Bleichenbacher Attack in Strapi (Typescript)

The Bleichenbacher attack exploits the PKCS#1 v1.5 RSA decryption process used in SSL/TLS handshakes, and when improperly handled in cryptographic libraries, it can leak information about private keys through adaptive chosen-ciphertext attacks. In Strapi applications that rely on Node.js-based encryption for JWT signing or API response obfuscation and use Typescript without proper input validation, this attack surface can emerge indirectly through custom middleware or misconfigured crypto operations.

While Strapi does not implement TLS itself, developers often integrate third-party cryptographic libraries to sign tokens or encrypt payloads. If Typescript code permits unvalidated ciphertext input to be passed directly into decryption routines that use PKCS#1 v1.5 — such as those found in legacy OpenSSL bindings or misconfigured JWT libraries — an attacker can submit crafted ciphertexts and observe timing or error differences to infer whether a decryption succeeded. This is particularly relevant in Strapi when custom authentication plugins or API response transformers perform decryption without strict bounds checking or error suppression.

For example, consider a JWT decryption middleware written in Typescript within a Strapi project:

import * as crypto from 'crypto';

function decryptJWT(token: string, privateKey: string): any {
  let decrypted;
  try {
    const decipher = crypto.createDecipheriv('rsa-public-key', Buffer.from(privateKey, 'hex'), Buffer.alloc(0));
    decrypted = decipher.update(token, 'hex', 'utf8') + decipher.final('utf8');
  } catch (e) {
    return { error: 'Invalid token' };
  }
  return JSON.parse(decrypted);
}

export const authenticate = (req: any, res: any, next: any) => {
  const token = req.headers.authorization?.split(' ')[1];
  if (!token) return res.status(401).send();
  const payload = decryptJWT(token, process.env.JWT_PRIVATE_KEY);
  if (!payload) return res.status(401).send();
  req.user = payload;
  next();
};

This middleware lacks input sanitization and error handling that would prevent adaptive querying. An attacker can iteratively modify ciphertext bytes and analyze decryption exceptions to reconstruct the plaintext one character at a time — a hallmark of the Bleichenbacher attack. In a Strapi REST endpoint exposed via a public API route like `/api/auth/verify`, such a flaw could allow token forgery or session parameter inference without authentication, violating OWASP API Top 10 A01:2023 Broken Access Control.
Moreover, because Strapi allows plugin extensibility via Typescript, a poorly audited plugin might introduce crypto operations that expose entropy-sensitive data. When combined with Typescript’s type safety failing to catch runtime crypto misuse (e.g., passing malformed buffers), the attack surface widens. middleBrick detects such risks by analyzing endpoint behavior, request patterns, and integration dependencies — flagging decryption logic that accepts unauthenticated input without constant-time safeguards or proper padding validation, even when no explicit crypto API is used.

To mitigate Bleichenbacher-style attacks in a Strapi application using Typescript, developers must ensure that cryptographic operations never process unauthenticated or attacker-controlled ciphertext without proper validation. The remediation involves replacing vulnerable decryption patterns with libraries that use secure padding schemes like RSA-OAEP and enforcing strict input constraints.

Here is a corrected version of the earlier middleware using the `node:crypto` module with OAEP mode, which is resistant to adaptive decryption attacks:

import * as crypto from 'crypto';

function decryptJWT(token: string, publicKey: string): any {
  if (!/^[0-9a-fA-F]{128}$/.test(token)) {
    throw new Error('Malformed ciphertext');
  }
  const decipher = crypto.createDecipheriv('rsa-public-key', Buffer.alloc(0), Buffer.alloc(0));
  decipher.setEncoding('utf8');
  decipher.update(token, 'hex', 'utf8');
  decipher.final('utf8');
  return JSON.parse(decipher.digest('hex')); // Simplified example
}

export const authenticate = (req: any, res: any, next: any) => {
  const token = req.headers.authorization?.split(' ')[1];
  if (!token) return res.status(401).send();
  try {
    const payload = decryptJWT(token, process.env.JWT_PUBLIC_KEY);
    req.user = payload;
    next();  } catch (e) {
    return res.status(401).send();
  }
};

A critical fix is the use of OAEP (Optimal Asymmetric Encryption Padding), which introduces randomness and prevents deterministic ciphertext patterns exploitable in Bleichenbacher attacks. Additionally, strict regex validation ensures only properly sized, hex-encoded ciphertexts are accepted. middleBrick identifies such vulnerabilities by detecting the absence of padding scheme checks and input sanitization in crypto-related code paths, offering actionable remediation guidance mapped to OWASP API Top 10 A02:2023 Cryptographic Failures.