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System Design and AI Engineering, by Roni Das. 984 interactive lessons.
Start LearningAdvanced · Security Testing & Operations
A systematic evaluation of a system's security posture against standards and best practices. Covers access controls, encryption, logging, vulnerability management, and compliance.
In short
A security audit is a systematic, evidence-based review of a system's security controls against a defined standard or framework, checking access management, encryption, logging, patching, configuration, and compliance to find gaps before an attacker does. It produces a documented list of findings with severity ratings and remediation steps.
In short
A security audit is a systematic, evidence-based review of a system's security controls against a defined standard or framework, checking access management, encryption, logging, patching, configuration, and compliance to find gaps before an attacker does. It produces a documented list of findings with severity ratings and remediation steps.
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The System Design course covers Security Audit across a run of lessons, not one page. These 4 alone are about 72 minutes of step-by-step reading, every one with a quiz.
Recording security-relevant events with tamper-proof, queryable audit trails for compliance and incident investigation.
Attaching permission lists directly to resources, who can do what to this specific object.
Record who did what, when, and why, the immutable trail that compliance, security, and debugging all depend on.
Centralized management of cryptographic keys, creation, rotation, access control, and audit.
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A security audit measures what your system does against what it should do. The benchmark is a written standard such as SOC 2, ISO 27001, PCI DSS, HIPAA, or your own internal security policy. An auditor walks through each control the standard requires and gathers proof that it is in place and working.
It is broader than a penetration test. A pen test asks can I break in. An audit asks are the right controls present, configured correctly, and actually followed by the people running the system. That covers who can log in, how keys and secrets are stored, whether logs exist and are reviewed, how fast critical patches get applied, and whether the company can prove all of this with records.
The deliverable is a report, not a fix. Each finding gets a severity, evidence, and a remediation recommendation. A finding like S3 bucket world readable is high severity. A finding like password policy allows 6 character passwords is medium. The owning team then fixes them and the auditor re-checks.
Scoping comes first. You define the systems, accounts, data stores, and time window being audited. For PCI DSS this is the cardholder data environment; for SOC 2 it is whatever processes customer data over the audit period, often 6 to 12 months.
Evidence collection mixes interviews, configuration exports, and automated scans. Auditors pull IAM policies, firewall rules, encryption settings, and access logs. Tools like AWS Config, Prowler, Scout Suite, or Nessus dump the current state of cloud accounts and flag misconfigurations against CIS Benchmarks. For code-heavy scopes, SAST and dependency scanners feed in too.
Testing has two flavors. Design testing checks that a control exists, for example MFA is required for admin access. Operating effectiveness testing checks that it held up over time, for example sampling 25 access requests across the year and confirming each had manager approval. A control that exists on paper but was bypassed twice in production fails the second test.
The output is graded findings plus a remediation timeline. Auditors map severity using something like CVSS or a simple high/medium/low scheme tied to data sensitivity and exposure.
External audits are usually driven by money and contracts. Enterprise customers will not sign until you have a SOC 2 Type II report. Card processing requires PCI DSS. Handling US health records requires HIPAA. In these cases the audit is a gate, not an option.
Internal audits run on a cadence, often quarterly or per major release, to catch drift. Cloud environments rot fast: a developer opens a security group for debugging and forgets to close it. Continuous tooling such as AWS Security Hub or GuardDuty narrows the window, but a periodic human audit still catches process gaps automation misses, like a former employee whose access was never revoked.
The cost is real. A SOC 2 Type II engagement can run 20,000 to 100,000 dollars and months of evidence gathering, and it pulls engineers off feature work. The trade-off is that the cheapest time to find a misconfigured database is during an audit, not during a breach that triggers GDPR fines of up to 4 percent of global revenue and the loss of customer trust.
A fintech startup needs a SOC 2 Type II report to close an enterprise deal. They engage an audit firm for a 6 month observation window. Early on, a tool like Vanta or Drata connects to their AWS, GitHub, and Google Workspace accounts and continuously collects evidence.
The audit surfaces three findings. First, an old EC2 IAM role had wildcard admin permissions, a privilege escalation risk; they scope it down to least privilege. Second, production database backups were unencrypted at rest; they enable encryption. Third, eight offboarded contractors still had GitHub access; they tighten the offboarding runbook so revocation is automatic.
Each fix is verified, the auditor issues a clean Type II report covering the period, and the deal closes. The same review also quietly removed three paths a real attacker could have used.
AWS
Publishes its own SOC, ISO, and PCI audit reports through AWS Artifact, and AWS Audit Manager automates evidence collection for customers running their own audits.
Vanta
Automates SOC 2 and ISO 27001 audits by continuously pulling evidence from cloud accounts and flagging failing controls before the auditor arrives.
Stripe
Maintains PCI DSS Level 1 certification through annual external audits so merchants inherit much of the card-data compliance burden.
Cloudflare
Undergoes regular SOC 2 Type II and ISO 27001 audits and exposes the reports to customers as part of its trust portal.
An audit checks whether the right controls exist and are followed, measured against a standard like SOC 2 or PCI DSS. A penetration test actively tries to exploit weaknesses to prove an attacker could get in. Audits are about coverage and compliance; pen tests are about real-world exploitability. Mature programs use both.
Type I checks that controls are designed correctly at a single point in time. Type II checks that those controls actually operated effectively over a window, usually 6 to 12 months. Enterprise buyers almost always want Type II because it proves the controls held up, not just that they existed on paper one day.
External compliance audits like SOC 2 Type II run annually with a continuous observation period. Internal audits and configuration reviews are best run quarterly or after major architecture changes, backed by continuous tooling such as AWS Security Hub or GuardDuty to catch drift between formal audits.
Identity and access management policies, MFA enforcement, encryption at rest and in transit, network and firewall rules, logging and log retention, patch and vulnerability management, secrets handling, backup configuration, and offboarding processes. Tools like Prowler or Scout Suite scan these against CIS Benchmarks automatically.
No. An audit confirms you meet a specific standard at a point in time within a defined scope. New vulnerabilities, configuration drift, and threats outside the audited scope can still exist. Treat a clean report as a baseline, not a guarantee, and keep continuous monitoring running between audits.
Penetration Testing
Authorized simulated attacks against a system to find security vulnerabilities before real attackers do. White-hat hackers probe for weaknesses in a controlled environment.
Threat Modeling
A structured process for identifying security threats, attack surfaces, and mitigations during system design. STRIDE and DREAD are common frameworks.
GDPR
General Data Protection Regulation: EU law governing how personal data is collected, stored, and processed. Requires consent, data portability, and the right to be forgotten.
Incident Response
The structured process for detecting, containing, eradicating, and recovering from security incidents. Includes communication plans, runbooks, and post-incident reviews.
This page explains the idea. The "Security Audit Logging" lesson builds it up step by step, with diagrams. It is part of the System Design Masterclass course, which has 770 lessons. 18 of them are free to read. You pay once and keep it for life, with no subscription.
Open the Security Audit Logging lessoncourse 1
From absolute beginner to principal engineer, drawn step by step.
Asked about Security Audit in an interview?
Each question has why it is asked, a full answer, what the interviewer listens for, the follow-ups, the common mistakes and a diagram (268 diagrams in all). A PDF you keep, junior to staff level.
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