The ubuntu‑pro‑client constructs APT source list files using data directly from a contract server response without sanitization. Because the Python str.format() call is used without validation or newline filtering, an attacker who can send a forged contract response containing newline characters can inject arbitrary configuration lines into root‑owned APT source files. The service also passes an unvalidated additionalPackages[] list into a root‑executed apt‑get install command. This combination permits an attacker to compel the client to fetch and install arbitrary packages, ultimately enabling full‑privilege code execution on the affected host.

Canonical Ubuntu releases 14.04 through 26.04 LTS and any system that installs the ubuntu‑pro‑client (ubuntu‑advantage‑tools) package are affected. The component ships preinstalled on supported Ubuntu server releases and is automatically enabled on cloud‑provider Ubuntu‑Pro images.

The vulnerability carries a CVSS score of 9, indicating high criticality. The EPSS score is 0.00342 (less than 1%), and the vulnerability is not listed in CISA’s KEV catalog. The likely attack vector requires an attacker to spoof or modify the contract response, which can be achieved through a compromised internal network component, interception of traffic presented with a trusted CA, or exploitation of a local logical defect that provides control over the contract data. Once such a response is delivered, the client writes malicious lines into APT sources and runs apt‑get install as root, delivering arbitrary code execution. Given the severity score and the ability to leverage trusted connections, the risk is significant even though publicly observed exploit data is not yet documented.