CVE-2014-6271: GNU Bourne-Again Shell (Bash) Arbitrary Code Execution Vulnerability

CVE-2014-6271 is an arbitrary code execution vulnerability in the GNU Bourne-Again Shell (Bash). It stems from how Bash handles certain environment variable values, allowing remote attackers to execute code. Because Bash is widely used as a system shell and in contexts that process untrusted input via environment variables, successful abuse can give an attacker control over the affected host. Confirm all version and configuration details against the vendor advisory.

CISA summarizes the issue as GNU Bash through 4.3 processing trailing strings after function definitions in the values of environment variables. The associated weakness is CWE-78 (OS command injection). Known ransomware use is not documented. The required action is to apply updates per vendor instructions.

How it works

This flaw belongs to the OS command injection class (CWE-78). Bash interprets environment variable values that contain function definitions. When those values include trailing strings after the function definition, older Bash behavior can cause those trailing strings to be executed rather than treated as inert data.

An attacker who can influence environment variables seen by a Bash process—commonly through network-facing services, CGI scripts, or other components that pass attacker-controlled data into the environment—may cause Bash to run attacker-chosen commands in the context of that process. Exact exploit mechanics and affected call paths vary by how the service invokes Bash; do not assume a single universal trigger. Confirm the precise conditions in the vendor advisory for your distribution and Bash build.

Am I affected? How to find it in your systems

Bash is the default or common interactive and non-interactive shell on many Linux and Unix-like systems. It appears on servers, workstations, appliances, containers, and embedded devices, and is often invoked by web servers, force-command SSH setups, DHCP clients, and other services that set environment variables from external input.

• Inventory hosts and images for the Bash package or binary (for example via package managers, configuration management, or filesystem searches for the bash executable).
• Record the installed Bash version and compare it to the fixed versions listed in your OS or vendor advisory. The CISA summary references GNU Bash through 4.3; treat that as a starting point only and verify against the advisory for your platform.
• Identify services that pass untrusted data into the environment and then invoke Bash or shell scripts (web CGI, certain agents, automated tools).
• Review logs for unusual process trees, unexpected shell invocations, or environment variables containing function-definition-like content followed by extra commands. Telemetry that captures process command lines and environment at execution time is especially useful. Absence of obvious log signs does not prove you were not targeted.

How to remediate

Patch first. Apply the vendor-supplied Bash update for your operating system or distribution exactly as described in the vendor advisory and in line with CISA’s direction to apply updates per vendor instructions. After updating, restart services or hosts as required so that running processes load the fixed Bash.

• Verify the installed version post-patch matches the fixed release in the advisory.
• Rebuild or refresh containers and golden images so new instances do not reintroduce the vulnerable binary.
• Where possible, reduce unnecessary use of Bash for processing untrusted input; prefer safer interfaces that do not evaluate environment content as shell code.
• Harden remaining shell usage: minimize setuid scripts, avoid passing raw external data into the environment, and run network-facing services with least privilege.

If you can't patch immediately

Compensate until you can patch. These measures reduce exposure but do not replace the vendor update.

• Segment and restrict network access to services that invoke Bash with attacker-influenced environment data; limit exposure to trusted networks only.
• If a web application firewall or reverse proxy is in path, apply virtual-patching rules that block or sanitize requests known to abuse environment-variable function definitions—tune carefully to avoid breaking legitimate traffic and confirm rule efficacy against the advisory.
• Temporarily disable or isolate affected features (for example, CGI or other handlers that spawn Bash) if business needs allow.
• Increase monitoring: alert on anomalous shell spawns, unexpected outbound connections from web or service accounts, and environment variables that contain function-definition patterns with trailing content. Retain relevant logs for incident review.

If your data may have been exposed

Actively exploited code-execution vulnerabilities can lead to full host compromise and subsequent data theft or lateral movement. If you have reason to believe systems were exposed before patching, follow your incident-response process: isolate affected hosts, preserve evidence, rotate credentials that may have been accessible, and assess what data the compromised context could reach. You can run a free exposure scan of your email addresses to check whether they appear in known breach datasets and prioritize further monitoring accordingly.