A significant security flaw has been identified in the FastCGI library, potentially allowing attackers to execute arbitrary code on embedded devices. This vulnerability, designated as CVE-2025-23016 with a CVSS score of 9.3, affects all FastCGI fcgi2 versions from 2.x through 2.4.4, posing substantial risks to devices utilizing this lightweight web server development library.
Understanding the FastCGI Integer Overflow Vulnerability
Discovered by Baptiste Mayaud from Synacktiv during an internal research project and disclosed on April 23, 2025, the vulnerability originates from an integer overflow in the `ReadParams` function within the `fcgiapp.c` file. This flaw can lead to a heap-based buffer overflow when processing specially crafted `nameLen` or `valueLen` values sent to the inter-process communication (IPC) socket.
The issue arises during the memory allocation for HTTP parameters. Specifically, when both `nameLen` and `valueLen` are set to 0x7fffffff, adding 2 to their sum causes an integer overflow on 32-bit systems. This results in a smaller memory allocation than intended, allowing attackers to write beyond the allocated buffer boundaries. Synacktiv explained that this overflow enables attackers to exploit the way FastCGI processes protocol parameters, leading to potential memory corruption and arbitrary code execution.
Technical Breakdown of the Vulnerability
The vulnerability exploits how FastCGI handles protocol parameters. The library reads parameter lengths from the incoming stream but fails to properly validate the addition operation before allocation. On 32-bit systems, while 0x7fffffff + 0x7fffffff + 1 equals 0xffffffff, adding one more byte causes a wraparound: 0x7fffffff + 0x7fffffff + 2 equals 0. This results in allocating a tiny buffer for what should be gigabytes of data, leading to a heap-based buffer overflow.
Impact and Affected Systems
This vulnerability predominantly affects embedded devices such as cameras and IoT equipment running the FastCGI library on 32-bit architectures. These systems often lack modern exploit mitigations like Address Space Layout Randomization (ASLR) or No Execute (NX) protections, making them particularly susceptible to attacks.
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.
Exploit Prerequisites and Methodology
A successful exploitation requires:
– Access to the FastCGI socket, potentially via Server-Side Request Forgery (SSRF).
– The ability to send 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.
Mitigation Strategies and Patch Information
To address this critical vulnerability, security experts recommend the following actions:
– Update the FastCGI Library: Upgrade to FastCGI library version 2.4.5 or later, which contains the fix for CVE-2025-23016.
– Configure Web Servers Appropriately: Use UNIX sockets instead of TCP sockets for FastCGI communication to reduce exposure.
– Restrict Access: Limit remote access to the FastCGI socket to prevent unauthorized exploitation.
– Implement Network Segmentation: Apply network segmentation to restrict access to potentially vulnerable embedded devices.
This vulnerability underscores the security challenges in long-established libraries used in embedded systems. Prompt action is essential to protect affected devices from potential exploitation.
