Buffer Overflow in Strapi with Hmac Signatures

A buffer overflow in Strapi when Hmac Signatures are used typically arises when input that includes signature-related data (e.g., a token or hash parameter) is copied into a fixed-size buffer without proper length checks. In C/C++ extensions or native addons that Strapi may rely on, if a developer reads a request parameter containing an Hmac Signature into a character array sized for an expected token length, an attacker providing an oversized signature can overflow the buffer. This can overwrite adjacent memory, including saved return addresses or function pointers, leading to arbitrary code execution or crashes.

For example, consider a native addon that validates an Hmac Signature by copying it into a 256-byte stack buffer. If the signature exceeds 256 bytes, a classic stack-based overflow occurs. Even in higher-level JavaScript logic within Strapi, unsafe concatenation or direct use of user-controlled signature values in operations that eventually interact with lower-level bindings can trigger out-of-bounds reads or writes if the host runtime or native bindings do not enforce strict size limits.

An attacker might send a long, crafted Hmac Signature in a query parameter or header to exploit this. If the overflow corrupts control data, it may bypass intended access controls or lead to remote code execution. The risk is compounded if the endpoint performing Hmac verification runs with elevated privileges or if the overflow allows instruction pointer redirection. Findings from middleBrick often highlight such input validation weaknesses under the Input Validation and Unsafe Consumption checks, especially when the signature is not bounded before processing.

Because Strapi plugins sometimes integrate native modules for performance or cryptographic operations, developers must ensure buffers are sized conservatively and signatures are validated using safe abstractions. middleBrick’s checks for BFLA/Privilege Escalation and Property Authorization can surface endpoints where signature handling intersects with authorization flaws, increasing the blast radius of a successful overflow.

To remediate buffer overflow risks when handling Hmac Signatures in Strapi, use fixed-size buffers with explicit length checks and prefer high-level cryptographic APIs that avoid manual memory management. Always validate the signature length before copying, and use functions that limit writes to the buffer size.

Example: Safe buffer handling in a native addon (C/C++)

#include <string.h>
#include <openssl/hmac.h>

#define MAX_SIGNATURE_LENGTH 256

int verify_hmac_signature(const char* input, const char* received_sig) {
    if (received_sig == NULL) {
        return 0;
    }
    size_t sig_len = strlen(received_sig);
    if (sig_len >= MAX_SIGNATURE_LENGTH) {
        return 0; // reject oversized signatures
    }
    char buffer[MAX_SIGNATURE_LENGTH] = {0};
    strncpy(buffer, received_sig, MAX_SIGNATURE_LENGTH - 1);
    // Perform HMAC verification using OpenSSL or similar
    // ...
    return 1;
}

Example: Strapi controller with safe signature validation (JavaScript)

const crypto = require('crypto');

module.exports = {\n  async verifyAction(ctx) {
    const { data, signature } = ctx.request.body;
    const expected = ctx.request.query.expected;
    const secret = process.env.HMAC_SECRET;

    if (!signature || typeof signature !== 'string') {
      ctx.throw(400, 'Invalid signature format');
    }
    if (signature.length > 512) {
      ctx.throw(400, 'Signature too long');
    }

    const hmac = crypto.createHmac('sha256', secret);
    hmac.update(data);
    const computed = hmac.digest('hex');

    // Use timing-safe compare to avoid side-channels
    const valid = crypto.timingSafeEqual(
      Buffer.from(computed),
      Buffer.from(signature)
    );

    if (!valid) {
      ctx.throw(401, 'Invalid Hmac Signature');
    }

    ctx.body = { result: expected };
  },
};

Example: Using middleBrick to detect risky signature handling

Run a scan with the CLI to surface related findings:

middlebrick scan https://your-strapi.example.com/api/endpoint

In the dashboard, review Input Validation and Property Authorization findings to confirm that signature handling is constrained and that there are no missing length checks.