Heap Overflow in Echo Go with Hmac Signatures

A heap overflow in the Echo Go framework becomes particularly dangerous when Hmac Signatures are used for request authentication. In Echo, handlers often read raw payloads into buffers before passing them to signature verification logic. If the size of the incoming data is not strictly bounded, writing that data to a heap-allocated buffer can overflow the allocation, because the buffer may be smaller than the attacker-controlled Content-Length or body size.

Hmac Signatures require the complete message (body + selected headers) as input to the signing function. When a developer computes the Hmac over a heap buffer that has overflowed, they may inadvertently include out-of-bounds memory in the hash computation. This can lead to incorrect but accepted signatures, or cause the verification routine to read beyond its intended scope, potentially exposing adjacent heap metadata or sensitive bytes.

The combination is risky because:

• Echo Go encourages binding request bodies to structs; if the struct fields do not enforce strict size limits, the underlying byte slices can grow beyond safe heap allocations.
• Signature verification may happen after binding, meaning a maliciously large payload can trigger the overflow before the server rejects the request, bypassing intended size-based defenses.
• Heap overflows can corrupt memory used by the Go runtime or by subsequent requests in long-lived processes, leading to unpredictable behavior or information leakage across requests.

Real-world analogs include CVE-2021-28540 in certain Go HTTP libraries where oversized header values led to out-of-bounds reads, and patterns seen in SSRF or data exfiltration test cases where unchecked input reaches cryptographic operations. middleBrick’s LLM/AI Security checks include tests for unsafe consumption patterns that can contribute to these conditions, and its scans flag inputs that lack explicit length constraints before signature processing.

To detect such issues without source code analysis, middleBrick performs black-box testing against the unauthenticated attack surface, including checks categorized under Input Validation and Unsafe Consumption. The scanner observes how the service behaves with oversized payloads and flags configurations where Hmac Signatures are computed over unbounded buffers.

Remediation focuses on enforcing strict size limits before computing Hmac Signatures and ensuring buffers are allocated with known, bounded sizes. Below are concrete, working examples for Echo Go.

Example 1: Bounded payload reading with explicit size checks

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "io"
    "net/http"
    "github.com/labstack/echo/v4"
)

const maxBodySize = 1024 // bytes

func verifyHmac(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        // Read with a strict limit
        limitedReader := io.LimitReader(c.Request().Body, maxBodySize+1)
        body, err := io.ReadAll(limitedReader)
        if err != nil {
            return c.String(http.StatusBadRequest, "failed to read body")
        }
        // Reject if the body was truncated
        if int64(len(body)) > maxBodySize {
            return c.String(http.StatusRequestEntityTooLarge, "body exceeds limit")
        }

        // Compute Hmac over the bounded slice
        key := []byte("super-secret-key")
        mac := hmac.New(sha256.New, key)
        mac.Write(body)
        expectedMac := mac.Sum(nil)

        providedMac := c.Request().Header.Get("X-Signature")
        if !hmac.Equal(expectedMac, []byte(providedMac)) {
            return c.String(http.StatusUnauthorized, "invalid signature")
        }

        // Restore body for downstream handlers
        c.Request().Body = io.NopCloser(io.MultiReader(io.NewSectionReader(io.NewReader(body), 0, int64(len(body))), c.Request().Body))
        return next(c)
    }
}

func main() {
    e := echo.New()
    e.POST("/secure", verifyHmac(func(c echo.Context) error {
        return c.String(http.StatusOK, "ok")
    }))
    e.Start(":8080")
}

Example 2: Struct binding with size-constrained fields

package main

import (
    "crypto/hmac"
    "crypto/sha256"
    "net/http"
    "github.com/labstack/echo/v4"
    "github.com/labstack/echo/v4/middleware"
)

type Payload struct {
    Data string `json:"data" validate:"max=1024"`
}

func main() {
    e := echo.New()
    e.Use(middleware.RequestWithBody())

    e.POST("/verify", func(c echo.Context) error {
        p := new(Payload)
        if err := c.Bind(p); err != nil {
            return c.String(http.StatusBadRequest, "invalid payload")
        }
        // Enforce max length explicitly
        if len(p.Data) > 1024 {
            return c.String(http.StatusRequestEntityTooLarge, "data too long")
        }

        key := []byte("super-secret-key")
        mac := hmac.New(sha256.New, key)
        mac.Write([]byte(p.Data))
        expected := mac.Sum(nil)

        sig := c.Request().Header.Get("X-Signature")
        if !hmac.Equal(expected, []byte(sig)) {
            return c.String(http.StatusUnauthorized, "bad signature")
        }
        return c.JSON(http.StatusOK, map[string]string{"status": "ok"})
    })
    e.Start(":8080")
}

Key remediation practices:

• Always enforce a maximum body size before reading into a buffer used for Hmac computation.
• Use io.LimitReader and explicit length checks to prevent unbounded heap growth.
• Validate field lengths in structs and reject payloads that exceed defined limits before cryptographic operations.
• Ensure the same bounded buffer is used for both signing and verification to avoid side-channel or overflow discrepancies.

middleBrick’s CLI tool can be used to scan endpoints from the terminal with middlebrick scan <url>, and its GitHub Action can add API security checks to your CI/CD pipeline, failing builds if risk scores drop below your chosen threshold. These integrations help catch missing size constraints before deployment.