cli/safeexec

safeexec

A Go module that provides a stabler alternative to exec.LookPath() that:

• Avoids a Windows security risk of executing commands found in the current directory; and
• Allows executing commands found in PATH, even if they come from relative PATH entries.

This is an alternative to golang.org/x/sys/execabs.

Usage

import (
    "os/exec"
    "github.com/cli/safeexec"
)

func gitStatus() error {
    gitBin, err := safeexec.LookPath("git")
    if err != nil {
        return err
    }
    cmd := exec.Command(gitBin, "status")
    return cmd.Run()
}

Background

Windows security vulnerability with Go <= 1.18

Go 1.18 (and older) standard library has a security vulnerability when executing programs:

import "os/exec"

func gitStatus() error {
    // On Windows, this will result in `.\git.exe` or `.\git.bat` being executed
    // if either were found in the current working directory.
    cmd := exec.Command("git", "status")
    return cmd.Run()
}

For historic reasons, Go used to implicitly include the current directory in the PATH resolution on Windows. The safeexec package avoids searching the current directory on Windows.

Relative PATH entries with Go 1.19+

Go 1.19 (and newer) standard library throws an error if exec.LookPath("git") resolved to an executable relative to the current directory. This can happen on other platforms if the PATH environment variable contains relative entries, e.g. PATH=./bin:$PATH. The safeexec package allows respecting relative PATH entries as it assumes that the responsibility for keeping PATH safe lies outside of the Go program.

TODO

Ideally, this module would also provide exec.Command() and exec.CommandContext() equivalents that delegate to the patched version of LookPath. However, this doesn't seem possible since LookPath may return an error, while exec.Command/CommandContext() themselves do not return an error. In the standard library, the resulting exec.Cmd struct stores the LookPath error in a private field, but that functionality isn't available to us.