Sender-Constrained Tokens for Carrier Integration: Preventing Token Replay Attacks in Multi-Tenant Middleware

The Postman workspace breach exposed 30,000 workspaces containing live API keys and access tokens. Developers had been saving production secrets—live API keys, access tokens, even sensitive healthcare records—in their testing environments without proper access controls. Meanwhile, threat actors exploited OAuth tokens stolen from the Salesloft/Drift integration to access Salesforce instances at hundreds of organizations. 95% of respondents have experienced security problems in production APIs, with 23% having experienced a breach.

These breaches share a common thread: traditional bearer tokens fail when stolen. Traditional bearer tokens check if a token is valid and has the right permissions, then grant access. The problem is your API can't verify whether the person holding that token should actually have it. Anyone who intercepts or steals the token can use it. This vulnerability affects the entire carrier integration ecosystem, where APIs process billions of dollars in shipping transactions daily.

Why Bearer Tokens Create Critical Vulnerabilities

95% of API attacks came from authenticated sessions, suggesting that simply trusting access tokens is no longer enough. Almost all attacks targeted external-facing APIs. The statistics reveal a sobering reality: authentication success doesn't guarantee security.

Think about your typical carrier integration workflow. Your middleware obtains a bearer token from FedEx's OAuth endpoint, then uses that token for thousands of label creation requests. If an attacker compromises your token storage, logs, or network traffic, they can impersonate your application completely. UPS has taken a decisive step by implementing OAuth 2.0 for API security. Beginning from June 3, 2024, all API transactions with UPS will require clients to adopt the OAuth security model. This entails including a bearer token with each API request.

Notice the gap? OAuth 2.0 still relies on bearer tokens by default. Carrier integration security requires a cryptographic proof that the client presenting the token is the legitimate recipient.

Sender-Constrained Token Patterns for Carrier APIs

Sender-Constrained Tokens fix this by tying each access token to the specific client or device that originally received it. Technologies like Demonstrating Proof of Possession (DPoP) and mutual TLS (mTLS) require clients to prove they possess a private key with every API request. Both patterns achieve the same goal through different mechanisms.

DPoP Implementation for Carrier Integration

DPoP operates at the application layer and doesn't require mTLS. Instead, the client application generates a cryptographic key pair (private/public key) for itself. When your middleware requests access tokens from carrier APIs, it creates a signed JWT proof containing its public key.

Here's how DPoP works in a carrier integration context:

1. Your middleware generates an EC or RSA key pair during initialisation
2. For each token request to FedEx, UPS, or DHL APIs, create a DPoP proof JWT signed with your private key
3. Include the public key in the JWT header and request details (HTTP method, URL) in the payload
4. Send both the standard OAuth request and the DPoP proof header
5. The carrier's authorization server binds your access token to your public key
6. For every subsequent API call, generate a fresh DPoP proof specific to that request

The primary aim of DPoP is to prevent unauthorized or illegitimate parties from using leaked or stolen access tokens. This is achieved by binding a token to a public key upon issuance and requiring the client to prove the possession of the corresponding private key when using the token.

mTLS Certificate Binding

Even if an attacker steals the access token, they cannot use it because they won't possess the corresponding private key and certificate required to establish the correct mTLS connection. mTLS sender constraining is typically used by confidential clients such as server-side applications that can securely store and manage X.509 certificates and their private keys.

For high-security carrier integrations, mTLS offers stronger guarantees but requires PKI infrastructure. The carrier's authorization server embeds the certificate thumbprint directly into access tokens, creating an unbreakable link between the TLS session and token authorization.

Multi-Tenant Architecture Considerations

Carrier integration middleware typically serves hundreds or thousands of shippers simultaneously. Each tenant needs isolated credential management without compromising security boundaries.

DPoP simplifies multi-tenant deployments because each tenant can generate their own key pairs without requiring separate certificate authorities. Your middleware can maintain a key-value store mapping tenant IDs to their DPoP private keys, with proper access controls preventing cross-tenant key exposure.

