A06:2025 – Insecure Design
Overview
Insecure Design represents a category of vulnerabilities that do not originate from implementation mistakes, missing patches, or misused APIs, but from fundamental flaws in how an application is conceived, modeled, and structured.
In the OWASP Top 10:2025, Insecure Design remains a critical risk because it reflects a deeper problem: security controls were never designed to exist in the first place.
Unlike injection or misconfiguration, insecure design cannot be fixed by:
input validation
configuration hardening
adding authentication
patching libraries
If a security control does not exist in the design, no amount of perfect implementation will compensate for it.
Insecure Design Is Not Insecure Implementation
A key distinction in A06 is the difference between design flaws and implementation flaws.
Insecure implementation means the right control exists, but it is implemented incorrectly.
Insecure design means the control was never defined, modeled, or required.
An application can be perfectly coded and still be insecure if:
abuse cases were never considered
threat modeling was skipped
business logic risks were ignored
trust boundaries were poorly defined
From an attacker’s perspective, insecure design creates structural weaknesses that are far more valuable than bugs.
Design Flaws Are Logic Flaws
Insecure design vulnerabilities almost always manifest as logic abuse.
Examples include:
workflows that can be skipped or reordered
missing limits on critical actions
trust in client-controlled state
assumptions about user behavior
missing validation of business rules
lack of defensive boundaries between components
These flaws do not break the application — they use it exactly as designed, just not as expected.
Why Insecure Design Is Hard to Detect
Insecure design is difficult to detect because:
there may be no errors
nothing “crashes”
logs may look normal
requests appear legitimate
functionality works as intended
Automated scanners struggle with insecure design because:
it is context-dependent
it requires understanding business logic
it involves intent, not syntax
exploitation looks like normal usage
This makes insecure design one of the most under-tested and under-reported categories.
Insecure Design Appears Early — and Persists
Design flaws are introduced:
during requirements gathering
during architecture decisions
when defining workflows
when modeling trust and identity
Once introduced, they tend to:
propagate across features
survive refactors
persist across versions
become “expected behavior”
Fixing insecure design later is often expensive because it may require:
redesigning workflows
breaking backward compatibility
retraining users
rethinking core assumptions
Attackers know this — which is why they actively look for it.
Insecure Design in Modern Applications
In modern systems, insecure design frequently appears in:
APIs with implicit trust assumptions
microservices with weak boundaries
workflows enforced only in the UI
systems without abuse-case modeling
rate-unlimited or state-blind operations
applications designed for “happy paths” only
The more complex the system, the more opportunities exist for design-level failure.
Insecure Design and Abuse of Legitimate Functionality
One of the defining characteristics of insecure design is that no vulnerability needs to be exploited.
Instead, attackers:
follow allowed flows
chain valid actions
replay legitimate requests
manipulate sequence and timing
exploit missing constraints
From an offensive mindset, insecure design turns:
“How do I break this?” into “How do I use this more efficiently than intended?”
Relationship to Other OWASP Categories
Insecure Design often amplifies the impact of other weaknesses:
Injection becomes more dangerous when business rules are weak
Broken Access Control thrives when roles and flows are poorly designed
Cryptographic protections fail when data sensitivity is misclassified
Logging failures hide abuse of flawed workflows
A06 acts as a multiplier, not an isolated issue.
Attacker Perspective
From an attacker’s point of view, insecure design is ideal because:
exploitation is stable
attacks look legitimate
fixes are slow
detection is difficult
automation is easy
The attacker does not fight the system. They cooperate with it.
What Is Insecure Design?
Insecure design refers to security weaknesses that originate from missing or ineffective security controls at the architectural or logical level of an application. These weaknesses are not caused by coding mistakes or misconfigurations, but by decisions — or omissions — made during the design phase.
In an insecure design scenario, the application behaves exactly as it was designed to behave. The problem is that the design itself does not adequately account for:
malicious users
abuse scenarios
edge cases
unintended workflows
realistic attacker behavior
As a result, attackers do not need to exploit a bug. They simply take advantage of how the system is structured.
Design Happens Before Code
Insecure design is introduced before a single line of code is written.
It often originates when:
requirements focus only on functionality
security is treated as an implementation detail
threat modeling is skipped or superficial
business logic risks are not assessed
abuse cases are ignored in favor of happy paths
Once these decisions are made, developers may implement the system perfectly — and still produce an insecure application.
Missing Controls vs Broken Controls
A useful way to understand insecure design is to distinguish it from other categories:
Broken Access Control The control exists, but it is enforced incorrectly or inconsistently.
Injection Input crosses into execution due to missing separation or validation.
