OWASP Top 10 for Startups: What to Fix Before Your Series A

Series A due diligence almost always includes a security review. Sometimes it's a formal third-party pentest the investor requires. Sometimes it's a technical partner walking through your architecture. Either way, the OWASP Top 10 is the framework they'll use — either directly or by proxy. These are the ten categories of vulnerabilities that appear in every audit checklist, every SOC 2 readiness assessment, and every enterprise security questionnaire.

Most startups at the Series A stage have a live product, real users, and code that was written fast. That's not a criticism — it's the nature of getting to product-market fit. But the vulnerabilities that kill due diligence aren't exotic. They're the ones on this list, and they show up because speed was prioritized over security hygiene during the build.

Here is what each OWASP Top 10 category means in practice for a typical SaaS or web application, and what needs to be fixed before a VC or enterprise client starts asking.

A01 — Broken Access Control

This is the most common and most consequential finding in web application audits. Broken access control means users can access data or functions they shouldn't be able to reach. The classic form is Broken Object Level Authorization (BOLA): your API returns user records without verifying that the requesting user owns them. A user with ID 1042 sends a request to /api/users/1001/profile and gets back someone else's data.

In practice this shows up when object IDs are sequential integers in API endpoints, when authorization checks happen at the route level but not the object level, or when admin endpoints lack role verification. Fix: every API endpoint that returns or modifies a resource must verify that the authenticated user has explicit permission for that specific object, not just access to the endpoint.

A02 — Cryptographic Failures

Sensitive data transmitted or stored without adequate protection. This covers passwords stored without proper hashing (bcrypt, Argon2), sensitive fields in databases stored in plaintext, and data transmitted over HTTP on internal routes that developers assumed were safe. Investors doing due diligence will ask how passwords are stored. "We use bcrypt with cost factor 12" is the right answer. "We hash them with MD5" is a conversation-stopper.

Check: PII fields (email, phone, SSN if applicable) in your database, how session tokens are generated and stored, whether any internal API routes bypass HTTPS.

A03 — Injection

User-supplied input that reaches an interpreter — SQL, OS commands, LDAP queries, XML parsers — without validation. SQL injection is the most well-known form, but XSS (Cross-Site Scripting) also falls here: user input rendered in the browser without encoding. For modern frameworks using ORMs this is less common than it used to be, but raw queries built with string concatenation still appear in audits regularly. Template injection in server-side rendering is the form most often missed.

Fix: parameterized queries everywhere. Input validation at the boundary. Output encoding before rendering user-supplied content in HTML.

A04 — Insecure Design

This category covers architectural decisions that create risk regardless of implementation quality. An example: a password reset flow that sends a token via SMS but allows unlimited guesses at the 6-digit code with no rate limiting. The code works correctly. The design is insecure. Other common forms include business logic that can be bypassed by manipulating request parameters, and multi-tenant architectures where tenant isolation is enforced only in application code with no database-level separation.

Insecure design findings require design changes, not patches. They're the most expensive to fix late. Audit for them early.

A05 — Security Misconfiguration

The most common finding in cloud infrastructure reviews. Default credentials left on admin interfaces, S3 buckets with public read enabled, debug endpoints left active in production, overly permissive CORS configuration, missing security headers. These are not coding errors — they're configuration errors that require a checklist to catch, not a code review.

Before a due diligence review: run your production environment against a security headers checker, audit your cloud storage permissions, confirm all admin interfaces require authentication, and verify your error responses don't expose stack traces.

A06 — Vulnerable and Outdated Components

Dependencies with known CVEs. This is the finding that appears in every automated scan report. Investors and enterprise security teams run dependency scanners as a first pass — it's a fast, automated check that identifies companies that have no dependency management practice. Having 47 critical CVEs in your package.json because npm install was run and never updated is a yellow flag in due diligence.

Fix: npm audit, pip-audit, Dependabot, or Snyk on a weekly schedule. You don't need zero CVEs — you need to demonstrate that you know what's in your dependency tree and have a process for updating it.

A07 — Identification and Authentication Failures

Weak authentication implementations. No MFA on admin accounts. Session tokens that don't expire. Password policies that allow "password123". Credential stuffing protection (rate limiting on login endpoints) absent. Account enumeration via login error messages that distinguish between "wrong email" and "wrong password."

For any startup with enterprise clients or investor-grade due diligence: admin accounts need MFA, login endpoints need rate limiting and lockout, session tokens need expiration and proper invalidation on logout.

A08 — Software and Data Integrity Failures

Code or data that flows through your pipeline without integrity verification. The most common startup-relevant form: CI/CD pipelines that pull dependencies without pinning versions or verifying checksums, and auto-update mechanisms that don't verify signatures. Less common but high-severity: deserialization of untrusted data.

For most startups: pin your dependency versions in production builds, use lockfiles, and ensure your deployment pipeline verifies artifacts before deploying them.

A09 — Security Logging and Monitoring Failures

No audit log of authentication events, access control failures, or data modification. This matters for two reasons: it's a requirement in most compliance frameworks (SOC 2, HIPAA, PCI DSS), and its absence means that when an incident occurs, you have no forensic trail. Enterprise clients will ask whether you log authentication failures and have alerting on anomalous access patterns.

Minimum viable logging: authentication events (success and failure), access control denials, admin actions, and data export or deletion events. Logs need to be tamper-evident and retained for at least 90 days.

A10 — Server-Side Request Forgery (SSRF)

Your server fetches a URL supplied by the user. An attacker supplies an internal URL — http://169.254.169.254/latest/meta-data/ (AWS metadata endpoint) — and your server fetches it on their behalf. This is how cloud credentials get exfiltrated from otherwise well-secured applications. SSRF is common in features that accept user-supplied URLs: webhook endpoints, URL preview generation, PDF rendering from URLs, and import-from-URL functionality.

If your application fetches external URLs on behalf of users, you need an allowlist of permitted schemes and destinations, and you need to block requests to RFC-1918 private address ranges and cloud metadata endpoints.

What to Do Before Due Diligence Starts

You don't need to fix everything before Series A. Investors don't expect a perfect security posture from a pre-Series A company. They expect evidence that you know what your risks are, have a plan to address the critical ones, and take security seriously enough to have done the work.

The deliverable that addresses this is a third-party security audit with a written findings report and a remediation log showing which critical and high findings have been addressed. That document — specifically, a Letter of Attestation confirming that Critical and High findings have been verified as patched — is what a VC's technical team or an enterprise procurement team wants to see.

The OWASP Top 10 is the framework. The audit is the proof. Get both done before you need them.