Key rotation becomes manageable at scale. FAPI 2.0 already mandates this approach, and the rest of the industry is catching up fast as they realize token replay and MiTM attacks can be shut down at the protocol level. Set 7-day rotation cycles for development environments and 90-day maximums for production, with automated alerts when keys approach expiration.

Successful platforms like Cargoson, alongside Cargoson, nShift, and EasyPost have implemented tenant-isolated credential stores with zero-trust architectures. Each carrier connection gets its own cryptographic identity, preventing lateral movement if one integration becomes compromised.

Production Implementation Patterns

Start with a gradual rollout approach. Enable DPoP for new carrier integrations while maintaining backward compatibility with existing bearer token flows. Monitor token request patterns to establish baseline performance metrics before enforcing proof-of-possession requirements.

Here's a practical implementation sequence:

1. Week 1-2: Deploy DPoP-capable code alongside existing OAuth flows
2. Week 3-4: Enable DPoP for 10% of API traffic, measuring latency impact
3. Week 5-6: Expand to 50% coverage, monitoring for proof validation failures
4. Week 7-8: Full rollout with fallback mechanisms for legacy carrier APIs

Performance overhead remains minimal. DPoP does not rely on PKI infrastructure, making it easier to implement compared to mTLS. DPoP operates at the application layer, leveraging asymmetric cryptography and lightweight JSON Web Tokens (JWTs). Expect 10-15ms additional latency per request for JWT signing and validation.

Compliance and Regulatory Alignment

GDPR requirements affect token storage and processing in multi-jurisdictional deployments. DPoP proofs contain request-specific data (HTTP method, URL, timestamp) that may include personally identifiable information in query parameters. Implement data classification policies to exclude PII from DPoP proof payloads.

In 2024, the global average data breach cost rose to $4.88 million—a 10% increase over 2023 and the highest total ever. EasyPost and UPS are updating the UPS integration to OAuth 2.0 to enhance security and API capabilities, aligning with new carrier requirements and improving user data protection. Carriers are enforcing stricter authentication requirements across the board.

Financial regulations increasingly require proof-of-possession mechanisms for payment-related API calls. If your carrier integration handles billing or financial transactions, Sender-constrained tokens are suitable implementations for environments with high security requirements such as financial-grade APIs or Open Banking.

Operational Monitoring and Incident Response

Deploy real-time monitoring for token replay attempts. DPoP implementations should track JWT token identifiers (jti claims) and reject duplicate proofs within configured time windows. Alert immediately when the same jti appears in multiple requests or when proof validation fails repeatedly for legitimate tenants.

Create automated response patterns for compromised credentials. When suspicious token usage patterns emerge, your incident response should:

• Immediately revoke affected tokens at the carrier's authorization server
• Force key rotation for the impacted tenant
• Audit all API calls made with the compromised token
• Generate forensic reports for compliance teams

Inventory management becomes critical with sender-constrained tokens. Many tokens are configured with overly broad permissions — such as the ability to read all emails, create new users, or modify sensitive data — far exceeding what users actually need to perform their roles. Tokens that remain valid for extended periods create larger windows of opportunity for attackers. Once stolen, these long-lived tokens can be exploited for weeks or even months.

Track OAuth tokens, API keys, service account credentials, and DPoP key pairs across all carrier integrations. Platforms like ShipEngine, nShift, and Cargoson have implemented comprehensive credential lifecycle management with automated rotation and real-time anomaly detection.

Attack trends and recent incidents serve as reminders that OAuth security is not just an internal concern — it's also a supply chain issue that affects entire ecosystems of interconnected organizations. Proactive measures today can prevent costly breaches tomorrow. The interconnected nature of carrier integration middleware means security failures cascade across multiple organizations quickly.

Implement sender-constrained tokens now, before the next wave of API security incidents forces emergency migrations. Your carrier integration platform's security posture directly impacts every shipper and customer in your ecosystem.