CVE-2025-66032

Executive Summary

CVE-2025-66032 is a critical vulnerability in Anthropic's Claude Code agentic coding tool (versions prior to 1.0.93). The flaw stems from improper parsing of shell commands involving the Internal Field Separator ($IFS) and short CLI flags. This parsing error allows an attacker to bypass the tool's read-only validation mechanism, leading to arbitrary code execution. The attack requires the ability to inject untrusted content into the Claude Code context window, making it a significant supply chain and prompt injection risk for users of the tool.

Supply Chain & Package Security

Claude Code is an agentic coding tool that operates within a user's development environment. The vulnerability is not in a traditional package manager (NPM, PyPI, Go, Ruby) but in the tool's own command parsing logic. However, the risk is amplified in supply chain scenarios where a developer might inadvertently include malicious content (e.g., from a compromised repository, code snippet, or AI-generated output) into the Claude Code context. This could lead to arbitrary code execution on the developer's machine, potentially compromising credentials, source code, or CI/CD pipelines.

CVSS Breakdown & Threat Level

The CVSS score is listed as 0.0, which is likely a placeholder. Based on the description, a reasonable CVSS v3.1 vector would be:

• Attack Vector (AV): Network (AV:N) - The attack can be initiated by injecting content into the context window, which could be delivered over the network.
• Attack Complexity (AC): Low (AC:L) - The exploit relies on a parsing error, not complex conditions.
• Privileges Required (PR): None (PR:N) - No authentication needed to inject content.
• User Interaction (UI): Required (UI:R) - The user must be using Claude Code and have the malicious content in the context.
• Scope (S): Unchanged (S:U) - The vulnerability affects the tool's execution environment.
• Confidentiality (C): High (C:H) - Arbitrary code execution can lead to data exfiltration.
• Integrity (I): High (I:H) - Code execution can modify files or system state.
• Availability (A): High (A:H) - Code execution can disrupt operations.

Estimated CVSS Score: 8.8 (High)

Threat Level: High. The vulnerability allows arbitrary code execution with low complexity, though it requires user interaction and the ability to inject content into the context window.

Deep Dive & Mechanics

The vulnerability is rooted in how Claude Code parses shell commands, specifically related to the $IFS environment variable and short CLI flags.

• $IFS Manipulation: The Internal Field Separator ($IFS) is a shell variable that determines how the shell splits words. By manipulating $IFS within a command, an attacker can alter how the command is parsed, potentially bypassing input validation or read-only checks.
• Short CLI Flags: Short flags (e.g., -f, -r) are often parsed differently than long flags (e.g., --force, --read-only). The vulnerability exploits a discrepancy in how these flags are handled, allowing an attacker to inject additional arguments or commands.
• Bypassing Read-Only Validation: Claude Code likely has a read-only mode that restricts certain operations (e.g., file writes, command execution). By crafting a command that uses $IFS and short flags, an attacker can cause the parser to misinterpret the command, bypassing the read-only check and executing arbitrary code.

Attack Scenario:

1. An attacker injects a malicious payload into the Claude Code context window (e.g., via a compromised code repository, a malicious AI-generated response, or a crafted file).
2. The user interacts with Claude Code, which processes the context.
3. The malicious payload contains a command that exploits the $IFS and short flag parsing error.
4. Claude Code executes the command, bypassing read-only validation, leading to arbitrary code execution.

Vulnerable vs. Patched Code

Vulnerable Code (Pre-1.0.93): The vulnerable code likely had a function that parsed shell commands without properly handling $IFS or short flags. For example:

def parse_command(user_input):
    # Vulnerable: Does not sanitize $IFS or short flags
    if user_input.startswith("!"):
        command = user_input[1:]
        # Directly executes without read-only check
        os.system(command)

Patched Code (1.0.93): The patch likely introduces proper sanitization and validation:

def parse_command(user_input):
    # Patched: Sanitize $IFS and validate flags
    sanitized_input = sanitize_ifs(user_input)
    if is_read_only_allowed(sanitized_input):
        command = sanitized_input[1:]
        # Execute with proper restrictions
        subprocess.run(command, shell=True, check=True)
    else:
        raise PermissionError("Read-only mode: command not allowed")

The exact patch details are not publicly available, but the fix likely involves:

• Stripping or escaping $IFS from user input.
• Normalizing short and long flags to a consistent format.
• Adding a whitelist of allowed commands in read-only mode.

