CVE-2026-29783

A security vulnerability has been identified in GitHub Copilot CLI's shell tool that could allow arbitrary code execution through crafted bash parameter expansion patterns. An attacker who can influence the commands executed by the agent (e.g., via prompt injection through repository files, MCP server responses, or user instructions) can exploit bash parameter transformation operators to execute hidden commands, bypassing the safety assessment that classifies commands as "read-only."

Details

The vulnerability stems from how the CLI's shell safety assessment evaluates commands before execution. The safety layer parses and classifies shell commands as either read-only (safe) or write-capable (requires user approval). However, several bash parameter expansion features can embed executable code within arguments to otherwise read-only commands, causing them to appear safe while actually performing arbitrary operations.

The specific dangerous patterns are:

1. ${var@P}, Prompt expansion: The @P parameter transformation operator evaluates its value as a prompt string, which interprets embedded command substitutions. This allows hidden command execution inside what appears to be a simple variable reference.

2. ${var=value} / ${var:=value}, Assignment side-effects: These forms assign values to variables as a side-effect of expansion. When chained with @P, an attacker can progressively build up a command substitution string across multiple expansions.

3. ${!var}, Indirect expansion: Dereferences an arbitrary variable name, which can be combined with other patterns to construct and execute commands dynamically.

4. Nested $(cmd) or <(cmd) inside ${...} expansions: Command substitution or process substitution embedded within parameter expansion default values (e.g., ${HOME:-$(whoami)}) executes the nested command.

Proof of Concept

The following command appears to run a harmless echo, but actually executes touch /tmp/pwned through chained parameter expansion:

echo ${a="$"}${b="$a(touch /tmp/pwned)"}${b@P}

How it works:

• ${a="$"} assigns the literal $ character to variable a
• ${b="$a(touch /tmp/pwned)"} expands $a to $, constructing the string $(touch /tmp/pwned) and assigning it to b
• ${b@P} applies prompt expansion to b, which evaluates the embedded $(touch /tmp/pwned) command substitution

Prior to the fix, the safety assessment would classify echo as a read-only command and allow execution without user confirmation, even in modes that normally require approval for write operations.

Affected Versions

• GitHub Copilot CLI versions prior to 0.0.423

Remediation and Mitigation

User Actions

1. Upgrade GitHub Copilot CLI to 0.0.423 or later.
2. Exercise caution when working in untrusted repositories or with untrusted MCP servers.
3. Review any shell commands suggested by the agent that contain complex parameter expansion patterns.

An attacker who can influence command text sent to the shell tool, for example, through:

• Prompt injection via malicious repository content (README files, code comments, issue bodies)
• Compromised or malicious MCP server responses
• Crafted user instructions containing obfuscated commands

could achieve arbitrary code execution on the user's workstation. This is possible even in permission modes that require user approval for write operations, since the commands can appear to be using only read-only utilities to ultimately trigger write operations.

Successful exploitation could lead to data exfiltration, file modification, or further system compromise.

Untrusted input reaches a shell command, allowing arbitrary commands to run on the host. Typical impact: code execution in the application's environment.

Kodem intelligence

The fix adds three layers of defense:

1. Parse-time detection: The shell safety assessment analyzes ${...} expansion nodes within bash commands, detecting dangerous operators (@P, =, :=, !) and nested command/process substitutions. Commands containing these patterns are downgraded from read-only to write-capable, ensuring they require user approval.

2. Unconditional blocking: Commands with dangerous expansion patterns are unconditionally blocked at the tool execution layer, regardless of permission mode (including --yolo / autopilot). This prevents exploitation even when all commands are auto-approved.

3. System prompt hardening: The bash shell tool's system prompt now includes explicit instructions for the LLM to refuse executing commands with these patterns, providing a defense-in-depth layer.

1. What is CVE-2026-29783? CVE-2026-29783 is a high-severity OS command injection vulnerability in @github/copilot (npm), affecting versions <= 0.0.422. It is fixed in 0.0.423. Untrusted input reaches a shell command, allowing arbitrary commands to run on the host.
2. Which versions of @github/copilot are affected by CVE-2026-29783? @github/copilot (npm) versions <= 0.0.422 is affected.
3. Is there a fix for CVE-2026-29783? Yes. CVE-2026-29783 is fixed in 0.0.423. Upgrade to this version or later.
4. Is CVE-2026-29783 exploitable, and should I be worried? Whether CVE-2026-29783 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
5. What actually determines whether CVE-2026-29783 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
6. How do I fix CVE-2026-29783? Upgrade @github/copilot to 0.0.423 or later.