I did a detailed privacy check of the Tiktok app and website. You can read my article æt Süddeutsche Zeitung. Tiktok commits multiple breaches of law, trust, transparency and data protection. Here are the technical and legal details
Long thread⤵️

This is my setup: I used mitmproxy to route all app traffic for analysis. See in this video how device information, usage time and watched videos are sent to Appsflyer and Facebook.

Hard to believe that this is covered by "legitimate interest" and transparency: Entered search terms are sent to Facebook.

Transfers to both companies break different rules of the GDPR: Facebook can't fulfill Art. 14 (information, deletion etc.) on this data.

Transfer to Appsflyer lacks transparency as it's unknown to which of the 4500+ Appsflyer partners the data will be transferred afterwards. Bytedance: "We don't show the contracts." Did they even read Art. 26 GDPR?

Most important: Fundamental rights are violated because PII is transfered to a company in an unsecure noneuropean country. The server location doesn't count, it is about where the company deciding about the data resides, says @malteengeler. Tiktoks Headquarter: Beijing 🇨🇳

I also checked the website which is important as all shared videos (via messenger or social media) are viewed there. The short URL e.g. vm[dot]tiktok[dot]com/9uTpDV will be resolved to a URL which contains the installation ID. Tiktok will be able to check who shared which video.

But they also track who is watching the video. Among common trackers (Google Analytics) they use the highly controversial method of device fingerprinting to set a mostly unique hash to the cookie s_v_webid. This is done by combining typical hardware and browser characteristics.

One of them: Canvas Fingerprinting. They draw an image in the background using vector graphic commands. Afterwards they save the image to a rasterized PNG. This data is quite unique among different devices depending on settings and hardware.

They also use audio fingerprinting to identify visitors. This doesn't mean they actually use your microphone or speaker. Instead they generate a sound internally and record the bitstream, which also differs from device to device. This is what it sounds like.

Bytedance told me that they use this fingerprinting to identify malicous browser behaviour. I don't believe, because the website still works if the script is blocked. Also they use Akamai's fingerprinting technology already on the server (which is another story to investigate).

The same fingerprinting script and cookie is used on Bytedance's news site Toutiao. What I found out for sure: If someone shares a video, Bytedance can
a.) tie the recipients of the video to the sender
b.) track recipients subsequently on Tiktok and Toutiao.