CVE-2021-40438: Insight into Apache’s mod_proxy SSRF Flaw

The world of cyber security is constantly evolving, with new vulnerabilities being discovered and exploited.

One such recent vulnerability is the CVE-2021-40438, associated with the Apache HTTP Server’s mod_proxy module.

This issue is a Server-Side Request Forgery (SSRF) vulnerability, which can impact the security of web servers globally if not addressed promptly.

What is CVE-2021-40438?

Common Vulnerabilities and Exposures, or CVEs, are a catalog of known security threats. Each threat is assigned a unique identifier, in this case, CVE-2021-40438. This specific vulnerability lies within the mod_proxy module of the Apache HTTP Server.

The mod_proxy module is designed to provide proxy capabilities for the server, essentially acting as an intermediary for requests from clients seeking resources from other servers.

However, due to a flaw in this module, it can be exploited to perform unauthorized actions, leading to potential data breaches.

A flaw in this module has been identified, which can be exploited by unauthenticated attackers to interact with internal network resources typically inaccessible externally.

This flaw allows attackers to send arbitrary values in the request URI-path, effectively turning the server into a gateway for unauthorized network requests.

This opens the door for Server-Side Request Forgery (SSRF) attacks, where an attacker can manipulate the server to make network requests to arbitrary domains, leading to potential data exposure and service disruptions.

Understanding SSRF

Server-Side Request Forgery (SSRF) is a type of vulnerability where an attacker can manipulate the server into making network requests to arbitrary domains. In essence, it tricks the server into acting as a proxy. This could lead to sensitive data exposure, service disruptions, or even remote code execution in some cases.

In the context of CVE-2021-40438, the SSRF vulnerability is present in the mod_proxy module, allowing attackers to make unauthorized network requests.

Proof-of-Concept (PoC)

One of the potential malicious uses for this vulnerability is to spoof the original IPs in Denial-of-Service (DoS) attacks. Attackers could potentially use this flaw to mask their IP addresses, making it more challenging to trace the source of the attack.

It’s crucial to note that servers in Russia, particularly those associated with Rostelecom, were found to be vulnerable by a GitHub user to this CVE when queried using the platform.

An example of a request exploiting this vulnerability could look something like this: curl "|".

Here, is the vulnerable server used to forward the request, and is the victim server receiving the spoofed request.

To demonstrate the Proof of Concept (POC) for this vulnerability, you would first need to build and run a server using the command go build src/main.go && ./src/main.go.

Following this, you should update the test/main.go file with the appropriate details.

The final step would be to run the test against potential targets using the command go run test/main.go.

The Impact of CVE-2021-40438

This vulnerability primarily affects Apache HTTP Server versions 2.4.48 and earlier.

If successfully exploited, an attacker could use the server as a proxy to carry out nefarious activities, potentially leading to unauthorized access to sensitive information, disruption of services, or even server takeover in severe cases.

Given the widespread use of the Apache HTTP Server, the potential impact of this vulnerability on global web security cannot be underestimated.

Mitigating the Threat

The Apache Software Foundation has released a security patch to address this issue in the form of Apache HTTP Server version 2.4.49. Server administrators are strongly advised to upgrade to this version or later to protect against this vulnerability.

Moreover, it’s crucial to regularly monitor server logs for any suspicious activity and employ intrusion detection systems to catch any potential attacks early on.


The discovery of CVE-2021-40438 serves as a reminder of the ever-present nature of cyber threats.

It underscores the importance of regular updates, vigilant monitoring, and proactive security measures in maintaining the integrity of servers and protecting sensitive data.

As we continue to rely more heavily on digital infrastructures, prioritizing cybersecurity becomes not just an option, but a necessity.

Remember, the digital world can be a dangerous place, but with knowledge and vigilance, we can keep our servers safe from threats like CVE-2021-40438.

As we continue to depend more on digital infrastructures, prioritizing cybersecurity is not an option but a necessity.