Web application security vulnerabilities are weaknesses or flaws in a web application's code, configuration, architecture, APIs, authentication mechanisms, or supporting infrastructure that attackers can exploit.
These vulnerabilities can expose sensitive information, allow unauthorized access, disrupt applications, compromise user accounts, or give attackers greater control over systems.
Because modern web applications continuously interact with users, APIs, databases, third-party services, and cloud infrastructure, securing them requires more than fixing individual coding errors. Organizations need a combination of secure development practices, vulnerability management, access controls, application and API protection, and continuous monitoring.
Where do web app vulnerabilities come from?
A web application vulnerability is a security weakness that can cause an application to behave in an unintended or insecure way. Vulnerabilities can originate in application code, but they can also occur elsewhere in the application stack.
Common sources include:
Application and API code
Authentication and authorization logic
Session management
Input validation
Application configuration
Cloud and server configuration
Third-party libraries and dependencies
APIs and integrations
Business logic
Data handling
Encryption and security controls
The potential consequences vary considerably. One vulnerability might expose information that should be private, while another could allow an attacker to impersonate a user, manipulate a database, execute unauthorized commands, or interfere with an application's availability.
Organizations therefore need to consider both the likelihood that a vulnerability can be exploited and the impact exploitation could have. Having a web application security strategy in place is critical.
Why are web applications vulnerable?
Web applications are attractive targets because they are designed to be accessible. Customers, employees, partners, applications, and APIs may need to interact with them over the public internet.
Several factors contribute to their attack surface.
Applications accept untrusted input
Web applications constantly process information supplied by users and other systems, from URLs and search queries to form submissions, API payloads, uploaded files, and HTTP headers. If an application doesn't validate or safely process that input, attackers may be able to manipulate it.
Modern applications are complex
A modern application might combine frontend frameworks, backend services, databases, APIs, open-source libraries, cloud infrastructure, authentication providers, and third-party integrations. Each component can introduce additional security considerations and dependencies.
Applications change frequently
Continuous integration and deployment allow teams to release software rapidly. But frequent changes can also introduce vulnerabilities or configuration mistakes. Security controls therefore need to keep pace with development rather than relying exclusively on periodic reviews.
APIs expand the attack surface
Modern web applications increasingly rely on APIs for communication between browsers, mobile apps, microservices, partners, and backend systems. Attackers can target those APIs directly rather than interacting with the visible website.
Access-control logic can be difficult to implement
Applications often need to determine not only whether someone is authenticated, but also what that particular user is authorized to see and do. Errors in these rules can expose another user's information or provide access to administrative functions.
Third-party components introduce dependencies
Open-source packages and commercial libraries accelerate development, but vulnerabilities discovered in dependencies can affect every application using them. That’s why keeping an inventory of dependencies and applying security updates is an important part of application security.
What are the most common web application security vulnerabilities?
The exact vulnerabilities organizations encounter vary, but several categories consistently represent significant application-security risks. The OWASP Top 10 is a widely used reference for understanding major web application security risks.
A01:2025 - Broken Access Control. OWASP’s Description: Access control enforces policy such that users cannot act outside of their intended permissions. Failures typically lead to unauthorized information disclosure, modification, or destruction of all data, or performing a business function outside the user's limits.
A02:2025 - Security Misconfiguration. OWASP’s Description: Security misconfiguration is when a system, application, or cloud service is set up incorrectly from a security perspective, creating vulnerabilities.
A03:2025 - Software Supply Chain Failures. OWASP’s Description: Software supply chain failures are breakdowns or other compromises in the process of building, distributing, or updating software. They are often caused by vulnerabilities or malicious changes in third-party code, tools, or other dependencies that the system relies on.
A04:2025 - Cryptographic Failures. OWASP’s Description: This weakness focuses on failures related to the lack of cryptography, insufficiently strong cryptography, leaking of cryptographic keys, and related errors.
A05:2025 - Injection. OWASP’s Description: An injection vulnerability is a system flaw that allows an attacker to insert malicious code or commands (such as SQL or shell code) into a program’s input fields, tricking the system into executing the code or commands as if it were part of the system. This can lead to truly dire consequences.
A06:2025 - Insecure Design. OWASP’s Description: Insecure design is a broad category representing different weaknesses, expressed as “missing or ineffective control design.”
