Critical FastCGI Bug Exposes Embedded Systems to Remote Attacks

CVE-2025-23016 is a critical vulnerability discovered in early 2025 within the FastCGI library (fcgi2), a C library widely used to connect web servers like NGINX or lighttpd to backend applications-especially on embedded and low-power devices such as cameras and IoT endpoints.

This flaw, present in versions up to 2.4.4, can lead to remote code execution via a heap-based buffer overflow triggered by an integer overflow in parameter parsing.

Technical Root Cause: Integer Overflow in ReadParams

The vulnerability resides in the ReadParams function of the FastCGI library.

When processing incoming HTTP parameters, the function reads the lengths of key-value pairs, which can be specified as either 8-bit or 32-bit integers depending on their magnitude.

The problematic code is:

cnameValue = (char *)Malloc(nameLen + valueLen + 2);

If an attacker supplies large values for nameLen and valueLen (e.g., both set to 0x7FFFFFFF), the sum plus 2 causes an integer overflow on 32-bit systems, wrapping the result to a small value (such as 0).

This leads Malloc to allocating a much smaller buffer than required.

When the library then copies the supplied data into this undersized buffer, a heap overflow occurs, allowing the attacker to overwrite adjacent memory structures.

Why 32-bit Systems Are at Risk

On 64-bit systems, the size calculations do not overflow due to a larger addressable space, but many embedded devices still use 32-bit architectures, making them especially vulnerable.

Exploitation and Real-World Impact

The exploit leverages the overflow to overwrite function pointers within the FastCGI internal stream structure (FCGX_Stream).

By carefully crafting requests, an attacker can:

• Overwrite the fillBuffProc function pointer with the address of the system function.
• Place a shell command (e.g., /bin/sh) at the start of the structure.
• Trigger a call to fillBuffProc, resulting in arbitrary command execution.

Example Exploit Flow:

1. Send multiple FastCGI parameter requests to manipulate heap allocations, ensuring a vulnerable pointer is positioned before the FCGX_Stream structure.
2. Exploit the integer overflow to allocate a tiny buffer and overflow into the stream structure.
3. Overwrite fillBuffProc with the address of system and insert a shell command.
4. Trigger the overwritten function pointer to achieve code execution.

Example Python Exploit Snippet:

pythonio.send(makeHeader(1, 1, 8, 0) + makeBeginReqBody(1, 0) + header +
        (p8(0x13) + p8(0x13) + b"b" * 0x26) * 9 +
        p8(0) * (2 * 2) + p32(0xffffffff) + p32(0xffffffff) +
        b"a" * (4 * 4) + b" /bi;nc -lve /bin/sh" +
        p32(0) * 3 + p32(exe.plt["system"]))

This sequence manipulates heap allocations and overwrites the function pointer to launch a reverse shell.

Mitigation and Recommendations

• Upgrade FastCGI: Update to version 2.4.5 or later, which contains the necessary patch.
• Restrict Socket Exposure: Use UNIX sockets instead of TCP for FastCGI communication and avoid exposing FastCGI ports directly to the network.
• Review Server Configuration: Ensure example or default configurations do not expose FastCGI sockets to untrusted networks.

CVE-2025-23016 highlights the persistent danger of subtle memory management bugs in foundational open-source infrastructure, especially as they impact embedded and IoT devices with limited security mitigations.

Prompt patching and secure deployment practices are essential to mitigate such high-impact vulnerabilities.

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