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Potential curl CVE-2023-38545 Exploitation

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Detects potential exploitation of curl CVE-2023-38545 by monitoring for vulnerable command line arguments in conjunction with an unusual command line length. A flaw in curl version ⇐ 8.3 makes curl vulnerable to a heap based buffer overflow during the SOCKS5 proxy handshake. Upgrade to curl version >= 8.4 to patch this vulnerability. This exploit can be executed with and without the use of environment variables. For increased visibility, enable the collection of http_proxy, HTTPS_PROXY and ALL_PROXY environment variables based on the instructions provided in the setup guide of this rule.

Rule type: eql

Rule indices:

  • logs-endpoint.events.process*

Severity: medium

Risk score: 47

Runs every: 5m

Searches indices from: now-9m (Date Math format, see also Additional look-back time)

Maximum alerts per execution: 100

References:

Tags:

  • Domain: Endpoint
  • OS: Linux
  • Use Case: Threat Detection
  • Use Case: Vulnerability
  • Tactic: Execution
  • Data Source: Elastic Defend
  • Resources: Investigation Guide

Version: 8

Rule authors:

  • Elastic

Rule license: Elastic License v2

Investigation guide

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Triage and analysis

Disclaimer: This investigation guide was created using generative AI technology and has been reviewed to improve its accuracy and relevance. While every effort has been made to ensure its quality, we recommend validating the content and adapting it to suit your specific environment and operational needs.

Investigating Potential curl CVE-2023-38545 Exploitation

Curl is a command-line tool used for transferring data with URLs, often employed in scripts and automation. A vulnerability in versions up to 8.3 allows a buffer overflow during SOCKS5 proxy handshakes, potentially leading to arbitrary code execution. Adversaries might exploit this by crafting specific command lines or environment variables. The detection rule identifies suspicious curl usage patterns, such as unusual command lengths and specific proxy settings, to flag potential exploitation attempts.

Possible investigation steps

  • Review the process command line arguments and environment variables to confirm the presence of SOCKS5 proxy settings, such as "--socks5-hostname", "--proxy", "--preproxy", or environment variables like "http_proxy=socks5h://", "HTTPS_PROXY=socks5h://", and "ALL_PROXY=socks5h://*".
  • Check the length of the command line to ensure it exceeds 255 characters, as this is a key indicator of potential exploitation attempts.
  • Investigate the parent process of the curl execution to determine if it is one of the known benign processes like "cf-agent", "agent-run", "agent-check", "rudder", "agent-inventory", or "cf-execd", or if it originates from directories like "/opt/rudder/", "/vz/root/", or "/var/rudder/*".
  • Examine the network activity associated with the curl process to identify any unusual or unauthorized data transfers, especially those involving SOCKS5 proxies.
  • Cross-reference the alert with other security logs and alerts to identify any correlated suspicious activities or patterns that might indicate a broader attack campaign.

False positive analysis

  • Legitimate administrative tools like cf-agent, agent-run, and rudder may trigger the rule due to their use of curl with SOCKS5 proxies. To mitigate this, add these tools to the exception list in the rule configuration.
  • Automated scripts running in environments like /opt/rudder or /var/rudder might be flagged. Exclude these paths from the rule to prevent false positives.
  • Processes executed within virtualized environments, such as those under /vz/root, can be mistakenly identified. Consider excluding these directories if they are known to host benign activities.
  • Regularly review and update the list of known safe parent processes and directories to ensure that legitimate operations are not disrupted by the detection rule.

Response and remediation

  • Immediately isolate the affected system from the network to prevent further exploitation or lateral movement by the adversary.
  • Terminate any suspicious curl processes identified by the detection rule to halt potential exploitation activities.
  • Upgrade curl to version 8.4 or later on all affected systems to patch the vulnerability and prevent future exploitation attempts.
  • Review and reset any potentially compromised environment variables, such as http_proxy, HTTPS_PROXY, and ALL_PROXY, to ensure they are not being used maliciously.
  • Conduct a thorough investigation of the affected system for signs of further compromise, such as unauthorized access or changes to critical files, and take appropriate remediation actions.
  • Notify the security team and relevant stakeholders about the incident for awareness and further analysis, ensuring that any findings are documented for future reference.
  • Enhance monitoring and logging for curl usage and SOCKS5 proxy connections to improve detection capabilities for similar threats in the future.

Setup

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Setup

This rule requires data coming in from Elastic Defend.

Elastic Defend Integration Setup

Elastic Defend is integrated into the Elastic Agent using Fleet. Upon configuration, the integration allows the Elastic Agent to monitor events on your host and send data to the Elastic Security app.

Prerequisite Requirements:

  • Fleet is required for Elastic Defend.
  • To configure Fleet Server refer to the documentation.

The following steps should be executed in order to add the Elastic Defend integration on a Linux System:

  • Go to the Kibana home page and click "Add integrations".
  • In the query bar, search for "Elastic Defend" and select the integration to see more details about it.
  • Click "Add Elastic Defend".
  • Configure the integration name and optionally add a description.
  • Select the type of environment you want to protect, either "Traditional Endpoints" or "Cloud Workloads".
  • Select a configuration preset. Each preset comes with different default settings for Elastic Agent, you can further customize these later by configuring the Elastic Defend integration policy. Helper guide.
  • We suggest selecting "Complete EDR (Endpoint Detection and Response)" as a configuration setting, that provides "All events; all preventions"
  • Enter a name for the agent policy in "New agent policy name". If other agent policies already exist, you can click the "Existing hosts" tab and select an existing policy instead. For more details on Elastic Agent configuration settings, refer to the helper guide.
  • Click "Save and Continue".
  • To complete the integration, select "Add Elastic Agent to your hosts" and continue to the next section to install the Elastic Agent on your hosts. For more details on Elastic Defend refer to the helper guide.

Elastic Defend integration does not collect environment variable logging by default. In order to capture this behavior, this rule requires a specific configuration option set within the advanced settings of the Elastic Defend integration. ## To set up environment variable capture for an Elastic Agent policy: - Go to “Security → Manage → Policies”. - Select an “Elastic Agent policy”. - Click “Show advanced settings”. - Scroll down or search for “linux.advanced.capture_env_vars”. - Enter the names of environment variables you want to capture, separated by commas. - For this rule the linux.advanced.capture_env_vars variable should be set to "http_proxy,HTTPS_PROXY,ALL_PROXY". - Click “Save”. After saving the integration change, the Elastic Agents running this policy will be updated and the rule will function properly. For more information on capturing environment variables refer to the helper guide.

Rule query

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process where host.os.type == "linux" and event.type == "start" and event.action == "exec" and process.name == "curl"
and (
  process.args like ("--socks5-hostname", "--proxy", "--preproxy", "socks5*") or
  process.env_vars like ("http_proxy=socks5h://*", "HTTPS_PROXY=socks5h://*", "ALL_PROXY=socks5h://*")
) and length(process.command_line) > 255 and not (
  process.parent.name in ("cf-agent", "agent-run", "agent-check", "rudder", "agent-inventory", "cf-execd") or
  process.args like "/opt/rudder/*" or
  process.parent.executable like ("/vz/root/*", "/var/rudder/*")
)

Framework: MITRE ATT&CKTM

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