A07:2025 - Authentication Failures. OWASP’s Description: When an attacker is able to trick a system into recognizing an invalid or incorrect user as legitimate.
A08:2025 - Software or Data Integrity Failures. OWASP’s Description: Software and data integrity failures relate to code and infrastructure that does not protect against invalid or untrusted code or data being treated as trusted and valid.
A09:2025 - Logging & Alerting Failures. OWASP’s Description: Without logging and monitoring, attacks and breaches cannot be detected, and without alerting, it is very difficult to respond quickly and effectively during a security incident. Insufficient logging, continuous monitoring, detection, and alerting to initiate active responses occur at any time.
A10:2025 - Mishandling of Exceptional Conditions. OWASP’s Description: Mishandling exceptional conditions in software happens when programs fail to prevent, detect, and respond to unusual and unpredictable situations, which leads to crashes, unexpected behavior, and sometimes vulnerabilities. This can involve one or more of the following 3 failings: the application doesn’t prevent an unusual situation from happening, it doesn’t identify the situation as it is happening, and/or it responds poorly or not at all to the situation afterwards.
How do attackers exploit web application vulnerabilities?
Attackers generally begin by looking for weaknesses in the application's exposed attack surface.
Reconnaissance
An attacker may first identify domains, applications, APIs, endpoints, technologies, and publicly accessible services. Automated tools can make this process extremely fast.
Scanning and probing
Attackers may send requests designed to identify known vulnerabilities, configuration problems, exposed files, weak endpoints, or unexpected application behavior. Bots can automate this activity across thousands of targets.
Manipulating application input
An attacker may change request parameters, headers, cookies, API payloads, object identifiers, or other inputs to see whether the application processes them insecurely.
Exploiting authentication
Attackers may attempt brute-force attacks, credential stuffing, session theft, or other techniques intended to gain access to user accounts.
Abusing application logic
Not every successful attack requires a traditional software vulnerability. Attackers may use legitimate application functionality in unintended ways: automatically creating accounts, scraping content, exhausting expensive API operations, or manipulating ecommerce workflows.
Chaining vulnerabilities
A relatively minor vulnerability can become much more serious when combined with another weakness. For example, an information disclosure issue might reveal details that make an access-control vulnerability easier to exploit.
This is one reason organizations should consider application security as a layered risk problem rather than evaluating vulnerabilities exclusively in isolation.
How can organizations identify web application vulnerabilities?
Finding vulnerabilities requires multiple techniques because no single testing method identifies every type of weakness.
Perform application security testing
Organizations can use automated scanners and manual testing to identify vulnerabilities before and after applications reach production.
Techniques can include:
Static application security testing (SAST)
Dynamic application security testing (DAST)
Software composition analysis (SCA)
API security testing
Penetration testing
Code review
Dependency scanning
Cloud and configuration scanning
Test APIs explicitly
API endpoints should be inventoried and tested rather than assumed to be covered by traditional web-application testing. Teams should understand which APIs exist, what data they expose, who should be able to call them, and which operations could be abused.
Monitor production traffic
Pre-production testing can't anticipate every real-world attack. Monitoring production requests can reveal scanning, injection attempts, bot activity, credential attacks, anomalous API usage, and other suspicious behavior.
Track dependencies and vulnerabilities
Maintain an inventory of third-party components and monitor them for newly disclosed vulnerabilities. When a critical vulnerability is announced, knowing which applications use the affected component can significantly accelerate remediation.
Use threat intelligence
Information about emerging attack techniques, malicious IP addresses, botnets, exploit activity, and newly discovered vulnerabilities can help organizations prioritize defenses.
How can organizations prevent web application vulnerabilities?
Preventing web application vulnerabilities requires security throughout the software lifecycle.
Build security into development
Secure coding standards, developer education, code review, threat modeling, and automated security tests can help prevent vulnerabilities before code reaches production.
Validate and safely process input
Applications should treat external input as untrusted. Use parameterized queries, context-appropriate output encoding, schema validation, safe framework functionality, and other established controls rather than attempting to filter individual malicious strings.
Enforce authorization server-side
Every sensitive request should be authorized according to the user, resource, and requested action. Don't rely on hidden buttons or frontend logic to prevent unauthorized operations.
