmation for high-risk actions influenced by retrieved material, and trace retrieval-to-tool causality. OWASP LLM Prompt Injection Prevention Cheat Sheet · OWASP RAG Security Cheat Sheet.
Authorize every proposed tool call outside the model
Before execution, application code should validate the tool name and arguments, confirm the principal’s tenant and action permissions, and check that the requested operation is consistent with the original task. Give each agent only the tools and data required for its job; use context-specific allowlists, require human approval for high-impact or irreversible operations, and use circuit breakers for anomalous tool behavior. These controls still apply when a model has classified the email correctly: classification is not authorization. OWASP AI Agent Security Cheat Sheet · OWASP LLM Prompt Injection Prevention Cheat Sheet · OWASP RAG Security Cheat Sheet.
Choose an isolation pattern for your risk and operating constraints
| Pattern | Trust boundary and blast radius | Trade-offs |
|---|---|---|
| Single model with constrained tools | The model handles untrusted content, so external argument validation, least privilege, and approval gates are essential. | Simplest architecture; safety depends on strong controls outside model output. OWASP AI Agent Security Cheat Sheet · OWASP LLM Prompt Injection Prevention Cheat Sheet. |
| Screening or guardrail model | Adds a layer to flag risky content or proposed actions, but the guardrail can also fail. | Can add latency and cost; it does not replace deterministic validation, least privilege, or approval. OWASP LLM Prompt Injection Prevention Cheat Sheet. |
| Quarantined parser plus privileged planner/interpreter | Separates untrusted-content parsing from tool access more strongly. | Requires careful policy design and capability tracking; isolation is not a guarantee against manipulation. OWASP LLM Prompt Injection Prevention Cheat Sheet. |
| Trusted-proxy identity context | Can carry ingress-established identity when the proxy controls the context and the application cannot be reached around it. | Requires stripping or overwriting inbound identity headers, blocking direct bypass, or verifying signed context with freshness and replay controls. It does not replace application authorization. OWASP Authentication Patterns Cheat Sheet. |
Log decisions and test the boundaries
Retain enough safe metadata to trace the authenticated principal, policy decision, classification, retrieval, and tool invocation. Minimize personal data, and never log credentials, tokens, or message content that is not needed for the audit purpose. A useful record makes it possible to determine which trusted identity and policy permitted an action without turning logs into another store of sensitive email. OWASP AI Agent Security Cheat Sheet · OWASP Email Validation and Verification in Identity Systems Cheat Sheet · OWASP RAG Security Cheat Sheet.
Exercise the system with dummy data and sandboxed tools. Include tests for:
- spoofed visible senders, conflicting identity headers, and attempts to bypass the trusted ingress;
- malformed addresses, Unicode lookalikes, and stale or replayed signed context;
- hidden instructions in bodies, attachments, quoted content, and retrieved documents;
- attempts to cross tenant boundaries or use unexpected tools and arguments; and
- high-impact or irreversible actions that should require approval.
These cases test whether identity comes from the trusted path, whether content stays data, and whether authorization remains effective when model output is wrong or adversarial. OWASP’s prompt-injection and authentication-pattern guidance supports testing those separate boundaries. OWASP LLM Prompt Injection Prevention Cheat Sheet · OWASP Authentication Patterns Cheat Sheet.
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