Shellshock in Chi with Basic Auth

Shellshock (CVE-2014-6271 and related variants) is a command injection vulnerability in the Bash shell that arises when function definitions are followed by additional code in environment variables. In Chi, a lightweight HTTP router for Go, developers often compose middleware that builds request-scoped context and headers. When Basic Authentication is used, it is common to extract credentials from the Authorization header, decode the base64 payload, and then pass username and password values into the runtime environment—typically via os.Setenv or by constructing an os.Environ-like map for downstream handlers or CGI-style behavior.

If the extracted username or password values are used to set environment variables that later invoke a Bash command (for example, through os/exec calling a shell, or via a system helper that ultimately calls sh), an attacker can inject malicious code by crafting credentials such as Authorization: Basic dGVzdD1$(malicious_cmd). Because Chi does not sanitize these values before they reach the shell, the injected code executes with the privileges of the running service. This pattern is especially dangerous in environments where the API is fronted by a gateway or reverse proxy that terminates TLS and forwards the Authorization header, potentially exposing a broader attack surface.

The combination of Chi, Basic Auth, and shell invocation creates a clear chain: an attacker sends a specially crafted Authorization header, the application decodes and forwards the credentials into the environment, and a Bash subprocess interprets the injected function body or command string. Because the scan tests unauthenticated attack surfaces, middleBrick can detect scenarios where authentication is weak or where credentials are reflected into system commands, surfacing risks such as CVE-2014-6271 alongside findings from the Authentication and Unsafe Consumption checks.

In a typical Chi-based service that uses Basic Auth and executes Bash, you might see code like this:

cmd := exec.Command("/bin/bash", "-c", "echo User: "+os.Getenv("BASIC_USER"))

If BASIC_USER is derived from an attacker-controlled Authorization header, the command becomes exploitable. middleBrick’s checks for System Prompt Leakage and Unsafe Consumption are designed to highlight these risky patterns, and the tool’s cross-referencing of OpenAPI specs with runtime behavior ensures that endpoints declaring authentication are scrutinized for unsafe data flows.

Remediation focuses on preventing credential values from ever reaching a shell and on avoiding the use of the shell when invoking external commands. The safest approach is to avoid Bash entirely and use Go’s standard library functions that do not invoke a shell interpreter. If you must use shell features, rigorously validate and sanitize inputs, and prefer passing arguments directly rather than via a single command string.

Example of vulnerable code that concatenates credentials into a shell command:

auth := parseBasicAuth(req) // returns username, password
os.Setenv("BASIC_USER", auth.Username)
cmd := exec.Command("/bin/bash", "-c", "curl -H 'X-User: $BASIC_USER' http://internal")

Secure alternative without Bash:

auth := parseBasicAuth(req)
req, err := http.NewRequestWithContext(ctx, "GET", "http://internal", nil)
if err != nil {
    // handle error
}
req.Header.Set("X-User", auth.Username)
client := &http.Client{}
resp, err := client.Do(req)

Secure alternative with strict input validation and explicit arguments (if shell is unavoidable):

auth := parseBasicAuth(req)
if !isValidUsername(auth.Username) {
    http.Error(w, "invalid credentials", http.StatusUnauthorized)
    return
}
cmd := exec.Command("/usr/bin/curl", "-H", "X-User:"+auth.Username, "http://internal")

Validation helper example:

func isValidUsername(u string) bool {
    return regexp.MustCompile(`^[a-zA-Z0-9._-]+$`).MatchString(u)
}

Additionally, prefer using Chi’s middleware to enforce authentication without exposing credentials to the environment. For example:

func BasicAuth(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        user, pass, ok := r.BasicAuth()
        if !ok || !validateUserPass(user, pass) {
            http.Error(w, "Unauthorized", http.StatusUnauthorized)
            return
        }
        ctx := context.WithValue(r.Context(), "user", user)
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

By combining these practices—removing shell invocation, validating inputs, and leveraging Chi’s idiomatic middleware—you reduce the attack surface significantly. The middleBrick CLI can be used in CI/CD to verify that no unsafe patterns remain, and the GitHub Action can fail builds if scans detect high-risk findings.