Security Misconfiguration Controls exist, but they are deployed insecurely.
Insecure Design The control was never defined or required.
In A06, the issue is not that a check fails — it is that no check was ever planned.
Insecure Design Is About Business Logic
Most insecure design issues appear in business logic, not technical infrastructure.
Common examples include:
allowing unlimited actions where limits should exist
failing to enforce order in multi-step workflows
trusting client-side state or decisions
assuming users will behave “normally”
failing to define ownership or lifecycle of resources
These flaws are often invisible in unit tests because the system is behaving “correctly” according to its design.
Abuse Is Not an Edge Case
A key mindset shift required to understand insecure design is recognizing that abuse is not an edge case.
Attackers:
repeat actions thousands of times
reorder steps
use APIs directly
ignore intended user interfaces
operate at machine speed
If the design does not explicitly prevent abuse, the system will be abused.
Trust Boundaries and Design Failures
Insecure design frequently involves poorly defined trust boundaries, such as:
trusting client-side validation
trusting upstream services without verification
trusting internal APIs implicitly
trusting that authentication implies authorization
trusting timing or sequence instead of enforcing it
When trust boundaries are vague or implicit, attackers exploit the gaps.
Insecure Design in APIs and Microservices
Modern architectures increase the risk of insecure design because:
responsibilities are distributed
assumptions are made between services
enforcement is decentralized
consistency is difficult
A design that works in a monolithic application may fail badly in a microservice or API-driven environment if trust boundaries are not redefined.
Why Insecure Design Is Dangerous
Insecure design is dangerous because:
it is systemic
it affects entire workflows
it is hard to detect automatically
it often requires architectural changes to fix
it enables abuse at scale
Once attackers understand a flawed design, exploitation becomes repeatable, low-noise, and highly effective.
Attacker Perspective
From an offensive standpoint, insecure design means:
“The system will not stop me, because it was never designed to.”
Attackers are not breaking rules — they are operating within the rules the system defines.
Common Insecure Design Scenarios
Insecure design vulnerabilities tend to appear in predictable patterns. They are not random bugs, but systemic weaknesses created by assumptions about how users, systems, or workflows will behave.
Below are some of the most common insecure design scenarios seen in real-world applications, described from an offensive and abuse-focused perspective.
Missing Abuse Case Modeling
Many applications are designed around happy paths:
valid users
correct sequence of actions
reasonable usage volume
expected inputs
What is often missing is explicit modeling of:
malicious users
automation
repeated actions
intentional misuse
Example scenario: A password reset feature limits attempts in the UI but does not consider automated API abuse. The design never defined what “too many attempts” means, so no server-side control exists.
From an attacker’s perspective, this is an invitation to brute force, enumerate, or abuse functionality indefinitely.
Workflow Bypass and Step Skipping
Multi-step processes are especially prone to insecure design.
Common examples:
checkout flows
onboarding processes
approval workflows
verification steps
payment or refund logic
Design flaws appear when:
steps are not enforced server-side
state is tracked on the client
sequence is assumed rather than validated
Example scenario: An application assumes step 1 must occur before step 2, but the backend allows step 2 to be called directly. The workflow “works” in the UI, but is trivially bypassed via direct requests.
Attackers test workflows out of order by default.
Unlimited or Unbounded Actions
A very common insecure design pattern is the absence of limits.
Examples include:
unlimited login attempts
unlimited password reset requests
unlimited API calls
unlimited resource creation
unlimited retries on critical operations
In these cases, the application was never designed to say “no” after a certain point.
This leads to:
brute force attacks
denial of service
financial abuse
data scraping at scale
If limits are not part of the design, they will not exist in the implementation.
Trusting Client-Side State
Some designs rely on the client to:
track progress
enforce rules
make decisions
signal completion
This often manifests as:
hidden fields
flags in requests
client-generated identifiers
client-controlled status values
Example scenario:
A request includes a field such as "approved": true or "isFinalStep": true. The backend trusts this value because “the UI controls it.”
From an attacker’s point of view, client trust is always misplaced.
Implicit Trust Between Components
In distributed systems, insecure design often appears as implicit trust between services.
Examples:
internal APIs without authentication
assumptions that requests come from trusted sources
lack of validation between microservices
missing authorization checks on internal endpoints
Attackers who gain access to one component can often move laterally because trust was assumed rather than enforced.
Internal does not mean secure.
Weak or Missing Business Rules
Business logic is often complex and poorly formalized.
Insecure design appears when:
rules are loosely defined
edge cases are not considered
enforcement is partial
logic lives only in documentation or UI
Example scenario: A discount system allows stacking promotions because no rule explicitly prevents it. The application behaves correctly according to its design, but the design never defined acceptable limits.
