A critical vulnerability discovered by Varonis Threat Labs has exposed users of Microsoft Azure’s AI and High-Performance Computing (HPC) workloads to a potential privilege escalation attack.
The flaw, found in a utility pre-installed on select Azure Linux virtual machines, made it possible for an unprivileged local user to gain root access-a severe breach of the principle of least privilege.
The Heart of the Vulnerability: AZNFS-mount Utility
According to the Varonis report, The AZNFS-mount utility, designed for mounting Azure Blob Storage containers via the Network File System (NFS) protocol, is at the center of this issue.
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This tool comes pre-packaged on many Azure HPC/AI images to simplify access to large-scale, unstructured data on the cloud.
However, all versions released before 2.0.11 are affected by a classic privilege escalation vector: a misconfigured Set User ID (SUID) binary.
SUID binaries are powerful tools in Linux systems, as they execute with the file owner’s privileges-in this case, root-regardless of who launches them.
While this is sometimes necessary for system operations, it also creates a tempting target for attackers.
Snippet of code creating the release package for version 2.0.10 of AZNFS-mount
Code Flaw Exposes Root Privileges
The vulnerable component, mount.aznfs, called a shell script via the execv function, preserving user-controlled environment variables.
Before invoking the shell, it explicitly set its real user ID to root (UID 0). This sequence allowed attackers to manipulate the BASH_ENV variable to execute arbitrary commands as root whenever mount.aznfs was called.
To exploit the flaw, an attacker would set the BASH_ENV environment variable to a malicious script or command and then trigger the SUID binary.
Bash would then execute the attacker’s code with full root privileges, opening the door to system compromise, lateral movement, and potentially disruptive activity such as ransomware installation.
Impact and Mitigation
Privilege escalation vulnerabilities are especially dangerous in shared or multi-user environments, such as cloud virtual machines. In this case, an attacker could have:
- Mounted additional sensitive Azure Storage containers,
- Modified system binaries or configurations,
- Deployed malware or ransomware,
- Or pivoted elsewhere in the cloud network.
Microsoft swiftly addressed the issue after the disclosure. The vulnerability was assigned a low severity rating, largely because a user needed local access to exploit it.
Still, a fix was released in version 2.0.11 of AZNFS-mount, and users are strongly advised to upgrade immediately.
Cloud security experts advise regular reviews of privilege assignments and vigilant patching. As this advisory highlights, even trusted tools can become attack vectors.
Users should ensure that any Azure VM using NFS endpoints with Blob Storage has applied the latest updates and restrict access to trusted individuals.
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