A critical vulnerability, CVE-2025-23016, has been discovered in FastCGI fcgi2 (aka fcgi) versions 2.x through 2.4.4. This post aims to provide an in-depth analysis of this vulnerability, its associated risks, and possible exploit scenarios. We will be discussing the integer overflow and resultant heap-based buffer overflow that affects the application when it processes maliciously crafted nameLen or valueLen values in data sent to the IPC (inter-process communication) socket. The vulnerability is found in the ReadParams function in the fcgiapp.c component of the FastCGI application.

Before we dive into the details, let's briefly discuss what FastCGI is and why it's essential. FastCGI is an open-standard protocol that enables web servers to efficiently interact with external applications. It is widely used in web development as a means to speed up the processing and handling of dynamic content on websites. With FastCGI, applications run independently from the web server, improving security and scalability.

Exploit Details
The vulnerability exists in the ReadParams function of fcgiapp.c when it processes nameLen and valueLen values from data sent to the IPC socket. Due to an integer overflow, the application fails to validate the size of these values properly, which can lead to a heap-based buffer overflow. To understand this better, let's analyze the code snippet in question:

static int ReadParams(FCGX_Stream *stream, unsigned char *pTo, int toSize) {
    int nameLen, valueLen;
    int bytesMoved = ;

    for (;;) {
        unsigned char c;
        if(FCGX_GetChar(&c, stream) < )
            return -1;
        if (c == )
            break;

        nameLen = (c >> 7) ? ((c & x7f) << 24) : (c);
        valueLen = FCGX_GetChar(&c, stream);

        if (nameLen + valueLen > toSize - bytesMoved) {
            // Error: insufficient space in the destination buffer.
            return -1;
        }

        memcpy(pTo + bytesMoved, &(nameLen), sizeof(nameLen));
        bytesMoved += nameLen;
        memcpy(pTo + bytesMoved, &(valueLen), sizeof(valueLen));
        bytesMoved += valueLen;
    }
    return bytesMoved;
}

In the above snippet, ReadParams reads a series of data consisting of nameLen and valueLen from the input stream into the buffer pTo. The vulnerability arises when there is an integer overflow in the computation of nameLen + valueLen, causing the application to incorrectly check for sufficient space in the destination buffer. This results in a heap-based buffer overflow, potentially leading to memory corruption and the execution of arbitrary code.

A successful exploit of this vulnerability could enable attackers to remotely execute arbitrary code in the context of the affected application, leading to a complete compromise of the system. Additionally, this vulnerability can be exploited to cause a denial of service (DoS) by crashing the affected process.

Original References

1. Vulnerability Details: NVD - CVE-2025-23016
2. FastCGI Official Website: www.fastcgi.com

Mitigation

As of now, there is no official patch or update provided by the vendor to address this vulnerability. Organizations using FastCGI fcgi2 versions 2.x through 2.4.4 are advised to closely monitor the development of this issue and apply any future official updates as soon as they become available.

Implement strict input validation for nameLen and valueLen values.

2. Enable relevant security features in the operating system to prevent the execution of arbitrary code (e.g., ASLR, DEP).
3. Isolate the affected application in a restricted environment to minimize the potential impact of a successful exploit.

Conclusion

CVE-2025-23016 is a critical vulnerability affecting FastCGI fcgi2 versions 2.x through 2.4.4, specifically in the ReadParams function of fcgiapp.c. A successful exploit of this vulnerability could lead to arbitrary code execution or denial of service, depending on the attacker's intentions. It is essential for organizations and developers to be aware of this issue and stay updated on any mitigations or patches released by the vendor.

Timeline

Published on: 01/10/2025 12:15:25 UTC