HAXcms: Private Key Disclosure via Broken HMAC Implementation

Summary

The hmacBase64() function in the HAXcms Node.js backend contains two critical cryptographic implementation errors that together allow any unauthenticated attacker to extract the system’s private signing key and forge arbitrary admin-level JSON Web Tokens (JWTs) allowing them to get full admin access with a single HTTP request.

Details

Bug 1: Hardcoded HMAC Key (line 2160): The function passes the literal string "0" as the HMAC signing key instead of the key parameter, making every HAXcms instance compute identical HMACs for the same input.

Bug 2: Private Key Appended to Output (lines 2161- 2163): After computing the HMAC, the function concatenates the real key parameter which is "this.privateKey + this.salt", the system’s master signing secret is directly onto the output. The combined buffer is base64-encoded and returned as the token.

Every base64url token produced has the same structure: 32 bytes HMAC keyed with "0" and N bytes of privateKey+salt. An attacker base64-decodes any token, discards the first 32 bytes, and reads the private key directly.

The /system/api/connectionSettings endpoint is unauthenticated and returns multiple tokens generated by this function. A single GET request to this endpoint exposes the private key.

The PHP backend (HAXCMS.php:1619-1631) implements this function correctly with the actual key and returns only the hash. The PHP version produces 44-character tokens whereas the broken Node.js version produces 139+ character tokens.

PoC

1. GET request to /system/api/connectionSettings endpoint and fetch the token.
2. Extract the private key from the fetched token. The hmacBase64() function produces 32 bytes with HMAC-SHA256 with hardcoded key "0" and the rest of the bytes are privateKey+salt (plaintext). Decode the Base64 token, discard the first 32 bytes, read the remaining bytes as UTF-8 (this is your extracted private key).
3. Since JWT's are signed with privateKey+salt, use this stolen private key to forge a JWT for admin using JWT.sign(payload, this.privateKey+this.salt). NOTE: the payload uses {id, user (set this as admin), iat (current timestamp), exp (expiration timestamp)}
4. The same key can also be used to create other tokens (user_token, base_token, form_token, etc).
5. Use these forged tokens to hit all authenticated endpoints (modify/delete/create etc) with admin privileges.

Impact

An unauthenticated attacker can perform the complete attack chain with a single HTTP request:

1. Extract private key: GET "/system/api/connectionSettings", base64-decode any token, discard first 32 bytes.
2. Forge admin JWT: sign arbitrary JWT payloads with the stolen privateKey+salt.
3. Forge all request tokens: compute valid user_token, site_token for any API call.
4. Full admin access: create/modify/delete sites, upload files, modify content.

This works even if the admin has changed the default credentials to a strong password. The forged tokens produce no login events in logs.