Strengthen authentication
Use appropriate password protections, secure session management, multi-factor authentication for sensitive use cases, and controls against automated credential attacks.
Patch vulnerable components
Maintain software inventories and processes for identifying and updating vulnerable dependencies. Critical vulnerabilities may require emergency remediation or temporary mitigating controls.
Apply least privilege
Applications, services, users, and infrastructure should receive only the permissions required to perform their intended functions. Limiting permissions can reduce the damage possible when one component is compromised.
Protect applications with a WAF
A web application firewall (WAF) can inspect incoming application requests and block or otherwise respond to malicious traffic. A WAF doesn't eliminate the need to fix vulnerable code, but it can provide an important additional layer of defense, particularly when a vulnerability can't be patched immediately.
Protect APIs
Apply authentication, authorization, schema validation, rate limiting, and other appropriate security controls to API traffic.
Manage malicious bots
Bot management can help identify and control automated clients involved in credential attacks, vulnerability scanning, scraping, and application abuse.
Deploy DDoS protection
Application security also includes availability. DDoS mitigation can help keep applications accessible when attackers attempt to overwhelm network or application resources.
What Fastly offerings can help protect web applications?
Fastly provides application and API security capabilities designed to complement secure development and vulnerability-remediation programs.
Fastly Next-Gen WAF
Fastly Next-Gen WAF protects web applications and APIs by identifying and blocking malicious requests. Fastly's WAF uses its SmartParse detection technology to analyze request parameters and identify attacks based on their intent rather than relying solely on traditional regular-expression matching.
Fastly Next-Gen WAF can help protect against attack classes including:
SQL injection
Cross-site scripting
Command injection
Path traversal
Other application-layer attacks
It can be deployed at the edge, in cloud environments, or alongside application infrastructure depending on organizational requirements.
Virtual Patching
When a vulnerability is discovered, organizations can't always deploy a permanent fix immediately. Fastly's virtual patching capabilities can provide protection against known vulnerabilities while teams test and deploy application-level fixes. This can help reduce the period between vulnerability disclosure and remediation.
Virtual patches aren't a substitute for updating vulnerable software, but they can provide an additional defensive layer during that window.
API protection
Modern applications increasingly expose functionality and data through APIs, making API security an important component of web application security. Fastly's application-security capabilities can inspect API traffic and help organizations identify and block malicious requests before they reach protected applications.
Bot Management
Automated tools are commonly used for reconnaissance, vulnerability scanning, credential attacks, scraping, and other forms of application abuse. Fastly Bot Management provides mechanisms for detecting and managing automated traffic, complementing WAF protection against application-layer exploits.
Organizations can apply different policies to unwanted automation while allowing legitimate users and approved bots to continue accessing applications.
DDoS Protection
Attackers don't always need to exploit a software vulnerability to make an application unavailable. Fastly's DDoS Protection helps defend applications and services from attacks designed to overwhelm infrastructure. Combining DDoS mitigation with WAF and bot-management capabilities provides protection against multiple categories of application-layer and network threats.
Edge-based security
Fastly's security capabilities operate on its edge cloud platform, allowing malicious traffic to be identified and mitigated before it reaches application origins. This helps protect backend infrastructure while reducing the compute, bandwidth, and application resources consumed by attack traffic.
How can Fastly help organizations reduce web application risk?
Web application vulnerabilities can't be eliminated by a security product alone. Organizations still need secure application design, code review, testing, patching, dependency management, strong authentication, and effective access controls.Runtime security adds another important layer.
Fastly brings together Next-Gen WAF, API protection, Bot Management, DDoS Protection, edge delivery, and security visibility to help organizations protect applications and APIs against attacks in production.
This layered approach can help organizations:
Block application attacks before they reach backend infrastructure
Protect applications while vulnerabilities are being remediated
Manage malicious automated traffic
Reduce exposure to DDoS attacks
Protect APIs alongside traditional web applications
Gain visibility into malicious traffic and attack patterns
Apply security controls at the edge without unnecessarily sacrificing application performance
Ultimately, effective web application security requires both reducing vulnerabilities in the application itself and limiting attackers' ability to exploit the weaknesses that remain. Fastly's application-security portfolio provides runtime defenses that can complement secure development and vulnerability-management practices.