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06/09/2026 3:45 pm
TL;DR: Substack's October 2025 breach exposed 697,313 user records through phishing, exposed APIs, and weak access controls, with leaked email addresses, phone numbers, and internal metadata increasing downstream phishing risk, according to FireCompass. The case shows how data exposure and identity abuse can combine when API permissions and token governance are too loose.
NHIMG editorial — based on content published by FireCompass covering the Substack data breach: Substack Data Breach
By the numbers:
- Attackers attempt access to publicly exposed AWS credentials within an average of 17 minutes.
- 72% of organisations have experienced or suspect they have experienced a breach of non-human identities.
Questions worth separating out
Q: What breaks when exposed APIs return more data than users should see?
A: Overexposed APIs turn a limited access event into a broader identity risk because attackers can collect email addresses, phone numbers, and internal metadata for reconnaissance and phishing.
Q: Why do weak API access controls increase phishing risk after a breach?
A: Weak API access controls let attackers harvest structured personal data at scale, which makes phishing messages more believable and targeted.
Q: How can security teams tell if API exfiltration is happening?
A: Look for sequential record access, repeated calls to the same endpoint, unusual request volume, and data pulls that exceed normal user behaviour.
Practitioner guidance
- Tighten API response minimisation Remove unused metadata fields from externally reachable endpoints and apply per-route field allowlists so that a stolen or abused token cannot expose more than the task requires.
- Revoke and rotate API tokens faster Shorten token lifetimes, enforce rapid revocation for suspected abuse, and block reuse patterns that let attackers continue scraping after initial access has been detected.
- Alert on enumeration and extraction behaviour Baseline normal request patterns, then alert on sequential access, high-frequency endpoint calls, and repeated retrieval of structured user records that indicate automated scraping.
What's in the full analysis
FireCompass's full analysis covers the operational detail this post intentionally leaves for the source:
- MITRE ATT&CK mapping for the phishing-to-exfiltration chain and how the attacker used API access patterns
- Remediation guidance for OAuth token revocation, rotation, and permission hardening across exposed endpoints
- IOC-style indicators from leaked email and phone data that can feed monitoring and triage
- FireCompass's notes on anomalous API call behaviour and token reuse vulnerabilities
👉 Read FireCompass's analysis of the Substack data breach and API access failures →
Substack data breach: what API access control gaps mean for teams?
Explore further
06/09/2026 3:45 pm
API access governance is now an identity problem, not just an application problem. When exposed APIs reveal personal metadata, the control failure sits at the boundary between authorization, session handling, and data minimisation. IAM teams should treat API scopes, token lifecycle, and response filtering as part of the identity control plane, not as a separate developer concern.
A question worth separating out:
Q: How should teams govern API access when regulated data is exposed through endpoints?
A: Teams should govern API access the same way they govern any other regulated access path: define ownership, classify data, set least-privilege permissions, and require evidence that authentication and authorisation are working in production. APIs often combine human, partner, and machine access, so lifecycle controls and review cadence matter as much as the code.
👉 Read our full editorial: Substack data breach exposes API and access control gaps