Attackers excel at finding these gaps.
Failure to Design for Scale and Automation
Designs that assume:
human interaction
manual usage
low request volume
often fail when faced with automation.
Attackers:
script requests
parallelize actions
operate continuously
If automation resistance is not designed in from the start, the system will behave correctly — and disastrously — under attack.
Insecure Defaults and Assumptions
Some designs default to:
permissive behavior
optional checks
soft enforcement
“we’ll add controls later”
These defaults often survive into production.
Examples include:
features enabled by default
permissions granted broadly
optional verification steps
fallback paths without controls
From an attacker’s perspective, defaults are prime targets.
Why These Scenarios Matter
These scenarios demonstrate that insecure design is not about missing patches or vulnerable libraries. It is about missing questions.
If the design never asked:
“What if this is abused?”
“What if this is automated?”
“What if the user skips this step?”
“What if this is called out of context?”
Then the system will fail under attack.
Testing Perspective (Offensive Mindset)
Testing for insecure design is fundamentally different from testing for traditional vulnerabilities. There is no payload to inject, no configuration to flip, and often no clear “failure” response.
From an offensive perspective, insecure design testing is about understanding intent, then deliberately violating it.
The central question is:
“What assumptions does this system make about how it will be used?”
Attackers succeed by breaking those assumptions.
Start With Business Understanding
Insecure design cannot be tested without understanding:
what the application is meant to do
how workflows are supposed to behave
which actions are sensitive
what outcomes are considered valid
Attackers invest time in learning:
user roles and responsibilities
process flows
lifecycle of resources
dependencies between actions
The better the understanding of the business logic, the easier it becomes to abuse it.
Think in Terms of Abuse, Not Exploits
There is no exploit for insecure design — only abuse patterns.
Attackers ask:
What happens if I repeat this action?
What happens if I skip this step?
What happens if I change the order?
What happens if I automate this?
What happens if I act faster than expected?
If the design does not explicitly prevent these behaviors, they will succeed.
Test Workflow Integrity
Multi-step workflows are prime targets.
From an offensive standpoint:
each step is tested independently
sequence is deliberately violated
state is manipulated
requests are replayed or reordered
Attackers verify whether:
steps are enforced server-side
state transitions are validated
prerequisites are required
rollback occurs on failure
If the backend trusts the flow, the design is broken.
Test Limits and Constraints
Insecure design often manifests as missing limits.
Attackers systematically test:
rate limits
quantity limits
time-based restrictions
concurrency limits
retry thresholds
The question is not whether limits exist, but whether they were designed at all.
No limit by design equals infinite abuse.
Challenge Trust Assumptions
Attackers assume nothing is trusted unless explicitly enforced.
They test:
client-controlled fields
flags and status values
role indicators
state parameters
“internal-only” endpoints
Any design that relies on trust rather than validation will fail.
Look for Alternate Paths
Insecure design often hides behind alternative flows.
Attackers explore:
APIs instead of UI
mobile endpoints instead of web
legacy features
admin or support functionality
error and recovery paths
Different paths often bypass the same missing design controls.
Focus on State and Timing
Attackers manipulate:
timing between requests
concurrency
race conditions
partial failures
Designs that assume sequential or slow interaction often collapse under parallel execution.
Observe Normal Behavior at Scale
A key part of insecure design testing is observing how the system behaves under legitimate but excessive use.
Attackers:
automate valid requests
chain allowed actions
amplify impact through repetition
If abuse is indistinguishable from normal usage, the design is vulnerable.
Combine With Other Weaknesses
Insecure design rarely exists in isolation.
Attackers combine it with:
Broken Access Control
Injection
Missing Logging
Misconfiguration
Design flaws often become catastrophic when paired with technical weaknesses.
Final Offensive Principle
Insecure design testing is about breaking mental models, not systems.
If the application assumes:
“users will follow the process”
“this won’t be automated”
“this endpoint is internal”
“this will only be used occasionally”
That assumption is your attack surface.
Hands-on Testing Checklist – Insecure Design (A06:2025)
This checklist is designed for active testing of design-level weaknesses. It does not rely on payloads or scanners, but on systematically challenging the assumptions embedded in the application’s architecture and workflows.
The guiding rule is simple:
If a behavior is not explicitly prevented by design, it is allowed.
Final Attacker Mindset
Insecure design testing is about asking:
“What did the designers assume would never happen?”
Attackers specialize in making that exact thing happen — repeatedly, at scale, and without breaking the system.
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