Definition

Exfiltration over Web Services is a technique where adversaries steal sensitive data by sending it to a legitimate web-based service or API instead of directly transferring it to attacker-controlled infrastructure.

This makes detection harder because traffic often blends in with normal HTTPS traffic to trusted cloud platforms.

 Real-World Usage

• APT29 (Cozy Bear): Known to leverage Dropbox and OneDrive for data exfiltration.

• ShinyHunters / UNC6040: Abuse of Salesforce APIs for mass extraction of CRM records.

• Malware Families: Some RATs and stealers (e.g., RedLine, Agent Tesla) push stolen logs to Telegram, Discord, or Slack webhooks.

 Attack Flow (Typical)

1. Staging → Adversary collects files, credentials, or logs locally.

2. Compression & Encryption → Data zipped/encrypted to evade inspection.

3. Transmission → Data sent to:

   • Cloud Storage: Google Drive, AWS S3, Dropbox, OneDrive.

   • Messaging APIs: Slack, Telegram, Discord bots.

   • SaaS APIs: Salesforce, Office365, GCP APIs.

4. Persistence → Data automatically synced to attacker’s accounts, often unnoticed by defenders.

 Detection Opportunities

• Network Telemetry

  • Monitor unusual upload sizes to cloud storage domains.

  • Alert on excessive outbound traffic to SaaS APIs during off-hours.

• Endpoint/EDR

  • Detect abnormal use of curl, PowerShell, Python scripts sending data to APIs.

  • Monitor sudden creation of archives (ZIP/RAR/7z) in staging directories.

• Cloud Logs

  • Salesforce / Office365: Look for Bulk API exports or ReportExportEvents outside normal patterns.

  • AWS S3: Detect PUTObject with high volume to unknown buckets.

 Defensive Recommendations

• Enforce CASB / DLP controls to inspect and block unauthorized uploads.

• Implement API allowlists (block unsanctioned SaaS integrations).

• Baseline normal SaaS activity (e.g., daily Salesforce exports) → hunt for deviations.

• TLS inspection where policy allows → to catch anomalous encrypted uploads.

• Zero Trust SaaS Access → vendor apps should never get full API scopes unnecessarily.

 Case Example – Google Salesforce Leak (2025)

Attackers abused Salesforce APIs to mass-export Google’s business contact records. The exfiltration occurred via normal Salesforce web services, bypassing traditional firewalls. Detection required API anomaly monitoring, not network perimeter alerts.

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