In April 2021, the Rubin Central Design Bureau of Marine Engineering, a key Russian defense entity based in Saint Petersburg, fell victim to a spear-phishing attack orchestrated by Chinese state-sponsored hackers. The attackers deployed a previously undetected backdoor malware, PortDoor, concealed within a malicious RTF file attachment. The primary objective of the campaign was to exfiltrate classified intelligence related to submarine designs developed by the bureau. Despite the sophisticated nature of the intrusion—leveraging tailored malware and likely advanced persistence techniques—no data breach, operational disruption, or tangible harm was confirmed. The attack was detected before the hackers could achieve their goal of stealing sensitive submarine-related documentation. While the incident underscored vulnerabilities in Russia’s defense industrial base, particularly against nation-state cyber espionage, the lack of actual data compromise or systemic impact limited its immediate consequences. The attack, however, highlighted the persistent threats posed by state-backed cyber groups targeting critical military and industrial secrets.