CWE Top 25 - (2021)

The CWE Top 25 Most Dangerous Software Weaknesses from 2021 is a list of the 25 most dangerous software vulnerabilities. These weaknesses pose serious security risks that can lead to data theft or unauthorized control over systems. This blog post covers all 25 vulnerabilities in detail and explains how to defend against them.

CWE TOP 25 - 2021

1. Out-of-bounds Write: This vulnerability occurs when a program writes data beyond the intended boundary of a buffer or memory structure. It can cause crashes or allow an attacker to execute arbitrary code.

2. Improper Neutralization of Input During Web Page Generation (Cross-Site Scripting): When user-supplied input is not properly sanitized before being included in a web page, attackers can inject malicious scripts. These scripts can steal user data or perform unauthorized actions in the victim's browser.

3. Out-of-bounds Read: This weakness arises when a program reads data beyond the intended boundary of a buffer or memory structure. It can cause crashes or expose sensitive information to an attacker.

4. Improper Input Validation: When user-supplied input is not properly validated before processing, an attacker can supply crafted input to crash the program or execute arbitrary code.

5. Improper Neutralization of Special Elements Used in an OS Command (OS Command Injection): If user input is not properly sanitized before being passed to an operating system command, an attacker can inject and execute arbitrary commands on the host system.

6. Improper Neutralization of Special Elements Used in an SQL Command (SQL Injection): When user input is incorporated into an SQL query without sufficient validation, an attacker can manipulate the query to access sensitive data or execute arbitrary code.

7. Use After Free: This vulnerability occurs when a program accesses memory that has already been freed. It can lead to crashes or allow an attacker to execute arbitrary code.

8. Improper Limitation of a Pathname to a Restricted Directory (Path Traversal): When a program fails to properly restrict file or directory access, an attacker can traverse the file system to reach sensitive data or execute arbitrary code.

9. Cross-Site Request Forgery (CSRF): This weakness occurs when a web application fails to verify that incoming requests originate from a trusted source. An attacker can trick an authenticated user into performing unintended actions.

10. Unrestricted Upload of File with Dangerous Type: When a web application allows users to upload files without restricting file types, an attacker can upload malicious code and trigger its execution.

11. Missing Authentication for Critical Function: If a critical function lacks proper authentication checks, an attacker can access sensitive data or operations without valid credentials.

12. Integer Overflow or Wraparound: When a program does not properly handle integer arithmetic, overflow conditions can occur, leading to crashes or arbitrary code execution.

13. Deserialization of Untrusted Data: When a program deserializes data from an untrusted source without validation, an attacker can exploit this to execute arbitrary code.

14. Improper Authentication: If a program's authentication mechanism is flawed, attackers can bypass it to gain unauthorized access to sensitive functions and data.

15. NULL Pointer Dereference: This vulnerability occurs when a program attempts to access memory through a null pointer. It can cause crashes or, in some cases, enable code execution.

16. Use of Hard-coded Credentials: When credentials are embedded directly in source code, attackers can extract them and gain unauthorized access to the system.

17. Improper Restriction of Operations Within the Bounds of a Memory Buffer: When memory operations are not properly constrained to the intended buffer boundaries, it can result in crashes or arbitrary code execution.

18. Missing Authorization: Without proper authorization checks, users can access functions and data they are not entitled to, potentially leading to data breaches or privilege escalation.

19. Incorrect Default Permissions: When a program sets insecure default permissions on files or directories, attackers can gain unauthorized access to sensitive resources.

20. Exposure of Sensitive Information to an Unauthorized Actor: When sensitive data is not adequately protected, unauthorized parties can access and exploit it.

21. Insufficiently Protected Credentials: If user credentials are stored or transmitted without sufficient protection, attackers can intercept them and gain unauthorized access.

22. Incorrect Permission Assignment for Critical Resource: When permissions for critical resources are misconfigured, attackers can gain unauthorized access to sensitive data or systems.

23. Improper Restriction of XML External Entity Reference: If a program does not properly restrict XML external entity references, an attacker can exploit this to read sensitive data or execute malicious code.

24. Server-Side Request Forgery (SSRF): This weakness allows an attacker to make the server send requests to internal or external systems, potentially exposing sensitive data or enabling further attacks.

25. Improper Neutralization of Special Elements Used in a Command (Command Injection): When user input is not properly sanitized before being incorporated into a command, an attacker can inject and execute malicious code on the system.

Conclusion

The CWE Top 25 from 2021 is an essential reference for the most common and dangerous software vulnerabilities. Compared to the 2020 edition, the 2021 list introduced new entries such as Command Injection, Server-Side Request Forgery (SSRF), and Incorrect Default Permissions. Several existing weaknesses were also re-ranked, including Cross-Site Scripting and OS Command Injection. For software developers and security professionals alike, the CWE Top 25 remains an invaluable resource for understanding the most critical risks and prioritizing effective countermeasures.