Six Most Common Vulnerabilities Found During Penetration Testing

Best practices

18 min read

03 Jun 2026

6 most common vulnerabilities found during penetration testing

According to a new report by Juniper Research, over 33 billion records are stolen as a result of cyberattacks. Because of the increasing cyberattacks, penetration testing is no longer an option but a necessity – especially for any business handling data, gathering financial details, or establishing personal intellectual property.

Tania Zhydkova Author

Denis Matusovskiy Reviewed

Key Takeaways

The six most common pen test vulnerabilities found in real engagements map closely to the OWASP Top 10: security misconfigurations, SQL injection, broken authentication, sensitive data exposure, cross-site scripting (XSS), and outdated components.
Penetration testing attacks simulate what a real threat actor would do. Finding a vulnerability through a pen test is always better than discovering it after a breach.
Most pen test vulnerabilities are not zero-days. They are well-known, well-documented weaknesses that persist because of skipped updates, weak configurations, and untested code paths.
Remediation without retesting is not remediation. Every fixed vulnerability should be verified in the same environment where it was found.
Vulnerability scanning finds known issues automatically. Penetration testing attacks the system the way an adversary would, confirming whether vulnerabilities are actually exploitable and what the real-world impact would be.

In this post, we will briefly cover penetration testing, along with discussing the six most common vulnerabilities that are found during this test.

Penetration Testing, or pen testing, is the replication of the actions taken by malicious hackers but in an ethical way. It is a systematic process used to evaluate the security of an infrastructure to exploit vulnerabilities, which may be present in operating systems, software, and hardware flaws, inadequate configurations, or employees’ susceptibility to phishing and social engineering attacks. In addition, such assessments are used to validate the effectiveness of defensive systems and users’ adherence to security policies. Essentially, organisations can prevent or limit the damage that attackers could cause if they successfully exploit security vulnerabilities.

To identify any weak points in a system’s defences that an attacker could take advantage of, different types of tests are performed, each with varying goals, scope, and requirements. These include, but are not limited to, network penetration testing (external, internal, and perimeter devices), web application pen testing, cloud penetration testing, database penetration testing, and mobile device penetration testing. Pen testers also conduct tests for various consumer-grade products like VPNs, for example, to identify any weaknesses and provide the companies with the information required to fix such issues and secure their product.

Pen testers look for ways of exploiting known vulnerabilities, in addition to using their skills to find out any specific weaknesses, or unknown vulnerabilities, in a company’s security arrangements. Here are the six most commonly reported vulnerabilities discovered during penetration testing:

Use of default credentials

Many a time, manufacturers set default credentials to enable first-time configuration. Penetration testers often find that the network devices and systems have been improperly configured and default login credentials are still in place. Attackers are aware that devices and networks are usually left with default credentials, and with some guesswork, they can quickly enter those systems to find and steal confidential information. What’s worse, they can even set their own credentials, locking actual administrators and users out in order to continue their plan.

SQL injection

SQL injection remains one of the most damaging pen test vulnerabilities because the exploitation path is direct: an attacker injects malicious characters into an input field and the database executes commands it was never meant to run. The TalkTalk breach in 2015 resulted in a £400,000 fine from a single SQL injection attack. It is a well-understood vulnerability with known fixes that still appears regularly in penetration testing reports.

This injection works by manipulating any flaw in software where user input is required to access database information. Simply by injecting malicious characters, attackers could alter the workflow of the SQL statement, causing a remote code execution to steal user data and/or otherwise harm the targeted company. It causes significant damage to the business; for instance, the TalkTalk data breach in 2015 by a SQL injection resulted in a £400,000 fine, as well as an adverse effect on the company’s brand image.

Now that you understand how these vulnerabilities work in theory, it’s time to put your knowledge to the test. The interactive challenge below lets you step into the shoes of a penetration tester and exploit real vulnerabilities in a safe, simulated environment. Try your hand at SQL Injection, XSS attacks, phishing campaigns, and more, each challenge reveals exactly why these exploits succeed and how to defend against them.

