Container Escape in Chi (Go)

Container escape occurs when an attacker breaks out of a container’s isolation to gain unauthorized access to the host system or other containers. While Chi is a lightweight, idiomatic Go HTTP router, it does not directly cause container escapes. However, misconfigurations in how Go applications built with Chi are deployed — particularly when combined with excessive privileges, unsafe volume mounts, or flawed input handling — can expose the host to escape vectors. For example, if a Chi-based service runs as root inside a container and mounts the host’s Docker socket (/var/run/docker.sock) or root filesystem, an attacker who achieves remote code execution (RCE) via an API vulnerability could use that access to escape the container.

Consider a Chi handler that naively executes shell commands based on user input without proper validation. If the container runs with CAP_SYS_ADMIN or has /proc or /sys mounted in a way that allows kernel module manipulation, an attacker could exploit command injection to load malicious modules or access host resources. Real-world parallels include CVE-2019-5736 (runc), where overwriting the host’s runc binary allowed container escape — though not directly applicable to Chi, it illustrates how host-accessible paths in containers enable escape when combined with code execution.

Chi itself does not introduce escape risks, but its common use in microservices means APIs built with it often run in containers. If those containers are over-privileged or expose sensitive mounts, and the Chi application has injection flaws (e.g., in os/exec calls), the attack surface expands. middleBrick detects such risks by scanning for signs of command injection, path traversal, and unsafe patterns in unauthenticated endpoints — helping teams identify where container escape might be feasible post-exploitation.

The primary defense against container escape is not application-level code alone, but minimizing the blast radius if RCE occurs. For Go applications using Chi, this means avoiding patterns that could lead to command injection or path traversal, and ensuring the application runs with least privilege. Never pass user input directly to os/exec.Command without strict validation. Instead, use Go’s native libraries to avoid shell invocation entirely.

For example, consider a vulnerable Chi handler that attempts to resize an image based on user-provided dimensions:

// VULNERABLE: Direct shell command with unsanitized input
func resizeImageHandler(w http.ResponseWriter, r *http.Request) {
    width := r.URL.Query().Get("width")
    height := r.URL.Query().Get("height")
    cmd := exec.Command("convert", "input.jpg", "-resize", width+"x"+height, "output.jpg")
    if err := cmd.Run(); err != nil {
        http.Error(w, "Image processing failed", http.StatusInternalServerError)
        return
    }
    // ... serve output.jpg
}

An attacker could supply width=100; rm -rf / to attempt command injection. While this alone won’t escape the container, if the container runs as root with Docker socket access, further exploitation could lead to escape. The fix avoids shell metacharacters by validating input as integers and using argument separation:

// FIXED: Validate input, avoid shell, use native libraries where possible
func resizeImageHandler(w http.ResponseWriter, r *http.Request) {
    widthStr := r.URL.Query().Get("width")
    heightStr := r.URL.Query().Get("height")
    width, err1 := strconv.Atoi(widthStr)
    height, err2 := strconv.Atoi(heightStr)
    if err1 != nil || err2 != nil || width <= 0 || height <= 0 || width > 5000 || height > 5000 {
        http.Error(w, "Invalid dimensions", http.StatusBadRequest)
        return
    }
    
    // Use Go's imaging library instead of shelling out (example)
    img, err := imaging.Open("input.jpg")
    if err != nil {
        http.Error(w, "Could not open image", http.StatusInternalServerError)
        return
    }
    img = imaging.Resize(img, width, height, imaging.Lanczos)
    
    buf := new(bytes.Buffer)
    err = imaging.Encode(buf, img, imaging.JPEG)
    if err != nil {
        http.Error(w, "Encoding failed", http.StatusInternalServerError)
        return
    }
    
    w.Header().Set("Content-Type", "image/jpeg")
    w.Write(buf.Bytes())
}

Additionally, run the container as a non-root user, drop unnecessary capabilities (CAP_SYS_ADMIN, etc.), and avoid mounting /var/run/docker.sock or host paths unless absolutely required. middleBrick’s scan identifies risky endpoint patterns (like unsanitized input to system commands) and flags over-privileged container configurations indirectly through findings related to input validation and access controls — guiding teams toward safer deployment practices without requiring agents or configuration.