ModHeader, a popular Chrome developer extension with over 800,000 users, was flagged and removed by Google for containing hidden spyware. Version 7.0.18 included a covert SDK disguised as a date library (dayjs) that harvested visited domain names, encrypted them using AES-GCM, and was configured to upload the data daily to api.stanfordstudies.com. Although the collection remained dormant due to an empty allowlist, the complete exfiltration infrastructure was present and operational. Additionally, the extension displayed active adware behavior, opening affiliate tabs on every update including on enterprise-managed machines. The malicious code shipped with official Chrome Web Store signatures, affecting both Chrome and Edge users. Forensic analysis revealed the extension locally stored 178MB of sensitive HTTP headers from all browsing activity, though no data was successfully exfiltrated from analyzed systems.