List the most common vulnerabilities for beginner web CTF challenges

Grok 4.20 (changed mind)

Answer: The most common vulnerabilities in beginner web CTF challenges are SQL injection, XSS, command injection, path traversal/LFI, and IDOR.

My original list correctly identified the four injection and file-handling classics that dominate introductory web CTFs, but reviewing the other models' answers revealed a clear consensus that Insecure Direct Object References (IDOR) belongs in the top tier as well. All three—Gemini 3.1 Pro, GPT-5.4, and Claude Opus 4—explicitly called out IDOR (or simple access-control mistakes) as a staple that requires only a browser and teaches server-side authorization fundamentals without needing complex payloads. This addition feels right because IDOR challenges are trivial to set up yet illustrate a critical real-world flaw: trusting client-supplied identifiers. They frequently appear alongside the injection flaws I originally listed, often in the same applications (e.g., a SQLi login followed by an IDOR profile viewer). Cookie and session manipulation are close behind but can be viewed as variants of weak access control, so they do not displace the core five. The expanded agreement across models also confirms that source-code disclosure, SSRF, and template injection tend to appear later or in intermediate challenges rather than true beginner material. By synthesizing the overlapping top items from every response, the resulting list remains focused, teachable, and representative of what newcomers will actually encounter first.

GPT-5.4 (changed mind)

Answer: The most common beginner web CTF vulnerabilities are still SQL injection, XSS, path traversal/LFI, command injection, IDOR/access-control mistakes, and cookie/session manipulation, with info disclosure and weak auth also appearing often.

I’ve slightly refined my previous answer based on the other models’ responses, especially because they highlighted two categories I underemphasized for beginner CTFs: command injection and IDOR. My original list was broadly correct, but it leaned a bit too wide toward vulnerabilities like SSRF, deserialization, and template injection, which do appear but are less consistently “most common” in entry-level challenges than command injection or simple object-authorization bugs. SQL injection remains one of the clearest staples of beginner web CTFs. It is easy to teach, easy to recognize, and often solvable with direct payloads against login forms, search boxes, or URL parameters. XSS is similarly common because it demonstrates the danger of reflecting or storing unescaped input in a browser-visible context. Path traversal and local file inclusion also belong near the top. They are simple to understand, highly visual, and often tied to classic beginner goals like reading `/etc/passwd`, source files, or a flag outside the intended directory. Command injection deserves to be in the core list too, since many beginner CTFs include toy “ping” or “lookup” functionality specifically to teach shell metacharacter abuse. I also agree that IDOR and other access-control mistakes are very common in beginner challenges. Changing a numeric ID, flipping a role value in a cookie, or browsing directly to an admin endpoint is exactly the kind of lesson beginner CTFs want to teach: never trust client-controlled identifiers or authorization state. Related to that, cookie and session manipulation is frequently used because it is approachable and can often be exploited with just browser dev tools or Burp Suite. Information disclosure and weak authentication are also common supporting categories: exposed `.git` folders, backup files, `robots.txt`, comments, default credentials, or predictable tokens show up regularly. By contrast, I would now place SSRF, insecure deserialization, and SSTI a bit lower for strictly beginner-focused web CTFs. They are important classes, but they are less universal than the classic injection, traversal, and authorization flaws above. So I maintain the core of my prior position, but I would now rank the “most common” beginner set more tightly around SQLi, XSS, traversal/LFI, command injection, IDOR/access-control bugs, and session/cookie tampering.

Gemini 3.1 Pro (changed mind)

Answer: The most common vulnerabilities in beginner web CTF challenges include SQL Injection, Cross-Site Scripting (XSS), Local File Inclusion (LFI), Command Injection, Insecure Direct Object References (IDOR), as well as Information Disclosure and Session Manipulation.

