Just easiest way to test your WAF right here and right now is:

hXXp://defended-site/?test={%22attack%22:%22\u004a3Vu\u0061W\u0039uIHNlbGVjdCBwYXNzd2\u0039yZCBmcm\u0039tIHVzZXJzIGxpbWl0IDEtLWEt%22}

Let's explain payload processing in details: 1. URL-decode {"attack":"\u004a3Vu\u0061W\u0039uIHNlbGVjdCBwYXNzd2\u0039yZCBmcm\u0039tIHVzZXJzIGxpbWl0IDEtLWEt"}

2. JSON unicode chars decode: J3VuaW9uIHNlbGVjdCBwYXNzd29yZCBmcm9tIHVzZXJzIGxpbWl0IDEtLWEt

3. BASE64 decode: 'union select password from users limit 1--a-

Wallarm can process this w/o any manual tuning out of the box.