Forge (also called node-forge) is a native implementation of Transport Layer Security in JavaScript. Prior to 1.4.1, node-forge did not validate the AES-GCM tagLength option passed to start() during decryption through forge.cipher.createDecipher(). When an application derived tagLength from received data or used an unset or zero-like configuration value, a value that converted to zero together with an empty or missing authentication tag caused the calculated and supplied tags to compare as empty, so decipher.finish() accepted unauthenticated ciphertext. Applications that omit tagLength to use the 128-bit default or pass a fixed valid tag length are unaffected. This issue is fixed in version 1.4.1.

Affected: npm: node-forge

### Summary

The AES-GCM mode in forge.cipher does not validate the tagLength option passed to start(). With a tagLength of 0 or a value that converts to 0 (such as null, false, '' or '0'), the authentication tag is empty, and decipher.finish() returns true for any ciphertext and additional data when the supplied tag is empty or missing. Fixed in node-forge 1.4.1.

### Details

`modes.gcm.prototype.start()` in `lib/cipherModes.js` stored the `tagLength` option without validation, and `lib/cipher.js` documented it as accepting "0-128". After processing the data, `afterFinish()` truncates the 16-byte tag with `truncate(16 % (tagLength / 8))`. For a `tagLength` that converts to 0, this is `16 % 0 = NaN`, and truncating by `NaN` empties the tag. On decryption, the start-time check that the supplied `tag` has `tagLength / 8` bytes passes for an empty tag, and the final tag comparison compares two empty strings, so `decipher.finish()` returns `true` for any ciphertext and additional data.

NIST SP 800-38D section 5.2.1.2 allows only tag lengths of 128, 120, 112, 104 and 96 bits, plus 64 and 32 bits for certain applications.

The fix validates `tagLength` in `start()`, for both encryption and decryption. A missing, `null` or `undefined` `tagLength` uses the default of 128 bits. Any other value is converted with `Number()` and must be one of 128, 120, 112, 104, 96, 64 or 32; otherwise `start()` throws "Invalid tag length.". Numeric strings such as `'128'` are still accepted. With a…

Affected: Ubuntu:20.04:LTS: node-node-forge, Ubuntu:22.04:LTS: node-node-forge

Forge (also called node-forge) is a native implementation of Transport Layer Security in JavaScript. Prior to 1.4.1, node-forge did not validate the AES-GCM tagLength option passed to start() during decryption through forge.cipher.createDecipher(). When an application derived tagLength from received data or used an unset or zero-like configuration value, a value that converted to zero together with an empty or missing authentication tag caused the calculated and supplied tags to compare as empty, so decipher.finish() accepted unauthenticated ciphertext. Applications that omit tagLength to use the 128-bit default or pass a fixed valid tag length are unaffected. This issue is fixed in version 1.4.1.