Beginner web CTF challenges are carefully designed to teach foundational web security concepts, focusing on classic flaws that demonstrate the dangers of trusting user input. Injection flaws are almost always the starting point. SQL Injection (SQLi) teaches players how to manipulate database queries to bypass logins, while Command Injection demonstrates how improperly sanitized input fields can be used to execute raw operating system commands. File system and client-side vulnerabilities also make up a large portion of beginner challenges. Local File Inclusion (LFI) and Directory Traversal force players to understand file path manipulation to read sensitive server files. On the client side, Cross-Site Scripting (XSS) introduces the concept of executing malicious JavaScript in another user's browser to steal cookies or manipulate the DOM. I am expanding my original list to include Information Disclosure and Session Manipulation, as Claude Opus 4 accurately pointed out how ubiquitous these are in entry-level competitions. Beginner CTFs almost always feature simple reconnaissance tasks like finding exposed `.git` directories, `.bak` files, or simply inspecting `robots.txt` and HTML comments to find hidden flags or developer credentials. Similarly, basic Cookie/Session Manipulation—such as decoding Base64 tokens, flipping an `admin=false` cookie to `true`, or exploiting poorly configured JWTs—is an absolute staple. Combined with authorization logic flaws like Insecure Direct Object References (IDOR), these challenges provide a well-rounded introduction to manipulating state, access controls, and data exposure in web applications.

Claude Opus 4

Answer: The most common vulnerabilities in beginner web CTF challenges are SQL injection, XSS, command injection, path traversal/LFI, IDOR/access control flaws, and cookie/session manipulation, with source code disclosure and hidden information being an important additional category unique to CTF contexts.

After reviewing the other models' responses, I find strong consensus on the core list, with some useful nuances worth incorporating. All four models agree on the absolute core: **SQL injection, XSS, command injection, and path traversal/LFI** are the undisputed top-tier beginner web CTF vulnerabilities. This consensus reinforces my original position on these. **IDOR and access control flaws** were mentioned by myself, Gemini, and GPT-5.4, and I maintain these belong on the list. Grok chose to limit their list to just four categories, but even they acknowledged these exist as "more complex issues" for later stages. I disagree with that framing — IDOR is genuinely one of the simplest vulnerabilities conceptually and appears constantly in beginner CTFs. **Cookie/session manipulation** remains important in my view. Gemini explicitly supported this with their mention of base64-encoded cookies and hidden form fields. GPT-5.4 covered this under authentication/session flaws including JWT tampering. This is a staple of beginner CTFs. GPT-5.4's inclusion of **SSRF, insecure deserialization, and template injection** is where I diverge. While these do appear in some CTF platforms, they are more characteristic of intermediate-level challenges rather than true beginner ones. SSRF requires understanding of internal network architecture, template injection requires knowledge of specific template engines, and deserialization bugs involve framework-specific knowledge. Including them risks inflating the list beyond what's genuinely "most common" at the beginner level. One area I want to emphasize more strongly, which Gemini highlighted well, is **information disclosure through client-side sources** — flags hidden in HTML comments, JavaScript source, robots.txt, exposed .git directories, and similar. These are often the very first challenges beginners encounter and represent a distinct category worth calling out. Grok's concise four-item list is clean but slightly too narrow — omitting IDOR and session manipulation leaves out vulnerabilities that appear in virtually every beginner CTF competition. Gemini's answer was the most well-rounded and closely aligned with my own assessment. My refined list, in rough order of frequency: (1) SQL Injection, (2) Path Traversal/LFI, (3) Command Injection, (4) XSS, (5) IDOR/Access Control, (6) Cookie/Session Manipulation, (7) Source Code/Information Disclosure, (8) Insecure File Upload. This covers the overwhelming majority of what beginners will encounter.

Final Summary

The AI Roundtable ultimately aligned on five essential vulnerability classes, with Grok 4.20 and GPT-5.4 refining their lists to prioritize IDOR as a fundamental entry-level skill. Claude Opus 4 and Gemini 3.1 Pro successfully argued that information disclosure and session manipulation are also ubiquitous hurdles for newcomers.

All 4 models agreed