5 Common Vulnerabilities Found in Every Web App Pentest

After running dozens of web application penetration tests across industries — from fintech dashboards to coaching platforms to internal enterprise tools — the same handful of issues keep showing up, almost regardless of tech stack or team size. Understanding these patterns is often more useful than reading a generic checklist, because they reflect what actually happens when real teams build real software under real deadlines.

Broken access control tops the list, consistently. APIs that don't properly verify a user's permission before returning data are everywhere, especially in apps built quickly without a dedicated security review. The classic example: changing an ID in a URL (invoice/1042 to invoice/1043) and suddenly viewing another customer's data. Developers correctly implement authentication but forget authorization at every single endpoint.

Second is misconfigured authentication — weak password policies, missing account lockouts, and session tokens that don't expire in a reasonable timeframe. We routinely find applications where a session token remains valid indefinitely, meaning a stolen token from months ago could still grant access today.

Third, sensitive data exposure: API responses that leak more fields than the frontend actually uses, often including internal IDs or other users' partial data. Developers often build APIs that return an entire database record and let the frontend pick which fields to display.

Fourth, outdated dependencies. A single unpatched library can open the door to a known exploit, even in an otherwise well-built application — particularly common in JavaScript-heavy frontends with dozens of npm dependencies.

Fifth, insufficient logging — when we ask "how would you know if this had already happened," the honest answer from most teams is "we wouldn't." Applications frequently log errors for debugging but don't log security-relevant events like failed login attempts or unusual data access patterns.

Beyond these five, we also frequently encounter business logic flaws that automated scanners simply can't catch — things like applying the same discount coupon multiple times, or bypassing a checkout flow by directly calling a later-stage API endpoint.

None of these require exotic attacks to fix. They require a testing process that treats security as part of the build, not an afterthought bolted on before launch — and an independent, manual penetration test that catches the chained, business-logic-aware issues internal reviews often miss.\n\nInterestingly, the fix for most of these issues isn't exotic security engineering — it's disciplined engineering practice applied consistently. Access control checks belong in a shared middleware layer, not scattered individually across every endpoint where a developer might forget to add them. Dependency scanning should be part of the CI/CD pipeline, flagging vulnerable packages before they ever reach production, not discovered months later during a pentest.\n\nWe also increasingly see these issues cluster around rapid feature launches — a new feature shipped under deadline pressure, with security review treated as a "we'll do it after launch" afterthought. Building a lightweight security checklist into your sprint process, reviewed before any feature touching user data goes live, catches a surprising number of these issues before they ever reach a pentest report.\n\nOrganizations sometimes ask how often this kind of testing should happen. Our general guidance: at minimum annually, and additionally after any major release that touches authentication, payment processing, or user data handling — since these are precisely the areas where new vulnerabilities are most likely to be freshly introduced, and where the consequences of an undiscovered issue are highest.\n\nWe also encourage clients to think about vulnerability disclosure from the other direction — having a simple, publicly listed way for security researchers to responsibly report issues they find in your systems. Many serious vulnerabilities get reported by well-meaning outside researchers, and having no clear channel for that report often means it goes nowhere, or worse, gets disclosed publicly out of frustration before you've had a chance to fix it.\n\nWorth mentioning too: severity ratings alone don't always predict how quickly a finding needs remediation in the real world. We've seen "medium" severity findings sit unpatched for months with no consequence, while a "low" severity information disclosure bug became the first domino in a chained exploit within weeks of being reported, simply because it happened to expose exactly the piece of information an opportunistic attacker needed. This is part of why we push clients toward fixing entire vulnerability classes rather than individual instances — if broken access control shows up in three endpoints during a test, the real fix is reviewing your authorization middleware globally, not patching those three specific routes and calling it done, since the next unreviewed endpoint added next sprint will likely carry the same flaw.