🎯 Hack the System - Interactive Challenge (Click to open)

Cross-Site scripting (XSS) attacks

XSS is the pen test vulnerability that turns your own application against your users. When untrusted input is rendered in a browser without proper validation, an attacker can inject scripts that hijack sessions, steal credentials, redirect users to malicious sites, or deface the application entirely.

Less known than SQL injection, cross-site scripting (XSS) is also among the common vulnerabilities found in web applications. The general use of XSS is to steal user sessions on malicious sites. It is done either by injecting code into a site or creating a malicious URL, which will intercept the session if an unsuspicious user opens the link. This attack can get dangerous when a user’s session has been taken via cross-site scripting. As a result, the attacker leverages user impersonation to access sensitive information, such as bank account details.

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Server security misconfiguration

Security misconfiguration is the most commonly found pen test vulnerability category in enterprise environments because it rarely comes from a single mistake. It accumulates quietly across networks, servers, and applications wherever default credentials go unchanged, unnecessary ports stay open, or error messages reveal internal system details to unauthenticated users.

Inadequate access privileges

Every extra user having access to enter and/or access sensitive security systems increased the vulnerability of these systems. Privileges should be allocated only to the people needing them as per the nature of their roles and responsibilities. Remember, it takes hackers to compromise only one such user computer to get a foothold in the network and navigate around servers in order to access more data and confidential information.

Phishing

Phishing is another form of exploitation that is becoming more widespread with every passing year. According to ESET-Studie 2021
, there has been an increase of 7.3% in email-based attacks in just 4 months of 2021, many of which were a part of phishing campaigns. Phishing emails have become far more effective since attackers understand that there are higher chances of clicking on links if the email comes from a sender the recipient knows or, even better, trusts. As a result, the user clicks an infected link or enters credentials on a fake website. The attacker then gets a foothold on the network and escalates the privileges.

How to Fix and Prevent Common Penetration Testing Vulnerabilities

Finding a pen test vulnerability is step one. Fixing it and preventing it from recurring is the work that actually reduces risk. The two should not be treated as separate projects.

Security misconfigurations: Run a configuration audit against your baseline after every deployment, not just annually. Automate the check for default credentials and unnecessary open ports. Treat misconfiguration findings as a process failure, not a one-off fix.

SQL injection: Use parameterized queries and prepared statements for all database interactions. No exceptions for internal tools or admin panels. Apply a web application firewall as a secondary control, not a primary one. Retesting after a fix is non-negotiable.

Broken authentication: Enforce MFA across all user-facing and admin interfaces. Implement login throttling with a maximum attempt limit. Rotate session tokens after every privilege change. Audit your session expiry policies on a schedule.

Sensitive data exposure: Audit what data your application stores and transmits and classify it. Anything that falls into a regulated category needs encryption at rest and in transit with current cipher standards. Remove data that does not need to be stored. Log access to sensitive data and alert on anomalies.

Cross-site scripting: Validate and sanitize all user input. Escape output for the specific rendering context. Implement a Content Security Policy (CSP) header. Test every input field that renders output in the browser, including those behind authentication.

Outdated components: Maintain a software bill of materials (SBOM) for every application. Subscribe to CVE feeds for your key dependencies. Integrate software composition analysis (SCA) into your CI/CD pipeline so vulnerable components are flagged before they reach production.

For all six categories, the prevention cycle is the same: fix, retest in the same environment, document the remediation, and add a regression test case so the same pen test vulnerability cannot return undetected.

Penetration Testing vs Vulnerability Scanning

These two terms are used interchangeably in many security conversations and they should not be. The difference matters for budgeting, planning, and understanding what you actually know about your security posture after each exercise.

Vulnerability scanning is automated. A scanner runs against your systems, compares what it finds against a database of known vulnerabilities, and produces a list of potential issues ranked by severity. It is fast, repeatable, and relatively cheap. It tells you what known issues exist on the surface of your infrastructure. It does not tell you whether those issues are actually exploitable in your specific environment, what the real-world impact of exploitation would be, or whether an attacker could chain multiple findings together to achieve a significant breach.

