Denmark experienced its largest-ever cyberattack in May, targeting the nation’s critical infrastructure. SektorCERT, the Danish cybersecurity authority, reported breaches in 22 companies within days. Many resorted to ‘island mode,’ isolating themselves from the internet to contain the threat.

SektorCERT has released a full report on the attack — here are some key takeaways:

Vulnerability exploitation: The attackers exploited unpatched flaws in Zyxel firewalls, a common device among SektorCERT-protected organizations. Some breaches involved sophisticated zero-day vulnerabilities, suggesting well-resourced assailants. SektorCERT suspects involvement from multiple hacker groups, including Russia’s GRU-affiliated Sandworm.

Many organizations were caught off-guard, assuming their new firewalls had the latest software or that vendors were responsible for updates. Some avoided updates due to additional costs, while others were unaware of Zyxel devices in their network. This oversight gave attackers ample time, even after SektorCERT’s alerts.

Waves of Attack

Initial breach and reconnaissance: Starting May 11, 16 energy organizations were targeted, exploiting a specific vulnerability (CVE-2023-28771). Eleven were immediately compromised, possibly due to reconnaissance efforts, as attackers harvested firewall configurations and credentials.

Coordinated assault: The first attack wave displayed remarkable coordination, requiring significant planning and resources. A 10-day quiet period followed, after which a second wave commenced. This included using an already-compromised organization to launch DDoS attacks as part of the Mirai botnet, targeting entities in the US and Hong Kong.

Continuous Compromises and Responses

Escalating threats: Following the initial attacks, SektorCERT tirelessly countered further breaches through Zyxel firewalls. In one case, a compromised firewall led to the loss of remote location visibility, forcing manual interventions.

A final wave of attacks, linked to the Russian cyber unit Sandworm, disrupted but did not significantly impact Denmark’s critical infrastructure. SektorCERT applauded the swift and professional response of its team and the affected organizations.

SektorCERT has emphasized the need to address systemic vulnerabilities to protect Denmark’s critical infrastructure from persistent foreign cyber threats.