A Web App Security Checklist Every Founder Should Demand

If you’re a founder or product lead commissioning a web app, you’re probably not the person who will patch the dependency or configure the access-control rules. But you are the person who signs the contract, sets the deadline, and answers to customers if something goes wrong. This web app security checklist is written for you: plain-English guidance on what to ask, what to demand, and what “secure enough” actually looks like for a startup or scaling SME — not a developer’s guide to writing code.

The trigger for writing this now is the release of the OWASP Top 10:2025, the industry’s periodically updated list of the most critical web application security risks, finalised in January 2026. It’s the closest thing the industry has to a shared checklist, and two of its ten categories are new — which tells you the threat landscape has genuinely shifted, not just been relabelled.

Security is a commissioning problem, not just a coding problem

Most founders assume security is entirely a developer’s job: write clean code, use a framework, done. In practice, a large share of real-world breaches trace back to decisions made before a single line of code was written — what gets logged, who has access to what, which third-party packages are trusted by default, how errors are handled when something goes wrong. Those are commissioning decisions as much as engineering ones. If you never ask about them, they tend not to get prioritised, because they’re invisible until they fail.

The good news is that you don’t need to understand cryptography to ask the right questions. You need a checklist, and the discipline to ask for evidence rather than reassurance. That’s true whether you’re briefing an in-house hire, a freelancer, or a studio — and it’s a big part of what a proper technical due diligence review is actually checking for.

The OWASP Top 10:2025, translated for founders

You don’t need to memorise all ten categories, but it’s worth knowing what’s actually changed and why, because it reflects how attacks have evolved.

• Broken access control (still #1). This is the classic “user A can see user B’s data” bug — usually caused by an API endpoint that doesn’t properly check who’s asking. It has held the top spot for several years running, and this year it absorbed server-side request forgery as a sub-category. Ask: does every request that touches user data get checked against who’s logged in, on the server, every time?
• Security misconfiguration (up from #5 to #2). Default admin passwords left in place, cloud storage buckets left public, debug modes left on in production. This is less about clever attacks and more about basic hygiene falling through the cracks under deadline pressure. Ask: who reviews configuration before launch, and against what checklist?
• Software supply chain failures (new). Modern apps are built on hundreds of open-source packages. A single compromised or abandoned dependency can quietly introduce a vulnerability — or, in the worst cases, deliberately planted malware — into your product without your team writing a line of the bad code. Ask: does the team track dependency versions and known vulnerabilities, and how quickly do they patch when one is disclosed?
• Cryptographic failures. Passwords or payment details stored badly, data sent over the wire without proper encryption. Ask: is sensitive data encrypted at rest and in transit, using current standards rather than whatever a tutorial suggested five years ago?
• Injection. The old classic — untrusted input reaching a database or command in a way that lets an attacker manipulate it. Well understood, well solved by modern frameworks, but still shows up in hand-rolled queries or legacy code.
• Authentication failures. Weak password policies, missing multi-factor options, session tokens that don’t expire properly. If you’re deciding whether to build this yourself or use a managed provider, it’s worth reading our build vs buy guide to authentication before committing engineering time to it.
• Security logging and alerting failures. If something does go wrong, does anyone find out — and how quickly? Plenty of breaches go undetected for months because nothing was logged, or the logs were never reviewed.

None of this is exotic. It’s mostly discipline, applied consistently, by a team that treats it as part of the job rather than an afterthought bolted on before launch.

A startup security checklist to hand your development team

Whether you’re briefing an in-house team or a studio, these are reasonable, specific things to ask for — and specific enough that vague answers should worry you.

1. Authentication and access control. Multi-factor authentication available for admin and user accounts; server-side checks on every data-touching request, not just hidden UI elements; sessions that expire and can be revoked.
2. Dependency management. An automated tool (many are free, such as GitHub’s Dependabot) flags known vulnerabilities in third-party packages, and there’s an actual process for reviewing and patching them — not just a dashboard nobody looks at.
3. Data handling. Sensitive data (passwords, payment details, personal information) is encrypted both in transit (HTTPS everywhere, no exceptions) and at rest. Passwords are hashed with a modern algorithm, never stored in plain text.
4. Environment separation. Production, staging and development environments are properly separated, with different credentials, so a mistake in a test environment can’t touch live customer data.
5. Error handling that fails safely. Errors shouldn’t leak stack traces, database structure, or internal file paths to end users. This is exactly the kind of “mishandling of exceptional conditions” the new OWASP category calls out — the failure mode itself becomes the vulnerability.
6. Logging and monitoring. Security-relevant events — failed logins, permission changes, unusual access patterns — are logged, and someone is actually watching, whether that’s a person or an automated alert.
7. A basic incident plan. If something does go wrong, is there a documented first step? Who gets told, how fast, and what do you say to customers? You don’t need a full disaster-recovery manual on day one, but “we’ve never thought about it” is a bad answer.
8. Independent review before launch. For anything handling real customer data or payments, a review by someone other than the person who wrote the code — even an experienced colleague doing a focused pass — catches problems that familiarity blinds the original author to. A structured code audit before you scale is the more formal version of this same principle.

