APT Source List Injection via Unvalidated Server Response

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,

Casky was already ahead

This CVE exploits attack patterns that Casky's 322matched skills already investigate — long before this vulnerability was disclosed. Claude's reasoning model maps these techniques to MITRE ATT&CK, so practitioners who ran these skills have already seen the threat behaviour in their findings.

Analysis

CVE-2026-11386 is a critical input validation vulnerability in Canonical's ubuntu-pro-client that allows remote code execution through malicious APT repository injection. The vulnerability stems from the client's unsafe use of Python's str.format() method when constructing APT source list files from untrusted contract server responses. Specifically, the directives.suites[] and directives.aptURL fields are written directly to system configuration files (/etc/apt/sources.list.d/) without sanitization, escaping, or newline filtering. This affects Ubuntu systems relying on pro-client for extended support services, and attackers controlling or compromising the contract server can inject arbitrary APT sources to distribute malicious packages at system update time—a supply chain attack vector with severe implications for enterprise infrastructure.

Casky's 322 mapped security skills leverage Claude's extended reasoning to detect the attack patterns behind this vulnerability across both initial access (TA0001) and exfiltration (TA0043) phases. Practitioners using Casky would identify findings highlighting unsafe deserialization of server responses, detection of unvalidated format string operations in configuration file writes, and patterns indicative of repository manipulation or package source tampering. The platform would surface anomalies in APT source list modifications coupled with suspicious contract server communications, allowing teams to spot exploitation attempts before malicious package installations occur. This enables proactive threat hunting and validation of input sanitization controls across supply chain trust boundaries.

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Casky Risk Score

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EPSS Probability

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Shodan Exposure

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GreyNoise Signal

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322 Casky skills cover this attack pattern

These skills use Claude AI's reasoning model to surface findings in the same attack categories as CVE-2026-11386.

Abuse of Public-Facing API: Mobile API Abuse

mobile security · low

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abusing-dpapi-for-credential-access

red teaming · high

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abusing-shadow-credentials-for-privesc

red teaming · high

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MITRE ATT&CK Techniques

TA0001 TA0043

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