Container Escape in Echo Go with Hmac Signatures

A container escape in the context of an Echo Go API using HMAC signatures occurs when an attacker who can influence or forge HMAC-signed requests is able to break out of the intended execution context and access the host or other containers. This typically maps to the BOLA/IDOR and Property Authorization checks in middleBrick, and can lead to sensitive data exposure or further lateral movement.

HMAC signatures are designed to guarantee integrity and authenticity: a server verifies that a request was signed with the expected shared secret. In Echo Go, if the signature is computed over an insufficient or mutable subset of the request—such as only the request body or only selected headers—an attacker may manipulate unsigned or weakly verified parameters (for example, a path or a header used for routing) to change the effective handler or the interpreted tenant/container. If the application uses the same key across containers or fails to bind the signature to a container-specific context (like a namespace or label), a forged request may be processed in a higher-privilege container than intended.

Consider an endpoint that processes container-specific operations using a path parameter like /containers/:id/exec. If the server computes the HMAC over the request body but does not include the :id path parameter in the signed data, an attacker can reuse a valid signature from one container while changing the ID to target another container. In a multi-tenant setup where each tenant’s workloads run in separate containers, this can enable an attacker to execute actions in a different tenant’s container, effectively achieving a container escape. Compounding this, if the application deserializes user-controlled values into structs without strict validation (e.g., using c.Bind without whitelisting fields), an attacker may inject fields that change routing or execution context, escalating privileges across container boundaries.

Echo Go applications that expose unauthenticated or weakly authenticated endpoints alongside HMAC-signed routes may inadvertently expose container escape risks during runtime. middleBrick scans detect such patterns by correlating OpenAPI/Swagger specifications with runtime behavior: it checks whether signature coverage aligns with authorization-sensitive parameters and flags cases where container or tenant identifiers are not part of the signed payload. These findings map to OWASP API Top 10 controls and common misconfigurations that enable vertical or horizontal privilege escalation across containerized workloads.

To remediate container escape risks when using HMAC signatures in Echo Go, ensure the signature covers all data that influences routing, authorization, and container context. The following patterns demonstrate secure approaches.

Example 1: Signing path, headers, and body together

Compute the HMAC over a canonical string that includes the HTTP method, the full path (including container identifiers), selected headers, and the request body. This prevents an attacker from swapping the container ID or other parameters without breaking the signature.

import (
	"crypto/hmac"
	"crypto/sha256"
	"fmt"
	"net/http"
	"strings"

	"github.com/labstack/echo/v4"
)

const sharedSecret = "your-256-bit-secret"

func computeHMAC(method, path, headers, body string) string {
	mac := hmac.New(sha256.New, []byte(sharedSecret))
	mac.Write([]byte(method + "\n" + path + "\n" + headers + "\n" + body + "\n"))
	return fmt.Sprintf("%x", mac.Sum(nil))
}

func VerifySignature(next echo.HandlerFunc) echo.HandlerFunc {
	return func(c echo.Context) error {
		req := c.Request()
		bodyBytes, _ := c.Get("requestBody").([]byte) // assume body is captured via middleware
		provided := c.GetHeader("X-API-Signature")
		if provided == "" {
			return c.String(http.StatusUnauthorized, "missing signature")
		}

		// Build canonical representation
		canonical := computeHMAC(
			req.Method,
			req.URL.Path,
			req.Header.Get("X-Tenant-ID")+"|"+req.Header.Get("Content-Type"),
			string(bodyBytes),
		)

		if !hmac.Equal([]byte(canonical), []byte(provided)) {
			return c.String(http.StatusForbidden, "invalid signature")
		}
		return next(c)
	}
}

func SecureHandler(c echo.Context) error {
	return c.String(http.StatusOK, "OK")
}

func main() {
	e := echo.New()

	// Middleware to capture body for HMAC verification
	e.Use(func(next echo.HandlerFunc) echo.HandlerFunc {
		return func(c echo.Context) error {
			buf := new(strings.Builder)
			tee := io.TeeReader(c.Request().Body, buf)
			defer c.Request().Body.Close()
			bodyBytes, _ := io.ReadAll(tee)
			c.Set("requestBody", bodyBytes)
			c.Request().Body = io.NopCloser(strings.NewReader(buf.String()))
			return next(c)
		}
	})

	e.POST("/containers/:id/exec", VerifySignature(SecureHandler))
	e.Logger.Fatal(e.Start(":8080"))
}

Example 2: Binding with strict field selection and including container ID

When using c.Bind, avoid binding all fields automatically. Instead, parse into a controlled struct that includes the container identifier and validate each field. Then incorporate the container ID into the HMAC computation to bind the signature to that specific container.

type ExecRequest struct {
	ContainerID string `json:"container_id" validate:"required"`
	Command     string `json:"command" validate:"required,max=256"`
	// other strictly allowed fields
}

func VerifyAndBind(next echo.HandlerFunc) echo.HandlerFunc {
	return func(c echo.Context) error {
		req := c.Request()
		bodyBytes, _ := io.ReadAll(req.Body)
		defer req.Body.Close()

		var payload struct {
			ContainerID string `json:"container_id"`
			Command     string `json:"command"`
			Signature   string `json:"signature"`
		}
		if err := json.Unmarshal(bodyBytes, &payload); err != nil {
			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid json"})
		}

		// Recompute HMAC over the canonical subset that includes ContainerID
		canonical := computeHMAC(
			req.Method,
			req.URL.Path,
			payload.ContainerID,
			payload.Command,
		)

		if !hmac.Equal([]byte(canonical), []byte(payload.Signature)) {
			return c.JSON(http.StatusForbidden, map[string]string{"error": "invalid signature"})
		}

		// Now bind only allowed fields to a validated struct
		var execReq ExecRequest
		if err := json.Unmarshal(bodyBytes, &execReq); err != nil {
			return c.JSON(http.StatusBadRequest, map[string]string{"error": "invalid payload"})
		}
		if execReq.ContainerID != payload.ContainerID {
			return c.JSON(http.StatusBadRequest, map[string]string{"error": "container ID mismatch"})
		}

		// Proceed with execReq which is constrained and verified
		return next(c)
	}
}

General recommendations

• Include container identifiers (namespace, pod name, tenant ID) in the HMAC input to bind the signature to the intended execution context.
• Use hmac.Equal to perform constant-time comparison and prevent timing attacks.
• Apply strict schema validation on incoming requests and avoid binding arbitrary JSON fields to structs.
• Rotate shared secrets periodically and use distinct keys per container or tenant where feasible.

These changes reduce the risk of container escape by ensuring that any change to container context invalidates the signature, which middleBrick can verify by checking signature coverage against the API specification and runtime behavior.