fpspy

A console script that instruments the Fingerprint agent on any page it runs on. Paste fpspy.js into the DevTools console. It works whether the agent has already loaded, is loading, or has not been injected yet.

Regenerate with npm run spy after editing tools/fpspy.src.js. The generator's one job is to inline the signal-id labels from reference/signals.json so a decoded payload is readable.

What it hooks#

fetch, XMLHttpRequest, sendBeacon, Blob and URL.createObjectURL, Worker, the collector-facing browser APIs, and the FingerprintJS global if one appears. The global's load() gets debug: true forced on, which turns on the agent's own request and response logger.

API#

__fpspy.dump()        // console report: request table, api events, workers, storage, probe counts
__fpspy.payloads()    // decoded request bodies, newest last
__fpspy.signals(p)    // flatten a payload's {s,v} pairs to id / label / status / value rows
__fpspy.probes()      // API call counts, most-called first
__fpspy.storage()     // local, session, cookie and indexedDB snapshot, framed values decoded
__fpspy.decode(x)     // decode a base64 string or byte array by hand
__fpspy.deepProps()   // also trace navigator and screen property reads
__fpspy.save()        // download the whole capture as JSON
__fpspy.stop()        // restore every original

The point of it is the request body. The agent frames every POST as [R] [tag[0]+R] [tag[1]+R] [padLen+R] [pad] [key] [payload ^ key], all bytes mod 256, tag 3,13 for a plain body and 3,14 for a deflate-raw compressed one. The script brute-forces the key length against a JSON-shaped result, so a change to the tag or key size does not break it. The same framing with a 7-byte key holds the <prefix>lr entry in localStorage, which __fpspy.storage() decodes as well.

Third-party traffic is filtered by host and path rather than by the whole URL, because analytics beacons carry Fingerprint in their query strings and a substring match logs all of them. Anything whose body unframes is captured regardless of host, which is what catches a customer proxying the endpoint under their own domain. __fpspy.opts.all = true logs everything.

Detectability#

Function.prototype.toString is patched to return the original source for every wrapper, so the hooks survive a source check. They are still detectable: a Function.prototype.toString that is not itself native is the obvious tell. deepProps() is worse, because it replaces property descriptors on navigator and screen, which is exactly what s118, s162 and s166 look at. The default is method wraps only.