Questions worth asking a software studio before you sign

Beyond the technical checklist, a few conversational questions tend to reveal a lot quickly, and they’re worth working into the broader process of choosing a software development agency in the first place:

• “Talk me through how you’d handle a user trying to access another user’s data by guessing an ID in the URL.” A team that’s thought about this will answer immediately and specifically.
• “What’s your process when a dependency you use gets a critical vulnerability disclosed?” Vague answers (“we’d look into it”) are a warning sign; a specific process is reassuring.
• “What happens to my data if we end the engagement?” This tells you how seriously they take data ownership and handover, not just build quality.

Security expectations change as you scale

What “secure enough” means shifts as a product grows, and it’s worth being explicit about which stage you’re at so the checklist above gets applied with the right level of rigour.

For an early-stage MVP with no paying customers and no real user data, the priority is getting the fundamentals right without over-engineering: hashed passwords, HTTPS, server-side access checks, and dependency scanning switched on from day one. That’s a few hours of setup, not a project. Once you’re handling real customer data, payments, or anything covered by GDPR, the bar moves up: encryption at rest becomes non-negotiable, you need an actual incident plan rather than a mental note, and an independent review before launch stops being optional. If you’re selling into larger businesses, expect security questionnaires as part of procurement — enterprise buyers increasingly want evidence of logging, access control and dependency hygiene before they’ll sign, and having it in place already is a genuine sales advantage rather than paperwork.

None of this means treating a five-person startup like a bank. It means matching the level of care to what’s actually at stake, and revisiting that judgement every time the product takes on more risk — more users, more data, more money moving through it.

What this actually costs

None of the above requires an enterprise security budget or a dedicated security team. It requires a senior engineering team that builds these habits in from the start rather than trying to retrofit them later — which is both cheaper and far more effective. Retrofitting access control or proper encryption into a live product with real users is a genuinely painful, expensive project; designing it in from the first sprint costs comparatively little.

This is also why fixed-scope, well-scoped early builds tend to be safer than “just get something out fast and fix it later” — speed and security aren’t actually in tension when the fundamentals are handled by people who’ve done it before, and it’s reflected in how we structure fixed-scope pricing for early builds.

At DELLIUX, security fundamentals like these are part of how we build every product, not an optional extra we price separately — because it’s far cheaper to get right the first time. If you’re commissioning a web app and want a second opinion on a technical scope or a security review of something already built, book a free consultation.

FAQ

Do I need a dedicated security engineer for an early-stage startup?

No. Most of what matters at an early stage — hashed passwords, HTTPS everywhere, server-side access checks, dependency scanning — is standard practice for any competent senior engineering team, not a specialist hire. A dedicated security role tends to make sense once you’re handling significant volumes of sensitive data or selling to enterprise customers who require it.

What is the OWASP Top 10 and why should a non-technical founder care about it?

It’s a periodically updated list, maintained by the Open Worldwide Application Security Project, ranking the most critical web application security risks based on real-world data. You don’t need to understand the technical detail, but it’s a useful shared checklist to hand your development team and ask “which of these are you actively guarding against?”

How much should a security review cost?

It depends heavily on the size and sensitivity of the product, so it’s worth asking for a fixed-scope quote rather than an open-ended day rate. A focused review of a specific area (authentication, an API, payment handling) is far cheaper than a full audit, and often enough to catch the highest-risk issues.

What’s the single most important thing to check first?

Broken access control — making sure every request that touches user data is checked against who’s actually logged in, on the server, every time. It’s been the most common real-world vulnerability for years, and it’s usually the most damaging when it’s missed.

Related: Data Protection by Design: A Practical Engineering Guide — a closer look at building GDPR compliant data handling into your architecture from the first sprint, rather than bolting it on before launch.