Understanding Injection Attacks in Application Security: Types, Tools, and Examples

How Injection Attacks Exploit Web Application Vulnerabilities

Injection attacks occur when malicious input is inserted into a web application, exploiting vulnerabilities in unvalidated user input to execute unintended commands. Attackers craft payloads that manipulate how the application processes data, often leading to unauthorized access, data leaks, or system compromise.

This article explores the most prevalent injection attacks targeting web applications and APIs, examines the underlying security weaknesses that enable these exploits, and provides effective detection and prevention strategies to mitigate risks.

Understanding Injection Attacks

Injection attacks are a category of cyber threats that exploit injection vulnerabilities, allowing attackers to insert malicious payloads into application code through unvalidated user input. These attacks are among the most severe application security risks, as highlighted in the OWASP Top 10 (2021), where injection vulnerabilities were ranked as the #3 overall security risk for web applications.

Although injection attacks come in various forms, they all share a common trait: attackers manipulate how an application processes data, potentially altering database queries, executing JavaScript, running system commands, or even injecting native application code. Depending on the vulnerability and attack vector, the consequences can range from minor data leaks to severe security breaches, including denial of service (DoS), authentication bypass, privilege escalation, remote code execution (RCE), or full system compromise. Understanding and mitigating these risks is essential for strengthening application security and protecting sensitive data.

SQL Injection (SQLi): The Most Prevalent Injection Attack

Many web applications rely on relational databases that use SQL (Structured Query Language) to store and retrieve data. SQL injection (SQLi) is a critical vulnerability that occurs when malicious SQL statements are embedded into user input fields, such as web forms, query parameters, comment sections, or other input channels accessible to users. If an application fails to properly validate or sanitize user input, attackers can manipulate SQL queries to extract sensitive data, alter database records, or even delete entire tables.

One of the most common SQLi attack strategies involves injecting an SQL query that grants privileged access, allowing attackers to create, modify, or escalate user permissions within the database. In cases where a vulnerable application does not return data directly, blind SQL injection techniques can be used to infer database information through indirect responses.

SQL injection vulnerabilities fall under CWE-89: Improper Neutralization of Special Elements Used in an SQL Command and ranked #3 on the CWE Top 25 for 2023, highlighting its severity in application security. Invicti’s DAST tools can automatically detect various forms of SQL injection, including in-band SQL injection (such as UNION-based attacks), blind SQL injection (Boolean-based queries), and out-of-band SQLi techniques, helping organizations identify and remediate SQL vulnerabilities before they can be exploited.

Cross-Site Scripting (XSS): A Critical Script Injection Attack

Although it doesn’t contain “injection” in its name, Cross-Site Scripting (XSS) is fundamentally an injection attack that exploits script execution vulnerabilities. XSS occurs when a web application fails to properly sanitize user-supplied input, allowing malicious JavaScript (or other scripts) to be injected into the application’s output. If a vulnerable application processes this unfiltered input, it may execute the attacker’s script in a victim’s browser, leading to session hijacking, credential theft, or further exploitation.

To launch an XSS attack, an attacker embeds a malicious script within a request parameter, form input, or URL query string. Instead of treating the input as standard user data, the application renders and executes the injected script in the user’s browser. While XSS is sometimes considered low-risk, its impact can extend far beyond a single user session, particularly when used as part of a larger attack chain. Furthermore, with the rise of full-stack JavaScript environments like Node.js, XSS vulnerabilities can also pose risks to server-side applications.

Simple input filtering is not enough to prevent XSS, as attackers can use various techniques to evade filters. To mitigate XSS risks, developers should follow secure coding practices, enforce proper input validation and output encoding, and implement Content Security Policy (CSP) to restrict the execution of unauthorized scripts.

In the CWE classification, XSS is identified as CWE-79: Improper Neutralization of Input During Web Page Generation and was ranked #2 in the CWE Top 25 for 2023. Invicti’s DAST tools can automatically detect and validate various types of XSS vulnerabilities, including reflected XSS, stored (persistent) XSS, and DOM-based XSS, helping organizations secure their applications against this widespread threat.

OS Command Injection: A High-Risk System Exploit

OS command injection, also known as shell injection, occurs when a web application fails to properly sanitize user input, allowing attackers to execute arbitrary system commands on the underlying server. Some web applications legitimately execute operating system commands—for example, to read or write files, run system utilities, or manage server processes. However, if user-controlled input is improperly handled within these commands, attackers can inject malicious system-level instructions, leading to data exposure, privilege escalation, or full system compromise.

Successful command injection attacks can be highly destructive, enabling attackers to:

• Retrieve server and system configuration details, helping them map out vulnerabilities.
• Escalate user privileges, gaining unauthorized administrative access.
• Execute arbitrary system commands, which can lead to file manipulation, malware deployment, or even complete server takeover.

How to Mitigate OS Command Injection

Due to the severe risks associated with OS command injection, it is best to avoid executing system commands that include user-controllable data whenever possible. If executing system commands is unavoidable, developers should:

• Strictly validate input to ensure only expected values are processed.
• Use parameterized execution instead of directly concatenating user input into commands.
• Restrict command execution to predefined functions that limit potential misuse.

OS command injection is categorized as CWE-78: Improper Neutralization of Special Elements Used in an OS Command and was ranked #5 in the CWE Top 25 for 2023, highlighting its high-risk nature. Invicti’s DAST tools can detect various command injection vulnerabilities, including blind and out-of-band command injection, helping organizations identify and mitigate these critical security threats before they can be exploited.

Code Injection (Remote Code Execution – RCE): The Ultimate Security Threat

Code injection, also known as remote code execution (RCE), is one of the most severe vulnerabilities in web applications. It occurs when an attacker successfully injects malicious application code into user input and gets the vulnerable application to execute it. Unlike OS command injection, which manipulates system commands, code injection directly targets the application’s execution environment, making it an extremely powerful attack.

