Node-Forge Denial of Service via modInverse(0)

The node-forge library, a widely used cryptographic tool in JavaScript environments, contains a denial-of-service vulnerability in its BigInteger implementation (jsbn.js). Specifically, the modInverse() function, which calculates the modular multiplicative inverse, enters an infinite loop when invoked with a zero value. This occurs because the Extended Euclidean Algorithm within modInverse() lacks input validation for zero values, leading to an unreachable exit condition. An attacker can exploit this vulnerability by crafting inputs that trigger the modInverse() function with a zero value, causing the Node.js process to hang and rendering the application unavailable. This issue affects all versions of node-forge, including the latest (v1.3.1 as of the report), and can impact applications that process untrusted cryptographic parameters. The vulnerability was reported in March 2026.

Attack Chain

1. Attacker identifies an application using node-forge and exposing cryptographic operations that involve modular arithmetic.
2. Attacker crafts a malicious input, specifically designed to result in a zero value being passed to the modInverse() function. This could be through DSA/ECDSA signature manipulation or custom RSA/Diffie-Hellman implementations.
3. The application receives the malicious input via a network request or other external data source.
4. The application processes the input, leading to a call to BigInteger.modInverse(m) in lib/jsbn.js with a zero value as the this argument.
5. The modInverse() function's Extended Euclidean Algorithm enters an infinite loop due to the missing zero-value check.
6. The Node.js event loop becomes blocked, preventing the application from processing further requests.
7. The application becomes unresponsive, resulting in a denial of service.
8. The server process consumes 100% CPU, exacerbating the DoS impact.

Impact

A successful attack can cause a complete Denial of Service (DoS). The Node.js process will hang indefinitely, blocking the event loop and rendering the application unresponsive to all subsequent requests. This vulnerability affects any application processing untrusted cryptographic parameters using node-forge. As node-forge has millions of weekly downloads on npm, this poses a significant risk to a large number of applications. The impact is primarily on availability, as the application becomes completely unusable until the process is manually restarted or a fix is deployed.

Recommendation

• Upgrade to node-forge version 1.4.0 or later, which includes a fix for this vulnerability (CVE-2026-33891).
• Implement input validation to prevent zero values from being passed to the modInverse() function in lib/jsbn.js. Specifically, add a check for this.signum() == 0 as described in the "Suggested Fix" section of this brief.
• Monitor CPU usage of Node.js processes for sustained high CPU utilization, which can be an indicator of this DoS attack. Deploy a Sigma rule based on process_creation and cpu_usage to detect abnormal CPU consumption by node processes.
• Implement timeouts for cryptographic operations that use modInverse() to limit the impact of a potential infinite loop.