Security Incident Report Incident Type: Credential-Stuffing Attack Against Customer Login API Industry Context: Financial Technology Platform (Mobile Payments) Regulatory Context: PCI DSS compliant environment Severity: High --- 1. Executive Summary (One-Page) Incident Overview On 2026-03-07, the security monitoring team identified anomalous authentication traffic targeting the customer login API. Traffic patterns indicate a credential-stuffing campaign in which attackers attempted to authenticate using large volumes of previously leaked username/password pairs. The attack originated from distributed IP infrastructure and automated tooling designed to evade rate-limits. Environment Context The organization operates a financial technology platform processing mobile payments. Customer authentication occurs via an API gateway that feeds identity verification workflows used by payment services. Systems operate in a PCI-DSS controlled cardholder data environment (CDE) with microservice architecture. Affected Systems System Function Customer Authentication API Gateway Handles login requests and session token issuance Fraud Monitoring Service Evaluates authentication anomalies and fraud signals Payment Authorization Microservice Processes card and wallet transaction approvals Key Findings (Preliminary) Attack pattern consistent with credential stuffing using breached credential lists. Authentication requests increased ~18× above baseline over a 45-minute window. Attackers leveraged rotating IPs and automation frameworks to bypass simple throttling. A small subset of accounts successfully authenticated, suggesting password reuse by users. No evidence (at time of report) of compromise of cardholder data systems or backend infrastructure. Impact Summary Category Assessment Business Impact Increased authentication latency and elevated fraud risk Customer Impact Limited number of accounts potentially accessed Data Impact Possible exposure of customer profile information (non-card data) Regulatory May trigger review obligations under PCI DSS incident response requirements Root Cause (Initial) Primary contributing factors: Customer password reuse across breached services Insufficient adaptive authentication controls during attack surge Rate-limit and bot detection thresholds insufficient for distributed attack patterns Containment Status Temporary rate-limit escalation and IP reputation blocking deployed Password resets forced for suspicious login events Fraud monitoring rules updated to detect automated login bursts Current Risk Level: High but contained, pending completion of account review and security hardening. Executive Risk Score: 7.4 / 10 – Elevated credential compromise risk with limited confirmed data exposure --- 2. Root-Cause Analysis Attacker Tactics, Techniques, and Procedures (TTPs) Phase Observed Behavior Initial Access Credential stuffing via customer login endpoint Infrastructure Distributed proxy/VPN network Automation Scripted login attempts at high velocity Evasion Rotating IP addresses, user-agent randomization Objective Account takeover for fraud or resale Likely automation tools include commodity credential-stuffing frameworks (exact tool unknown). --- Indicators of Compromise (IOCs) IOC Type Example Indicator Notes IP Address 185.231.xxx.xxx (rotating subnet) Associated with proxy infrastructure User Agents python-requests/2.x, modified Chrome UA Indicates scripted traffic Endpoint /api/v1/auth/login High request concentration Behavior 10–20 login attempts/sec per account Password spraying pattern Session Pattern Rapid login success followed by API token generation Possible account takeover --- 3. Impact Assessment (Prioritized) Business Impact 1. Increased fraud exposure through potential account takeover. 2. Authentication service performance degradation. 3. Potential reputational impact if customer accounts accessed. Data Impact Possible exposure of: Customer names Email addresses Phone numbers Transaction history (read-only access) No confirmed exposure of: Card numbers Payment authorization tokens Backend secrets (Verification ongoing.) Regulatory Impact Potential triggers: PCI DSS incident response requirements Internal fraud reporting thresholds Possible consumer notification obligations depending on confirmed access Legal review recommended if >500 accounts accessed. --- 4. Chronology of Events (Preliminary Timeline) Time (UTC) Event 01:12 Authentication failure rate begins increasing 01:15 Fraud monitoring flags abnormal login volume 01:19 Security alert generated (credential stuffing signature) 01:25 Automated IP rate-limit policy triggered 01:31 SOC analyst begins investigation 01:38 Suspicious successful logins identified 01:42 Temporary authentication throttling applied 01:50 Incident declared (Severity High) 02:05 Forced password resets for flagged accounts 02:30 Additional WAF bot detection rules deployed --- 5. Structured Evidence Log Evidence ID Source Description Integrity Method EV-01 API Gateway Logs Authentication requests & response codes Log hash validation EV-02 WAF Logs IP source distribution & rule triggers SIEM export checksum EV-03 Fraud Monitoring System Login anomaly detection events Signed event records EV-04 Identity Service Logs Session token generation logs Timestamp correlation EV-05 Network Telemetry Traffic spikes to login endpoint NetFlow records Evidence retained in forensic storage with restricted access. --- 6. Containment, Eradication, Recovery Playbooks Containment Immediate actions: Increase authentication rate-limits Block high-risk IP ranges Enforce CAPTCHA or challenge mechanisms Suspend suspicious sessions Target SLA: <30 minutes detection-to-containment --- Eradication Actions: 1. Force password resets for impacted accounts 2. Revoke all active tokens from affected sessions 3. Remove attacker infrastructure from allow lists 4. Patch authentication logic to enforce stricter throttling Target completion: 24–48 hours --- Recovery 1. Restore normal authentication thresholds 2. Monitor login success anomaly metrics 3. Notify impacted customers if confirmed access 4. Conduct infrastructure security validation Target recovery SLA: 72 hours --- 7. Stakeholder Responsibilities Role Responsibility SOC Team Detection and monitoring Incident Response Forensic investigation Platform Engineering Authentication control updates Fraud Team Review suspicious transactions Legal/Compliance Regulatory notification review Customer Support Customer communication --- 8. Communications Plan Internal Audience: Executive leadership Security leadership Engineering leadership Cadence: Initial notification within 1 hour Situation updates every 4 hours --- External Potential recipients: Impacted customers Payment partners Regulatory bodies if thresholds met Communication conditions: Confirmed unauthorized access Evidence of data exposure --- Legal Legal review required before: Customer notifications Regulatory filings Public statements --- 9. Post-Mortem Metrics Track: 1. Detection time (MTTD) 2. Response time (MTTR) 3. Account compromise rate 4. Login failure ratio 5. Rate-limit effectiveness 6. Bot detection accuracy 7. Fraud losses from compromised accounts 8. Customer password reset completion rate --- 10. Actionable Remediation Checklist (Ticket-Ready) 1. Deploy adaptive rate-limiting on login API. 2. Enforce mandatory multi-factor authentication (MFA) for high-risk logins. 3. Implement credential breach monitoring against leaked password datasets. 4. Introduce bot detection / behavioral authentication controls. 5. Deploy IP reputation filtering and ASN-level blocking. 6. Force password resets for all accounts with suspicious activity. 7. Enable real-time alerting for login success anomalies. 8. Improve SIEM correlation rules for credential stuffing patterns. 9. Conduct customer password reuse awareness campaign. 10. Run tabletop incident simulation for account takeover scenarios. --- 11. Known Unknowns / Assumptions Unknowns: Total number of compromised accounts Exact credential dataset used by attackers Whether compromised accounts performed fraudulent transactions Assumptions: Attack used publicly available credential breach lists Authentication infrastructure was not directly exploited --- If helpful, I can also produce: a SOC-ready detection engineering rule set (SIEM queries for credential stuffing), a MITRE ATT&CK mapping, or a board-level risk briefing slide deck based on this report.