Hackers Exploit Critical Xen Orchestra Auth Bypass Flaw to Gain Admin Access

Vulnerability / Cyber Attack French software company Vates VMS has warned of a critical security vulnerability in Xen Orchestra that it said is being actively exploited in the wild. The authentication bypass flaw, which does not yet have a CVE identifier, is tracked as VSA-2026-062. "REST cookie parsing turns authenticationToken=j:%7B%7D into {}," Vates VMS said in an alert. "REST authentication passes that object to the XO token provider without requiring a string." "The token collection treats {} as an unfiltered lookup and returns its first token; the provider authenticates as that token's owner if it is unexpired. The authenticated /rest/v0/users/me/authentication_tokens route then returns reusable login token IDs." In other words, the vulnerability can allow an attacker to bypass authentication by sending a specially crafted cookie and perform arbitrary actions with "Admin" privileges, ultimately resulting in arbitrary code execution on managed pools and a full XCP-ng node takeover as root. Vates VMS said it's seeing the vulnerability being actively exploited by a threat actor from the following IP address and TOR domain - 91.211.251[.]112:8001, which has been flagged as a RedLine Stealer command-and-control (C2) server on VirusTotal ustar3rqxgnskzgnwkgpofgtvxevm6cx3lxdbwt4mndtveqyaohid[.]onion:3333 It's unclear when the activity started and if the vulnerability was exploited as a zero-day. Other indicators of compromise are listed below - HTTP requests containing authenticationToken=j:%7B%7D or authenticationToken=j:{} A running virtual machine named pivot2 Virtual machines configured using "hvm_serial: file" parameter Suspicious files named /etc/cron.d/zz-ustar2, /etc/cron.d/zz-ustar, /.ustar5, and /.ustar6 The issue has been addressed in Xen Orchestra 6.9.1 and 6.10.0 for the stable and latest channels, as well as xo-server versions 5.211.2 and 5.213.1. If immediate patching is not an option, the following mitigations are advised - Avoid exposing Xen Orchestra instances on the internet Block the IP address and domain associated with malicious activity Block the empty token in a web application firewall (WAF) or a reverse proxy Rotate all passwords and secrets in Vates VMS if indicators of compromise are detected Filip Magic (@mad31k) and Christoffer Olsen from TDC NET Security have been credited with discovering and reporting the flaw. Found this article interesting? Follow us on Google News, Twitter and LinkedIn to read more exclusive content we post.