CVE-2021-43837 in vault-cli
Summary
by MITRE • 12/16/2021
vault-cli is a configurable command-line interface tool (and python library) to interact with Hashicorp Vault. In versions before 3.0.0 vault-cli features the ability for rendering templated values. When a secret starts with the prefix `!template!`, vault-cli interprets the rest of the contents of the secret as a Jinja2 template. Jinja2 is a powerful templating engine and is not designed to safely render arbitrary templates. An attacker controlling a jinja2 template rendered on a machine can trigger arbitrary code, making this a Remote Code Execution (RCE) risk. If the content of the vault can be completely trusted, then this is not a problem. Otherwise, if your threat model includes cases where an attacker can manipulate a secret value read from the vault using vault-cli, then this vulnerability may impact you. In 3.0.0, the code related to interpreting vault templated secrets has been removed entirely. Users are advised to upgrade as soon as possible. For users unable to upgrade a workaround does exist. Using the environment variable `VAULT_CLI_RENDER=false` or the flag `--no-render` (placed between `vault-cli` and the subcommand, e.g. `vault-cli --no-render get-all`) or adding `render: false` to the vault-cli configuration yaml file disables rendering and removes the vulnerability. Using the python library, you can use: `vault_cli.get_client(render=False)` when creating your client to get a client that will not render templated secrets and thus operates securely.
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Analysis
by VulDB Data Team • 12/22/2021
The vulnerability identified as CVE-2021-43837 affects vault-cli, a command-line interface tool and python library designed for interacting with Hashicorp Vault. This tool enables users to manage and retrieve secrets from Vault while providing templating capabilities through Jinja2. The flaw stems from the tool's interpretation of secrets that begin with the specific prefix '', which triggers Jinja2 template processing. This functionality presents a critical security risk as Jinja2, while powerful for legitimate templating purposes, is not designed to safely process untrusted or arbitrary templates. The vulnerability manifests when an attacker can manipulate secret values stored in Vault, particularly in environments where Vault contents are not fully trusted.
The technical implementation of this vulnerability involves the automatic processing of templated content without proper sanitization or security boundaries. When vault-cli encounters a secret starting with '', it treats the remainder as a Jinja2 template and attempts to render it. This rendering process can be exploited to execute arbitrary code on the system running vault-cli, creating a remote code execution vulnerability. The risk is particularly severe because the attack vector requires only manipulation of secret values in Vault, which may be accessible to attackers through various means such as compromised accounts, misconfigured access controls, or other attack surfaces. The vulnerability aligns with CWE-94, which describes "Improper Control of Generation of Code ('Code Injection')" and represents a classic template injection attack pattern.
The operational impact of this vulnerability extends beyond simple code execution to encompass potential system compromise and data exfiltration. An attacker who can control Vault secret values can execute arbitrary commands with the privileges of the vault-cli process, potentially leading to full system compromise. This risk is particularly concerning in enterprise environments where Vault is used for credential management, configuration storage, and other sensitive operations. The vulnerability affects all versions prior to 3.0.0 and demonstrates the importance of proper input validation and template engine security in security tools. Organizations using vault-cli in production environments may be exposed to significant risk if their threat models include potential compromise of Vault secrets or if access controls are insufficient.
The remediation approach implemented in version 3.0.0 addresses this vulnerability by completely removing the templating functionality from the codebase. This represents a defensive security measure that eliminates the attack surface entirely rather than attempting to secure the template processing. Users can also implement several workarounds to mitigate the risk in older versions, including setting the environment variable VAULT_CLI_RENDER=false, using the --no-render flag, or configuring render: false in the vault-cli yaml configuration file. For developers using the python library directly, the vault_cli.get_client(render=False) method provides a programmatic approach to disable template rendering. These mitigation strategies align with the principle of least privilege and demonstrate the importance of providing secure defaults in security tools. The vulnerability's resolution reflects industry best practices for handling template engines in security contexts, emphasizing the need for explicit opt-in rather than automatic processing of potentially dangerous content. The fix also reinforces the ATT&CK framework concept of privilege escalation through command execution and highlights the importance of secure coding practices in security tool development.