CVE-2026-11386 in ubuntu-pro-client
Summary
by MITRE • 07/16/2026
An input validation and injection vulnerability exists in Canonical ubuntu-pro-client (formerly ubuntu-advantage-tools). The client constructs APT source files (such as /etc/apt/sources.list.d/ubuntu-.list or their DEB822 equivalents) using data received directly from the contract server response via the directives.suites[] and directives.aptURL fields. Because the client utilizes Python's str.format() to write these files without performing escaping, validation, or newline character filtering, a malicious or tampered contract response containing embedded newline (\n) characters can successfully inject arbitrary, attacker-controlled deb configuration lines into root-owned APT sources. When combined with the unvalidated additionalPackages[] field—which is passed positionally into a root-executed apt-get install command—an attacker capable of spoofing or manipulating the contract response (e.g., via a compromised internal infrastructure, an intercepted connection utilizing a trusted CA, or local logical bugs) can force the client to fetch and install malicious packages. This ultimately leads to arbitrary code execution with root privileges on the affected system. This component is preinstalled on supported Ubuntu Server releases and auto-attaches by default on cloud provider Ubuntu Pro images.
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Analysis
by VulDB Data Team • 07/16/2026
This vulnerability represents a critical input validation and injection flaw in Canonical's ubuntu-pro-client software that affects Ubuntu Server deployments across cloud environments. The vulnerability stems from improper handling of data received from the contract server, specifically within the directives.suites[] and directives.aptURL fields that are directly used to construct APT source files without adequate sanitization or escaping mechanisms. The client utilizes Python's str.format() method to write these configuration files to system locations such as /etc/apt/sources.list.d/ubuntu-.list or their DEB822 equivalents, creating a path for malicious input to be interpreted as legitimate system configuration rather than user-supplied data. This design flaw creates an environment where attacker-controlled input can be seamlessly integrated into the system's package management infrastructure.
The technical exploitation occurs through manipulation of newline characters within the contract response data, specifically embedding \n sequences that cause the str.format() method to inject additional deb configuration lines into the generated APT source files. These injected lines are executed with root privileges since they modify system-level package repositories that are subsequently processed by root-executed apt-get commands. The vulnerability is particularly dangerous because it operates at the system configuration level, allowing attackers to manipulate package sources without requiring direct user interaction or elevated privileges beyond what's already present in the compromised infrastructure. This type of vulnerability aligns with CWE-74 (Improper Neutralization of Special Elements in Output Used by a Downstream Component) and CWE-94 (Improper Control of Generation of Code) as it involves both injection of executable code through configuration files and improper handling of untrusted input that leads to privilege escalation.
The operational impact extends beyond simple package installation to full system compromise, as the additionalPackages[] field within the contract response is passed positionally into apt-get install commands executed with root privileges. This creates a complete attack chain where an attacker can not only modify package sources but also directly control which malicious packages are installed on the system. The vulnerability affects all supported Ubuntu Server releases and is automatically enabled on cloud provider Ubuntu Pro images, meaning that systems can be compromised without any specific user action or awareness. Attackers capable of spoofing contract responses through various means including compromised internal infrastructure, man-in-the-middle attacks using trusted certificate authorities, or logical bugs in the system can exploit this vulnerability to achieve arbitrary code execution with root privileges.
The implications of this vulnerability are particularly severe in cloud environments where Ubuntu Pro images are commonly deployed and where network security controls may be insufficient to prevent contract response manipulation. The default auto-attachment behavior means that systems are vulnerable immediately upon deployment without any additional configuration or activation steps. Mitigation strategies must address both the input validation issues within the ubuntu-pro-client software and the broader infrastructure security considerations including network segmentation, certificate pinning, and monitoring for unauthorized contract response modifications. Organizations should implement strict validation of all data received from contract servers, employ proper input sanitization techniques, and consider alternative deployment models that reduce exposure to this class of vulnerability. The ATT&CK framework classification includes T1059 (Command and Scripting Interpreter) and T1068 (Exploitation for Privilege Escalation) as the attack executes commands with elevated privileges and leverages system configuration manipulation to achieve its objectives.