How SSRF Can Compromise Internal Systems: Lessons from CVE-2021-40438
By:
Raul Fanti
On
09/01/2025Reading time:
8 min
Summary:
In this article, we explore Server-Side Request Forgery (SSRF), a vulnerability that allows attackers to trick servers into making unintended requests to internal resources. CVE-2021-40438 in Apache exploits improper URL validation, enabling attackers to redirect servers to external resources. To prevent SSRF, developers should implement strict input validation, network segmentation, whitelisting, and ensure regular software updates.
What is SSRF and Why Does it Matter?
Server-Side Request Forgery (SSRF) is a vulnerability where an attacker tricks a server into making unintended requests, often to internal resources. This happens when user input, such as a URL or IP address, is used by the server without proper validation.
SSRF is dangerous because it can allow attackers to access sensitive internal systems, like databases or cloud services, that are not exposed to the outside world. By exploiting this vulnerability, attackers can steal data, execute commands, or escalate their attacks.
In short, SSRF is a critical threat because it can bypass security measures and expose confidential resources, leading to potential data breaches and system compromise.
The Vulnerability: CVE-2021-40438
CVE-2021-40438 is a vulnerability in Apache HTTP Server versions 2.4.49 and 2.4.50. It allows Server-Side Request Forgery (SSRF), where an attacker can trick the server into making HTTP requests to internal resources, such as localhost or internal APIs, which should not be exposed externally. Apache HTTP Server fails to properly validate URLs in certain requests, such as redirects or proxy configurations. This flaw allows attackers to send specially crafted URLs to the server, forcing it to access internal services (like databases or admin interfaces) that are typically protected by firewalls or other security measures.
PoC
The sequence of "A"s (7701 characters) is used to manipulate the path construction process in Apache, exploiting a flaw in how Unix Domain Sockets (UDS) are handled. This sequence causes an error in path validation, setting the UDS path to NULL. As a result, the server bypasses internal socket handling and makes external requests (e.g., http://google.com/). This allows the attacker to redirect the server to an unintended external resource, exploiting the SSRF vulnerability in Apache HTTP Server.
In this case, I insert the address http://169.254.169.254. This is a link-local address used in Amazon Web Services (AWS) to provide metadata for instances running in the cloud, allowing me to access the AWS credentials for the instance running the application:
(These credentials are from a controlled environment and are not valid)
Server-Side Request Forgery (SSRF) can be exploited in various web applications, including image upload functionality, external URL fetching services, webhooks, proxy configuration settings, redirects, and URL handling. Invalid URLs can trigger SSRF and access internal services or sensitive metadata. External URLs may not filter internal URLs, allowing attackers to access internal resources. Webhooks can be manipulated to target internal systems or cloud metadata endpoints. Proxy configuration settings can also be exploited by injecting internal addresses into the server's configuration.
Lessons and Prevention
The CVE-2021-40438 vulnerability highlights the importance of validating user input and ensuring proper access controls. To prevent SSRF attacks, developers should implement strict input validation, allowing only trusted URLs or IP addresses, and avoid blindly trusting user-supplied data.
Other key prevention measures include:
Network Segmentation: Isolate sensitive internal systems from external access.
Whitelisting: Limit outgoing requests to trusted sources only.
Blacklisting: While not as secure as whitelisting, blacklisting can be used to block known malicious IP addresses or URLs (e.g., http://127.0.0.1, http://169.254.169.254, http://localhost). However, blacklisting alone is not foolproof and should be combined with other security measures.
Use of Security Tools: Employ web application firewalls (WAFs) and security scanners to detect and block SSRF attempts.
Regular Updates: Keep software up to date to patch known vulnerabilities.
By following these practices, developers can significantly reduce the risk of SSRF and protect sensitive resources.
In conclusion, SSRF vulnerabilities, like CVE-2021-40438, demonstrate the risks of insufficient input validation and the importance of securing internal systems. By understanding how SSRF works and taking proactive steps such as input validation, whitelisting, network segmentation, and regular software updates, developers can reduce the likelihood of successful attacks.
SSRF remains a significant security concern, and securing APIs and web applications should be a priority to protect sensitive data and prevent unauthorized access.