Penetration testing is manual and adversarial. A tester is given a defined scope and uses the same techniques, tools, and thinking as a real attacker to actively exploit vulnerabilities. Penetration testing attacks go beyond scanning by testing business logic, chaining vulnerabilities, and demonstrating actual impact. A pen tester might confirm that an SQL injection is exploitable and then show exactly which data tables are accessible as a result. A scanner would only flag that the injection point exists.

The practical differences:

Vulnerability scanning: automated, fast, low cost, run frequently (weekly or monthly). Finds known CVEs and common misconfigurations. Cannot confirm exploitability.
Penetration testing: manual, thorough, higher cost, run periodically (quarterly or annually). Confirms exploitability, demonstrates impact, and finds logic flaws that scanners miss.

Both belong in a mature security testing programme. Scanning gives you continuous visibility into known issues. Penetration testing attacks give you a realistic picture of what a motivated adversary could do. Neither replaces the other.

Bottom Line

Application and infrastructure vulnerabilities are a major focus of penetration testing and vulnerability reports executed during the security management life cycle. The vulnerabilities listed above illustrate only the key ways in which hackers can get access to the network, gain administrative privileges, and use them to do unimaginable damage to the business. With pen testing, organisations can establish weaknesses in their network, patch them or source a cybersecurity consultancy to do that, thereby improving overall security.

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FAQ

What is Penetration Testing?

Penetration testing is a security check-up for your systems. It’s where experts intentionally try to hack into your software, network, or apps—but with your permission. They must find vulnerabilities before real attackers do.

Here’s how it works:

They simulate real-world attacks and test for weak spots.
Once they find the issues, they give you a clear report with steps to fix them.

Why does this matter? Because one small vulnerability can lead to a massive breach—costing you money, trust, and reputation.

Why is it Important to Continuously Conduct Penetration Testing for a Strong Security System?

In especially heavily-regulated industries, you need to conduct penetration testing continuously, for these reasons:

New Threats Don’t Stop: Cybercriminals evolve; regular testing keeps you ahead.
Systems Change Over Time: Updates and new features can create vulnerabilities.
Compliance Matters: Meet GDPR, HIPAA, PCI-DSS or face fines.
Avoid Costly Breaches: Fix issues before they cost millions.
Peace of Mind: Secure systems let you focus on growth.

What does a penetration testing company do?

A penetration testing company simulates cyberattacks on your system. They aim to find vulnerabilities there. Then, they provide detailed reports and actionable fixes to strengthen your security.

How should penetration testing findings be prioritized?

Penetration testing findings should be prioritized by exploitability and business impact, not just CVSS severity score alone. A critical-rated finding that requires physical access to exploit is less urgent than a medium-rated finding that is remotely exploitable and exposes customer payment data. The standard prioritization framework groups findings into critical (immediate remediation required before next release), high (remediated within the current sprint or release cycle), medium (scheduled into the next two to three sprints), and low (documented and addressed in the backlog). Within each tier, prioritize by: how easily penetration testing attacks can be executed, what data or system access is gained through exploitation, whether the vulnerability can be chained with others to escalate impact, and whether it affects a compliance-relevant system or data category. Always start remediation with the findings that a real attacker would use first.

What is the difference between penetration testing and vulnerability scanning?

Vulnerability scanning is automated and identifies known issues by comparing your systems against databases of published vulnerabilities. It is fast and should be run frequently. Penetration testing is manual and adversarial: a tester actively tries to exploit the vulnerabilities it finds, chains findings together to demonstrate real-world attack paths, and identifies logic flaws that no scanner can detect. A scan tells you what might be wrong. A pen test tells you what is actually exploitable and what the impact of exploitation would be. Both belong in your security testing programme, but they answer different questions. Vulnerability scanning gives you continuous coverage of known issues. Penetration testing attacks give you a ground-truth picture of your actual security exposure. Many organizations make the mistake of treating a clean scan as evidence of security. A pen test is the only way to verify that assumption.