CVE-2023-1786: Cloud-init Information Disclosure Flaw

CVE-2023-1786 Overview

CVE-2023-1786 is an Information Leakage vulnerability affecting Canonical's cloud-init, a widely-used tool for initializing cloud instances across major platforms. The vulnerability allows sensitive data, including hashed passwords, to be exposed in cloud-init log files. An attacker with local access could leverage this information disclosure to obtain credential hashes and potentially escalate their privileges within the affected system.

Critical Impact

Sensitive data including hashed passwords may be exposed in cloud-init logs, enabling attackers with local access to harvest credentials and potentially escalate privileges across cloud infrastructure.

Affected Products

• Canonical cloud-init (versions prior to 23.1.2)
• Canonical Ubuntu Linux (16.04 ESM, 18.04 ESM, 20.04 LTS, 22.04 LTS, 22.10, 23.04)
• Fedora 38

Discovery Timeline

• April 26, 2023 - CVE-2023-1786 published to NVD
• November 21, 2024 - Last updated in NVD database

Technical Details for CVE-2023-1786

Vulnerability Analysis

This vulnerability stems from improper handling of sensitive information within cloud-init's logging functionality (CWE-532: Insertion of Sensitive Information into Log File). Cloud-init processes user data and vendor data during instance initialization, which may contain sensitive configuration details including password hashes, authentication tokens, and other credentials.

Prior to version 23.1.2, cloud-init did not properly sanitize or restrict the logging of this sensitive data. When cloud-init processes data sources from various providers (including LXD and Vultr as shown in the patch), user and vendor data containing credentials would be written to log files with permissive read access.

The attack requires local access to the system, allowing an attacker to read the cloud-init log files (typically located at /var/log/cloud-init.log and /var/log/cloud-init-output.log) and extract hashed passwords or other sensitive configuration data. This information could then be used for offline password cracking or to further compromise the system.

Root Cause

The root cause lies in cloud-init's data source handlers not treating user data and vendor data as sensitive information. The logging mechanisms would output this data without redaction, and log files were created with overly permissive file permissions allowing unauthorized users to read their contents. The fix implemented in version 23.1.2 marks user/vendor data as sensitive and removes inappropriate log permissions.

Attack Vector

The attack vector is local, requiring the attacker to have some level of access to the affected system. The exploitation flow involves:

1. An attacker gains initial low-privilege access to a cloud instance running a vulnerable version of cloud-init
2. The attacker reads cloud-init log files which may contain sensitive initialization data
3. Hashed passwords and other credentials are extracted from the logs
4. The attacker performs offline cracking of password hashes or uses exposed credentials
5. Successful credential recovery enables privilege escalation or lateral movement

# Security patch in cloudinit/sources/DataSourceLXD.py
# Make user/vendor data sensitive and remove log permissions (#2144)
 import time
 from enum import Flag, auto
 from json.decoder import JSONDecodeError
-from typing import Any, Dict, List, Optional, Union, cast
+from typing import Any, Dict, List, Optional, Tuple, Union, cast
 
 import requests
 from requests.adapters import HTTPAdapter

Source: GitHub Cloud-Init Commit

# Security patch in cloudinit/sources/DataSourceVultr.py
# Make user/vendor data sensitive and remove log permissions (#2144)
 # Vultr Metadata API:
 # https://www.vultr.com/metadata/

+from typing import Tuple
+
 import cloudinit.sources.helpers.vultr as vultr
 from cloudinit import log as log
 from cloudinit import sources, stages, util, version

Source: GitHub Cloud-Init Commit

Detection Methods for CVE-2023-1786

Indicators of Compromise

• Unusual read access to /var/log/cloud-init.log or /var/log/cloud-init-output.log by non-root users
• Evidence of password hash extraction patterns in system audit logs
• Suspicious offline authentication attempts following cloud instance access
• Unauthorized access to cloud-init log directories from non-administrative accounts

Detection Strategies

• Monitor file access patterns to cloud-init log files using auditd rules targeting /var/log/cloud-init*
• Implement file integrity monitoring (FIM) to detect unauthorized access to sensitive log locations
• Review cloud-init log file permissions across fleet instances to identify vulnerable configurations
• Correlate local access events with subsequent privilege escalation attempts

Monitoring Recommendations

• Enable verbose audit logging for read operations on /var/log/cloud-init.log and related files
• Deploy SentinelOne agents configured to detect credential harvesting behaviors and log file enumeration
• Establish baseline access patterns for cloud-init logs and alert on deviations
• Integrate cloud instance telemetry with SIEM platforms to correlate local access with credential abuse

How to Mitigate CVE-2023-1786

Immediate Actions Required

• Upgrade cloud-init to version 23.1.2 or later across all affected systems
• Review and restrict permissions on existing cloud-init log files (chmod 600 /var/log/cloud-init*.log)
• Audit cloud instances for evidence of log file access by unauthorized users
• Rotate any credentials that may have been exposed in cloud-init logs prior to patching

Patch Information

Canonical has released cloud-init version 23.1.2 which addresses this vulnerability by marking user and vendor data as sensitive and implementing proper log file permission restrictions. The fix is available through the GitHub Cloud-Init Commit.

Ubuntu users should apply the updates detailed in Ubuntu Security Notice USN-6042-1. Fedora users should refer to the Fedora Package Announcement for patched packages.

For additional technical details, see the Launchpad Bug Report.

Workarounds

• Restrict read access to cloud-init log files by setting appropriate file permissions
• Configure cloud-init to minimize logged data where possible pending upgrade
• Implement host-based access controls to limit which users can access log directories
• Consider log rotation with secure deletion to minimize exposure window of sensitive data

# Restrict permissions on cloud-init log files
sudo chmod 600 /var/log/cloud-init.log
sudo chmod 600 /var/log/cloud-init-output.log

# Set restrictive permissions on log directory
sudo chmod 750 /var/log/cloud-init/

# Verify cloud-init version
cloud-init --version

# Update cloud-init on Ubuntu
sudo apt update && sudo apt install cloud-init

# Update cloud-init on Fedora
sudo dnf update cloud-init