How Code Injection Works

The injected code must match the application’s programming language. For example:

• A PHP-based application with a code injection flaw would be vulnerable to malicious PHP code execution.
• A Java-based web application could be exploited using Java-based injection payloads.
• If an application flaw allows both code injection and OS command execution, an attacker could escalate from application-level compromise to full system control.

Why RCE is Considered Critical

Remote Code Execution (RCE) is one of the most dangerous security vulnerabilities because it often results in full system compromise. Attackers with RCE capabilities can:

• Execute arbitrary code on the server.
• Modify, delete, or exfiltrate data from the application.
• Deploy malware or backdoors for persistent access.
• Escalate privileges and gain administrative control over the system.

Even though some code injection vulnerabilities require additional steps to exploit, RCE is almost always classified as critical, as it provides attackers with unrestricted access to a compromised system.

How to Prevent Code Injection Attacks

• Never allow user-controlled input to be executed as code—always validate and sanitize input strictly.
• Use parameterized functions or sandboxed execution environments to restrict the scope of code execution.
• Apply proper input filtering and encoding to prevent untrusted code from being executed.

Detection and Classification

Code injection is classified as CWE-94: Improper Control of Generation of Code and remains one of the most sought-after vulnerabilities in application security testing. Invicti’s vulnerability scanner is capable of detecting and often automatically confirming dozens of code execution and evaluation vulnerabilities across multiple programming languages and frameworks, helping organizations identify and remediate critical security risks before they can be exploited.

XXE Injection: Exploiting XML Parser Vulnerabilities

Rounding out the top five injection attacks is XML External Entity (XXE) injection, a vulnerability that targets web applications handling XML inputs. If an application supports legacy document type definitions (DTDs) and is configured with weak XML parser security, attackers can manipulate malformed XML documents to execute XXE attacks. These exploits can lead to directory traversal, server-side request forgery (SSRF), or even remote code execution (RCE) in severe cases.

How XXE Injection Works

Unlike other injection attacks that stem from user input validation failures, XXE vulnerabilities arise from insecure XML parser configurations. By injecting external entity references into XML documents, attackers can trick the parser into loading external files, making unauthorized requests, or exposing sensitive system data.

Why XXE is Dangerous

• Can be used for directory traversal, allowing attackers to access restricted files.
• Enables SSRF attacks, tricking the server into making unintended external requests.
• In some cases, XXE can lead to remote code execution, allowing complete system compromise.
• Difficult to detect, as it exploits insecure configurations rather than traditional coding flaws.

Preventing XXE Attacks

If your application processes XML data, the best way to prevent XXE vulnerabilities is to:

• Disable support for DTDs entirely in your XML parser.
• If DTDs are required, disallow external entities to prevent unauthorized access.
• Use secure XML parsers that adhere to modern security best practices.

XXE Detection and Classification

XXE vulnerabilities fall under CWE-611: Improper Restriction of XML External Entity Reference. While XXE was ranked #4 in the OWASP Top 10 (2017), it was later merged into the Security Misconfiguration category in the 2021 OWASP Top 10, reflecting its nature as a configuration-based vulnerability.

Invicti’s web vulnerability scanner can detect and confirm multiple forms of XXE injection, including out-of-band (OOB) XXE attacks, helping organizations secure their XML processing workflows and eliminate risky parser misconfigurations.

Other Notable Injection Attacks

While the top five injection vulnerabilities pose the most significant risks to web applications and APIs, several less frequent—but still dangerous— injection attacks are also worth noting. These attack types exploit different input channels and target various backend systems, including databases, APIs, template engines, and HTTP headers.

NoSQL Injection

Similar to SQL injection (SQLi), NoSQL injection manipulates database queries—but instead of targeting SQL-based relational databases, it exploits NoSQL databases like MongoDB, Cassandra, or Elasticsearch. Since NoSQL databases do not use a standard query language, injection payloads must be tailored for each database type, often exploiting unvalidated JSON input or JavaScript-based queries to extract or manipulate data.

JSON Injection

Closely related to cross-site scripting (XSS), JSON injection allows attackers to manipulate JSON data sent or received by a web application. This is particularly relevant for REST APIs, where JSON is the dominant data format. By injecting or modifying JSON payloads, attackers can alter API behavior, steal sensitive data, or execute unauthorized actions.

Server-Side Template Injection (SSTI)

SSTI attacks exploit server-side template engines that dynamically generate HTML or code. If an application improperly handles user input within a template system, attackers can inject malicious expressions, causing the server to execute arbitrary code. Expression language (EL) injection is a related attack, targeting expression parsers within web frameworks instead of template engines, often leading to code execution or unauthorized data access.

HTTP Header Injection (CRLF Injection)

HTTP header injection, also known as CRLF (Carriage Return Line Feed) injection, occurs when an application fails to sanitize newline characters (\r\n) in user input before inserting it into an HTTP response header. Since HTTP uses newline characters to separate headers from the body, an attacker can inject their own headers or modify the response, potentially replacing the page content with a malicious XSS payload or altering security policies.

Final Thoughts

While these injection attacks are less common than SQL injection, XSS, OS command injection, code injection, and XXE, they still pose serious risks when applications fail to validate and sanitize user input properly. Modern security best practices, including input validation, output encoding, parameterized queries, and strict API security controls, are essential for mitigating these threats.

Organizations should adopt automated security testing solutions, such as Invicti’s DAST scanner, to detect and remediate injection vulnerabilities before they can be exploited.

THE AUTHOR

Acunetix

Acunetix developers and tech agents regularly contribute to the blog. All the Acunetix developers come with years of experience in the web security sphere.