QNAP NAS HBS 3 - Information Disclosure (CVE-2021-28799)

CVE-2021-28799 is a critical improper authorization vulnerability in QNAP HBS 3, allowing remote attackers to log in to affected NAS devices without authentication. Exploitation of this flaw could lead to full compromise of the device, resulting in unauthorized access, data theft, or further attacks on the network. This vulnerability poses a severe risk, as it enables attackers to bypass security controls and potentially exfiltrate sensitive information or disrupt backup operations.

CrowdSec has been tracking this vulnerability and its exploits since 14th of January 2026.

Insights from the CrowdSec network reveal that the attackers trying to exploit CVE-2021-28799 are composed of a fairly even mix of opportunistic and targeted actors. Some attackers employ preliminary reconnaissance, while others use indiscriminate scanning. CrowdSec network telemetry also shows that exploitation of CVE-2021-28799 has significantly declined over the past week. Attack volumes are well below the long-term average, suggesting attackers are rapidly losing interest. The vulnerability appears to be falling out of active use across most threat landscapes.

Attackers exploit this vulnerability by sending crafted POST requests to the /cgi-bin/backup/hbs_mgnt.cgi endpoint, attempting to execute unauthorized commands and access sensitive files without proper authentication.