Summary
A command injection vulnerability in shell-quote's quote() function can allow shell syntax in a later string token to escape intended quoting when preceded by a comment token. If an attacker controls the relevant parsed or combined input, they may cause additional shell commands to execute in the invoking shell.
Affected context
The issue occurs in applications that use shell-quote to construct shell command lines from partially trusted data, particularly when { comment } tokens are combined with later string tokens. It is also relevant when parse() output, which may represent an embedded # as a comment token, is concatenated with another untrusted argument before being passed to quote().
Exploit preconditions
- Attack surface: Application code that generates shell command strings with
quote()and executes them through a shell - Required access: Ability to influence a parsed command, comment-related token, or string token occurring after a comment token
- Trigger point: Shell interpretation of the command line produced by
quote() - Why exploitation is possible: A comment token can suppress the opening quote of a later string, while a line terminator in that string ends the comment and exposes the remaining text as shell syntax
Why this happens
The vulnerable behavior results from treating a comment token as text that comments out the remainder of the shell line without accounting for line terminators in subsequent tokens. When a later string contains a line terminator, the shell can resume parsing after the comment and interpret the remainder as commands.
This is CWE-78: Improper Neutralization of Special Elements used in an OS Command, because data intended to remain an argument can become active shell syntax after command construction and parsing interact in an unsafe way.
Potential impact
- An attacker may execute arbitrary commands in the application process context.
- The vulnerability could expose files, credentials, environment variables, or application data.
- A compromised process may be used for lateral movement or further system access.
- Command execution may affect build, deployment, automation, or data-processing workflows.
General mitigation guidance
Mitigation generally focuses on avoiding shell interpretation and preserving strict separation between command arguments and executable syntax.
Common defensive patterns include:
- Prefer direct process-execution APIs that accept an argument array rather than a shell command string.
- Apply consistent, context-aware validation to every token, including values following comment tokens.
- Reject or safely handle line terminators and other shell metacharacters in untrusted arguments where they are not required.
- Avoid concatenating parser output with independent untrusted strings unless token types and trust boundaries are explicitly enforced.
- Use safe command-construction abstractions with secure-by-default handling of comments and control characters.
- Run command-executing processes with least privilege and restricted filesystem, network, and credential access.
How teams detect issues like this
Detection can combine code analysis, review, and runtime policy controls:
- Search for calls to
quote()followed by shell execution APIs or subprocess functions configured to invoke a shell. - Review data flows from
parse()or user-controlled input into arrays containing{ comment }tokens and later string values. - Flag command construction that concatenates parser output with untrusted arguments before quoting.
- Add static-analysis rules for shell invocation, dynamic command strings, line terminators, and untrusted shell metacharacters.
- Use policy enforcement to restrict shell access, command allowlists, and process capabilities in CI and production environments.
- Automated code-review and software-composition tools may help identify vulnerable usage patterns and dependency exposure.