Exploitation & Lab Setup

Red Team

Objective: Demonstrate arbitrary code execution by injecting a malicious payload into the Claude Code context.

Lab Setup:

• Install Claude Code version < 1.0.93 (e.g., 1.0.92).
• Set up a controlled environment (e.g., a Docker container or VM) to avoid real damage.
• Prepare a payload that exploits the $IFS and short flag parsing error.

Exploit Payload Example:

# Exploit using $IFS to bypass read-only validation
!$IFS/bin/bash$IFS-c$IFS'echo "PWNED" > /tmp/pwned.txt'

Steps:

1. Start Claude Code in a terminal.
2. Inject the payload into the context (e.g., by pasting it as a user message or including it in a file).
3. Trigger the command execution (e.g., by asking Claude to run a command).
4. Verify that /tmp/pwned.txt was created, indicating successful code execution.

Note: This is a conceptual example. The actual exploit may require specific syntax to trigger the parsing error.

Blue Team

Objective: Detect and prevent exploitation of CVE-2025-66032.

Detection:

• Monitor for unusual shell commands that include $IFS or short flags in Claude Code logs.
• Look for commands that attempt to bypass read-only mode (e.g., commands that write to sensitive directories).
• Use endpoint detection and response (EDR) tools to monitor for child processes spawned by Claude Code.

Prevention:

• Update Claude Code to version 1.0.93 or later.
• Implement strict input validation for any content injected into the Claude Code context.
• Use a sandboxed environment for running Claude Code (e.g., a container with limited privileges).
• Educate users about the risks of including untrusted content in the context window.

Indicators of Compromise (IOCs)

• File Artifacts:

  • /tmp/pwned.txt (or similar files created by the exploit)
  • Unusual files in the user's home directory or /tmp/

• Process Artifacts:

  • Child processes spawned by Claude Code (e.g., /bin/bash, /bin/sh, /usr/bin/python)
  • Processes with unusual arguments containing $IFS or short flags

• Network Artifacts:

  • Outbound connections from Claude Code to unknown IPs (if the exploit exfiltrates data)
  • DNS queries for suspicious domains

• Log Artifacts:

  • Claude Code logs showing commands with $IFS or short flags
  • System logs (e.g., auth.log, syslog) showing unexpected command executions

Detection Engineering

YARA Rule

rule CVE_2025_66032_Claude_Code_Exploit {
    meta:
        description = "Detects potential exploitation of CVE-2025-66032 in Claude Code"
        author = "Valters IT"
        date = "2025-04-10"
        reference = "CVE-2025-66032"
    strings:
        $ifs_pattern = /\$IFS/
        $short_flag = /-[a-zA-Z]{1}/
        $shell_cmd = /(!|sh|bash|cmd)\s*/
    condition:
        $ifs_pattern and $short_flag and $shell_cmd
}

Sigma Rule

title: Detection of CVE-2025-66032 Exploitation in Claude Code
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects commands containing $IFS and short flags, indicative of CVE-2025-66032 exploitation
author: Valters IT
date: 2025-04-10
logsource:
    category: process_creation
    product: linux
detection:
    selection:
        CommandLine|contains: '$IFS'
        CommandLine|re: '-[a-zA-Z]{1}'
    condition: selection
falsepositives:
    - Legitimate use of $IFS in shell scripts
level: high

Suricata Rule

alert tcp any any -> any any (msg:"CVE-2025-66032 Claude Code Exploit Attempt"; flow:to_server,established; content:"$IFS"; nocase; pcre:"/-[a-zA-Z]{1}/"; classtype:attempted-admin; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Update Claude Code: Upgrade to version 1.0.93 or later immediately.
2. Restrict Context Content: Avoid including untrusted content (e.g., from unknown sources, AI-generated code) in the Claude Code context window.
3. Monitor Logs: Review Claude Code logs for any suspicious commands containing $IFS or short flags.

Long-Term Mitigations:

1. Input Validation: Implement strict input validation for all commands processed by Claude Code.
2. Sandboxing: Run Claude Code in a sandboxed environment (e.g., Docker container, VM) with limited privileges.
3. User Education: Train users on the risks of prompt injection and untrusted content in AI tools.
4. Regular Updates: Keep Claude Code and other AI tools updated to the latest versions.

Workarounds (if patching is not immediately possible):

• Disable read-only mode if not needed, or implement additional manual review of commands.
• Use a web application firewall (WAF) or proxy to inspect and block malicious payloads.

This report was generated by Valters IT DevSecOps & CTI Division.