CVE-2025-23016: Critical FastCGI Heap Overflow Threatens Embedded Devices, PoC Releases
Ddos 
Security researcher Baptiste Mayaud from Synacktiv has detailed a critical vulnerability in the FastCGI library, tracked as CVE-2025-23016 (CVSS 9.4). The flaw, which stems from improper handling of parameter lengths, can lead to an exploitable heap overflow, particularly affecting low-power devices such as cameras and embedded systems.
FastCGI is a C library designed to bridge web servers like Nginx or Apache with third-party web applications. It’s widely used in environments where lightweight, compiled applications are needed. While PHP-FPM reimplements FastCGI and is unaffected, many embedded technologies directly use the vulnerable library.
The vulnerability resides in the ReadParams() function. During the processing of incoming HTTP parameters, the FastCGI library incorrectly calculates the total size needed for memory allocation:
“Probably to store the ‘=’ character between the key and the value in addition to a null byte at the end of the string, a ‘+2’ is added to the final allocation calculation,” Baptiste Mayaud explains.
On 32-bit systems, this addition can cause an integer overflow, resulting in a small memory allocation despite very large intended data sizes. When data is then copied into the undersized buffer, a heap overflow occurs.
An attacker who can control HTTP request parameters could:
- Trigger a heap overflow.
- Corrupt memory structures.
- Potentially execute arbitrary code on affected devices.
This is particularly dangerous because embedded systems often lack modern exploit mitigations.
In a demonstration, Mayaud set up a vulnerable lighttpd server using FastCGI. By exploiting the bug, he successfully manipulated heap memory to overwrite function pointers within the FastCGI stream structure. Ultimately, he achieved arbitrary code execution by hijacking the fillBuffProc function pointer.
The exploitation strategy is based on obtaining a vulnerable pointer preceding the FCGX_Stream structure, then overriding its buffer to rewrite the structure, replacing fillBuffProc with system’s PLT entry. Mayaud published a proof-of-concept exploit code for this flaw.
Users are recommend to upgrade the FastCGI library to version 2.4.5 or later, where the bug has been fixed.
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