Prototype Pollution in Echo Go with Hmac Signatures

Prototype pollution in Go handlers using the Echo framework becomes more nuanced when Hmac Signatures are involved. In typical Echo applications, developers bind incoming JSON payloads directly to structs or maps. If the application merges user-supplied objects into a prototype (for example, using mapstructure or iterating with for key, value := range), an attacker can supply keys like __proto__, constructor, or other special properties to modify the behavior of shared objects or downstream logic.

When Hmac Signatures are used—commonly to verify the integrity of query parameters, form fields, or webhook payloads—the risk shifts to how the signature is computed and validated. If the server includes user-controlled fields in the data that is hashed for the Hmac, an attacker can attempt to inject additional properties that change the canonical representation of the object. For example, if the server builds a string or byte representation of a map for signing by iterating over keys in an uncontrolled order, the same payload with a polluted prototype may serialize differently, leading to signature mismatch or, worse, to the server accepting a modified object because the pollution changes how the comparison is performed.

Consider an Echo route that validates an Hmac signature over JSON body fields without strict schema enforcement:

func handleWebhook(c echo.Context) error {
    m := make(map[string]interface{})
    if err := c.Bind(&m); err != nil {
        return err
    }
    secret := []byte(os.Getenv("WEBHOOK_SECRET"))
    receivedSig := c.Request().Header.Get("X-Signature")
    body, _ := json.Marshal(m)
    mac := hmac.New(sha256.New, secret)
    mac.Write(body)
    expectedSig := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
        return c.String(http.StatusUnauthorized, "invalid signature")
    }
    // further processing of m
    return nil
}

If an attacker sends a payload that includes a polluted key such as { "__proto__": { "admin": true }, "data": "legit" }, and the application later iterates over m to build business logic, the polluted property may affect behavior before or after the Hmac check. Even if the Hmac itself remains valid because the attacker cannot forge the signature without the secret, the polluted object may still cause unintended authorization or logic changes after validation, especially if the server uses type assertions or reflection that follow the prototype chain.

In the context of the 12 security checks run by middleBrick, Prototype Pollution is categorized alongside Input Validation and BOLA/IDOR. The scanner does not assume the presence of Hmac Signatures; it flags cases where user input is merged into objects or used in reflection without canonicalization, and it highlights weak signature construction such as non-deterministic key ordering or inclusion of untrusted fields in the signed payload. These findings align with the OWASP API Top 10 and common weaknesses in webhook and API authentication designs.

Remediation focuses on strict input handling, canonical serialization, and avoiding the propagation of untrusted keys into application logic. For Echo handlers, define an explicit struct for expected fields and reject any additional properties. Compute the Hmac over a deterministic representation—such as a sorted JSON object or a concatenated string of known fields—so that prototype pollution cannot alter the serialized bytes used for signing.

Example of a secure handler that rejects extra fields and uses canonical JSON for Hmac verification:

type WebhookPayload struct {
    Event string `json:"event" validate:"required"`
    Data  string `json:"data" validate:"required"`
}

func handleWebhookSecure(c echo.Context) error {
    var p WebhookPayload
    if err := c.Bind(&p); err != nil {
        return err
    }
    if err := validator.New().Struct(p); err != nil {
        return c.String(http.StatusBadRequest, "invalid payload")
    }

    secret := []byte(os.Getenv("WEBHOOK_SECRET"))
    receivedSig := c.Request().Header.Get("X-Signature")

    canonical, err := json.Marshal(p)
    if err != nil {
        return c.String(http.StatusInternalServerError, "serialization error")
    }

    mac := hmac.New(sha256.New, secret)
    mac.Write(canonical)
    expectedSig := hex.EncodeToString(mac.Sum(nil))

    if !hmac.Equal([]byte(expectedSig), []byte(receivedSig)) {
        return c.String(http.StatusUnauthorized, "invalid signature")
    }

    // safe processing of p.Event and p.Data
    return nil
}

If you must accept dynamic keys, normalize the map by removing any keys that could influence prototype behavior (e.g., strings like __proto__, constructor, prototype) and enforce a deterministic JSON marshal using a sorted key encoder. Do not include user-controlled fields in the signed payload unless they are strictly typed and validated.

For continuous assurance, middleBrick’s CLI can be integrated into scripts or the GitHub Action to scan endpoint behavior and flag inconsistent signature schemes. The Pro plan enables continuous monitoring so that any change in request surface—such as new parameters that could affect canonicalization—triggers an alert before deployment.