A critical vulnerability in the FastCGI library could allow attackers to execute arbitrary code on embedded devices.

The flaw, tracked as CVE-2025-23016 with a CVSS score of 9.3, affects all FastCGI fcgi2 (aka fcgi) versions 2.x through 2.4.4 and poses significant risks to devices using the lightweight web server development library.

FastCGI Integer Overflow Flaw

Baptiste Mayaud from Synacktiv discovered the vulnerability during an internal research project and disclosed it on April 23, 2025.

It stems from an integer overflow in the ReadParams function located in fcgiapp.c, which can lead to a heap-based buffer overflow when processing crafted nameLen or valueLen values sent to the IPC socket.

The issue occurs during memory allocation for HTTP parameters:

When both nameLen and valueLen equal 0x7fffffff, adding +2 causes an integer overflow on 32-bit systems, resulting in a smaller memory allocation than intended. This allows attackers to write beyond the allocated buffer boundaries, Synacktiv said to Cyber Security News.

The vulnerability exploits how FastCGI processes protocol parameters. The library reads parameter lengths from the incoming stream, but fails to validate the addition operation before allocation properly:

On 32-bit systems, while 0x7fffffff + 0x7fffffff + 1 = 0xffffffff, adding one more byte causes a wraparound: 0x7fffffff + 0x7fffffff + 2 = 0. This results in allocating a tiny buffer for what should be gigabytes of data.

Risk Factors	Details
Affected Products	FastCGI fcgi2 (aka fcgi) versions 2.x through 2.4.4, especially embedded devices such as cameras and IoT equipment running 32-bit systems.
Impact	Heap-based buffer overflow leading to arbitrary code execution
Exploit Prerequisites	– Local or network access to the FastCGI IPC socket – Ability to send crafted nameLen and valueLen parameters- 32-bit system architecture – Vulnerable FastCGI library version (≤ 2.4.4)
CVSS 3.1 Score	9.3 (Critical)

Impact and Affected Systems

This vulnerability predominantly affects embedded devices like cameras and IoT equipment running the FastCGI library on 32-bit architectures.

These systems often lack modern exploit mitigations such as ASLR or NX protections, making them particularly vulnerable.

It’s important to note that this vulnerability does not affect PHP-FPM, which reimplements the FastCGI protocol.

Researchers have successfully exploited heap memory to overwrite function pointers within the FCGX_Stream structure.

By targeting the fillBuffProc function pointer, attackers can hijack execution flow and execute arbitrary commands.

A successful exploitation requires:

Accessing the FastCGI socket (potentially via SSRF)
Sending crafted parameter lengths to trigger the integer overflow
Overwriting the FCGX_Stream structure to redirect execution

The proof-of-concept exploit released by Synacktiv demonstrates code execution by replacing the fillBuffProc pointer with the system function’s address and passing shell commands as parameters.

Patch Released

Security experts recommend the following actions:

Update to FastCGI library version 2.4.5 or later, which contains the fix for CVE-2025-23016
Configure web servers to use UNIX sockets instead of TCP sockets for FastCGI communication
Limit remote access to the FastCGI socket to prevent exploitation
Apply network segmentation to restrict access to potentially vulnerable embedded devices

This vulnerability highlights the security challenges in long-established libraries used in embedded systems.

The fix implemented in version 2.4.5 addresses the integer overflow by adding proper bounds checking before allocation.

Organizations using FastCGI in their infrastructure, particularly in embedded contexts, should prioritize patching affected systems immediately.

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