Subject: Mitigating CVE-2014-6271: Shellshock Vulnerability in Bash
Tech Stack
- Technology: Bash (Bourne Again Shell)
- Affected Versions: Bash versions 1.14 through 4.3
Date(s) Issued
- Vulnerability Publication Date: 09/24/2014
- Patch Publication Date: 09/24/2014
Criticality
- CVSS v3 Base Score: 10.0 (Critical)
- Vector: CVSS:3.0/AV
/AC
/PR
/UI
/S
/C
/I
/A
- CVSS v2 Base Score: 10.0 (High)
- Vector: CVSS2#AV
/AC
/Au
/C
/I
/A
- Exploitability: Widely exploitable, with numerous public exploits available; can be exploited remotely by unauthenticated attackers.
Overview
CVE-2014-6271, also known as Shellshock, is a critical remote code execution (RCE) vulnerability affecting Bash, a widely used Unix shell. It allows attackers to exploit the way Bash handles environment variables. By sending specially crafted variables, attackers can inject and execute arbitrary commands, compromising the confidentiality, integrity, and availability of systems that use Bash in network-exposed services like web servers, SSH, or DHCP.
Solution/Mitigation
To mitigate this vulnerability, follow the steps outlined below:
1. Upgrade Bash
- Ensure that Bash is updated to the latest patched version provided by the OS vendor. Versions patched post-09/24/2014 address the Shellshock vulnerability.
- For specific patch versions:
- Debian-based systems: Run
sudo apt-get update && sudo apt-get install --only-upgrade bash.
- Red Hat-based systems: Run
sudo yum update bash.
- SUSE systems: Run
sudo zypper up bash.
2. Alternative Measures (If Immediate Upgrade is Not Feasible)
- Disable Unnecessary Bash Usage: Restrict access to services using Bash where possible or replace Bash with other shells that are not vulnerable (e.g., Dash or Ash).
- Apply Input Validation: Use strict input validation in CGI scripts and other Bash-dependent services to minimize attack vectors.
- Deploy Workarounds: Implement temporary workarounds such as using a WAF to block malicious requests targeting Bash via HTTP headers or environmental variables.
3. Additional Recommendations
- Restrict Network Access: Limit the exposure of Bash-dependent services to untrusted networks through network segmentation and firewall rules.
- Monitor System Logs: Regularly review system logs for unusual patterns or anomalies that could indicate Shellshock exploitation attempts.
- Apply Principle of Least Privilege: Configure services using Bash with minimal privileges to reduce potential impacts of exploitation.
Confirmation & Additional Information
- Verification: Run
env x='() { :;}; echo vulnerable' bash -c "echo test" to check if the system is still vulnerable. If "vulnerable" is displayed, it means the patch hasn't been applied.
- Continuous Monitoring: After patching, continuously monitor for new patches or versions released to address any related vulnerabilities that may arise.
- Official References:
