Securing serverless functions against event injection attacks

Serverless computing removes the need to manage underlying servers, but it does not remove application attack paths. A function may trust a webhook payload, queue message, object name or scheduled-event field and pass it into a shell, database query, template or HTTP client. That is the core of event injection attacks: untrusted event content changes what the function does.

For Australian engineering and security teams, the control must fit managed cloud delivery without slowing releases. Workloads in Sydney and Melbourne regions face the same malicious inputs as global platforms, while local governance determines how evidence is accepted. A practical design combines strict input validation, constrained runtime identities, observable logs and a repeatable patch path.

Treat every event as hostile input

Start by drawing the trust boundary around every trigger. API gateways, message queues, event buses, storage events and schedules can all deliver data created outside the function’s code. Even a correctly signed event may contain dangerous values if an upstream application allows users to influence its content. Authenticate the publisher separately, then treat the payload as untrusted after verification.

Inventory the events each handler accepts, including headers, metadata, filenames, identifiers and nested objects. Decide which fields have a fixed type, length or permitted format, and reject anything outside that contract. This applies to direct invocation as well as public webhooks because internal publishers can also be compromised or misconfigured.

Validate data before it reaches business logic

Validation should occur inside the handler, before parsing or transformation reaches privileged operations. Apply an allowlist schema that checks data types, string lengths, enumerations, ranges and unexpected properties. Reject unknown fields where practical, and enforce payload-size limits to prevent resource exhaustion. Gateway validation can provide defence in depth, but it cannot protect functions invoked through every other route.

Use safe interfaces for each destination. Parameterise database queries, pass shell arguments without constructing command strings, avoid dynamic evaluation, and validate URLs against permitted schemes and hosts. Sanitise filenames before joining them to storage paths, and reject path traversal sequences. Findings from these tests should enter a discovery-to-patch workflow, with severity based on exploitability and business impact.

Apply least privilege to runtime identities

Give each function a dedicated execution role or managed identity with only the permissions required for its task. Separate identities for unrelated workloads so one compromised function cannot read another function’s queues, buckets or secrets. Avoid wildcards in IAM policies, and review permissions whenever code, triggers or data destinations change.

Constrain outbound access as well. Functions that fetch user-supplied links can become serverside request forgery tools, including paths toward cloud metadata services and internal administration interfaces. Deny access to link-local addresses, restrict egress through controlled gateways or private endpoints, and place sensitive workloads in networks with limited reachability. Store secrets in managed services, rotate them and retrieve only the values needed during each operation.

Control dependencies and execution paths

Serverless applications still depend on operating systems, language runtimes and third-party packages. Produce a software bill of materials, lock dependency versions, scan packages for known vulnerabilities and remove unused components. Keep runtime images and provider-managed components on supported versions, with deployment gates that prevent a critical finding from reaching production without an accepted exception.

Control how code is invoked. Require authentication and authorisation at both the event source and handler where appropriate, and do not rely on obscurity in event names or URLs. Use idempotency keys, timestamps and replay windows for payment, account or order events. Set concurrency, memory and timeout limits so repeated malicious events cannot exhaust budget or block legitimate processing.

Monitor behaviour and prove resilience

Capture security-relevant telemetry without storing sensitive payloads unnecessarily. Log event identifiers, source identity, validation failures, invoked destinations, privilege denials and unusual function errors. Correlate these records with cloud audit logs and send actionable alerts to a SIEM. Useful detections include attempts to invoke internal addresses, repeated schema failures, access to secret stores and abrupt changes in invocation volume.

Map the controls to the Australian Centre for Security Collaboration’s Essential Eight and the Australian Signals Directorate’s cloud guidance. Financial institutions must also account for APRA CPS 230 requirements concerning information security, incident response and testing. Evidence should show who owns each control, when it was tested and how failed remediation was managed.

For healthcare and online-commerce organisations, privacy teams following OAIC expectations under the Australian Privacy Act need visibility into where event data is processed and retained. Communication can remain direct: in Australian boardrooms, a clear statement such as “the malicious payload was blocked and the evidence was retained” is more useful than security jargon. Regular exercises should verify that alerts reach the right people and that containment steps work across cloud, identity and third-party systems.

Operationalise protection continuously

Turn the controls into automated checks in CI/CD and infrastructure-as-code workflows. Test validation rules with malformed, oversized and adversarial event fixtures, and scan function policies for wildcard permissions or newly opened network paths. Track rejected events as security signals rather than routine application noise. Scheduled reviews should confirm that old functions, unused identities, exposed endpoints and vulnerable dependencies are removed promptly.

Begin with a scoped, trial-based review of your highest-impact functions. Infiziant Security can assess event handling, runtime permissions and cloud configuration, then provide a free VAPT report to help your team prioritise verified findings and move them into remediation.