Travel and expense systems look like ordinary SaaS applications, but their APIs coordinate money movement, corporate-card data, employee identity, travel itineraries, receipts, approval authority, policy exceptions, and third-party booking services. Many high-impact attacks therefore use valid API syntax and valid accounts. Security has to protect both data access and the business sequence that turns an expense into an approved reimbursement or booking.
The T&E API attack surface
Employee and mobile APIs
Profiles, itineraries, receipts, expenses, card transactions, and reimbursement status.
Approver APIs
Review, reject, delegate, override, and policy-exception workflows.
Finance / ERP integrations
General ledger, payroll, reimbursement, tax, and accounting synchronization.
Travel and card partners
Air, hotel, rail, booking, corporate-card, bank, and travel-management APIs.
The security boundary extends beyond the primary application because third-party services can inject data, trigger workflows, or receive sensitive records through trusted integrations.
Protect approval workflows from sequence and role abuse
Expense fraud often depends on workflow, not malformed input. An attacker may try to self-approve, skip required review, split expenses to stay below thresholds, replay reimbursement actions, abuse delegated approval, or change an expense after approval.
| Workflow risk | Example control |
|---|---|
| Self-approval | Prevent submitter and final approver from being the same principal where policy requires separation |
| State skipping | Allow only defined transitions such as draft → submitted → approved |
| Replay | Use idempotency and transaction identifiers for reimbursement actions |
| Post-approval mutation | Lock financial fields or require re-approval after material changes |
| Delegation abuse | Time-bound delegation, role checks, and visible audit history |
Handle corporate-card and payment data carefully
T&E platforms may receive card transaction feeds or payment-related identifiers. Minimize cardholder data, use tokenized references where possible, restrict fields by role, and align any environment that stores, processes, or transmits payment-card data with applicable PCI DSS responsibilities.
Never place full card details, bank account data, access tokens, or receipt contents into verbose logs simply because the API request is useful for debugging.
Receipts and attachments are untrusted inputs
Receipts may arrive as images, PDFs, email attachments, or files fetched from external URLs. Treat every upload as hostile: enforce content type and size, scan where appropriate, isolate parsing, randomize storage names, and prevent user-controlled paths. If the platform fetches a receipt or itinerary from a URL, apply SSRF protections and strict outbound destination rules.
Third-party travel APIs create inherited risk
Travel platforms rely on external availability, booking, identity, card, and itinerary providers. OWASP classifies unsafe consumption of APIs as a dedicated risk because developers often trust third-party responses more than direct user input.
- Authenticate every provider and validate server identity.
- Validate response schemas, sizes, URLs, and redirect behavior.
- Use separate credentials and scopes per integration.
- Set timeouts, retries, circuit breakers, and maximum response sizes.
- Monitor changes in provider endpoints and data shapes.
- Plan how to revoke or isolate one provider without taking down the entire T&E platform.
Protect scarce inventory and pricing workflows from automation
Travel-search and booking APIs can be expensive and can interact with scarce inventory. Automated scraping, repeated holds, fare checking, or account abuse can create backend cost even when requests are technically valid. Rate controls should consider user, tenant, device, operation, and business state rather than only source IP.
Runtime behaviors that deserve attention
Cross-employee access
One identity begins reading expenses or trips for unrelated employees.
Approval anomalies
A user approves unusually high volume or bypasses expected review stages.
Reimbursement duplication
Similar expenses or transaction IDs are submitted repeatedly.
Partner drift
An integration starts returning new redirect targets, fields, or abnormal response sizes.
Correlating API activity with identity, finance, and workflow state helps distinguish ordinary travel behavior from fraud or account compromise.
Travel and expense API security checklist
- Inventory employee, mobile, partner, ERP, card, and administrative APIs.
- Enforce object, property, and function authorization.
- Model approval and reimbursement state transitions explicitly.
- Use idempotency for financial actions.
- Minimize payment, identity, itinerary, and receipt data exposure.
- Harden upload and remote-fetch workflows.
- Apply rate and resource controls to search and booking operations.
- Validate third-party API responses as untrusted input.
- Monitor cross-user access, approval anomalies, and duplicate financial actions.
Frequently asked questions
What makes travel and expense APIs high risk?
They combine employee identity, itinerary data, financial transactions, corporate-card information, approvals, reimbursements, and third-party providers, so one API weakness can affect both privacy and money movement.
What is the most important authorization control?
Verify access to the exact expense, trip, receipt, transaction, or report object after authentication. Roles alone are not enough if object ownership or tenant boundaries are not checked.
How should reimbursement APIs prevent duplicate payments?
Use idempotency keys or transaction identifiers, enforce state transitions, reject replayed requests, and reconcile against finance-system records.
Are receipt URLs an SSRF risk?
Yes. If the server fetches user-supplied receipt or itinerary URLs, restrict protocols and destinations, block internal and metadata ranges, limit redirects, and apply size and timeout controls.
How should third-party travel APIs be trusted?
Authenticate providers but still treat responses as untrusted data. Validate schemas, redirects, sizes, and URLs and use narrowly scoped credentials and monitored integration behavior.
Sources and further reading
- OWASP API Security Top 10 — authorization, business-flow, SSRF, inventory, and unsafe-consumption risks
- OWASP File Upload Cheat Sheet — defensive handling of uploaded receipts and files
- OWASP SSRF Prevention Cheat Sheet — safe remote-resource fetching guidance
- PCI Security Standards Council — payment-card security standards and guidance
Protect APIs with runtime context, not just static rules
Ammune helps security teams discover APIs, understand normal behavior, detect abuse and authorization anomalies, and apply runtime protection across modern API environments.