Prototype Pollution in Echo Go with Api Keys

Prototype pollution in Go APIs often arises when user-controlled data is merged into a base object used to construct responses or validate requests. In Echo, this can occur when developers bind JSON payloads directly to generic map[string]interface{} structures and later use those maps to configure behavior or template rendering. If an attacker can inject properties such as __proto__, prototype, or constructor into the payload, and those properties are later read by code that influences object creation or access checks, the attacker may escalate their influence over the application logic.

When API keys are involved, the risk pattern becomes more specific: keys are commonly passed as headers or query parameters, deserialized into a request context, and then used to gate authorization or rate-limiting decisions. If the code merges request-scoped data (e.g., JSON body or URL query parameters) with configuration derived from the API key owner’s profile or permissions, an attacker who can pollute the merged object may try to modify effective permissions or bypass intended constraints. For example, a handler that binds a key lookup result into a map and then also binds user JSON into the same map creates a shared prototype chain that can be abused to overwrite inherited properties.

Consider an Echo route that looks up an API key and attaches account-level settings to the context, then binds a JSON body to the same context value without isolation:

// Risky: merging key-based settings with user body into the same map
func handler(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    settings, err := getSettingsByKey(key) // returns map[string]interface{}
    if err != nil {
        return echo.NewHTTPError(http.StatusUnauthorized)
    }
    var body map[string]interface{}
    if err := c.Bind(&body); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest)
    }
    // Danger: mutating the shared map that also holds settings
    for k, v := range body {
        settings[k] = v
    }
    // Later code may read settings["admin"] or settings["rateLimit"] influenced by attacker
    return c.JSON(http.StatusOK, settings)
}

If body contains {"__proto__": {"admin": true}}, and settings is a plain map that participates in prototype-style checks elsewhere (e.g., type assertions or inherited field resolution), the injected property may affect downstream logic. In Go, maps themselves do not have prototypes like JavaScript, but the pattern of conflating configuration maps with user data can still lead to privilege escalation or data exposure when the merged map is used inconsistently across packages or templates.

The LLM/AI Security checks in middleBrick specifically look for scenarios where system prompts or agent tooling could be influenced by uncontrolled data. While this does not apply directly to Go backend objects, the same principle applies: ensure that keys and runtime data are kept in separate, validated structures rather than merged into a single mutable object that downstream code treats as authoritative.

To prevent prototype-style pollution and privilege escalation when handling API keys in Echo, isolate key-derived data from user-controlled input, validate types strictly, and avoid mutating shared maps. Use distinct structures for configuration and request payloads, and enforce least-privilege access at the handler level.

1) Keep key-based settings in a dedicated, non-extensible struct and do not merge with user body:

// Safe: dedicated type prevents accidental property overwrite
type KeySettings struct {
    RateLimit int `json:"rateLimit"`
    Scopes    []string `json:"scopes"`
    Admin     bool   `json:"admin"`
}

func handler(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    settings, err := getSettingsByKey(key)
    if err != nil {
        return echo.NewHTTPError(http.StatusUnauthorized)
    }
    var body map[string]interface{}
    if err := c.Bind(&body); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest)
    }
    // Use settings as-is; do not merge with body
    // Validate body against expected shape separately
    return c.JSON(http.StatusOK, map[string]interface{}{
        "settings": settings,
        "input":    body,
    })
}

2) If you must work with maps, copy user input into a new map instead of mutating the key-derived map:

func handler(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    settings, err := getSettingsByKey(key)
    if err != nil {
        return echo.NewHTTPError(http.StatusUnauthorized)
    }
    var body map[string]interface{}
    if err := c.Bind(&body); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest)
    }
    // Safe: create a new map for combined output
    combined := make(map[string]interface{}, len(settings)+len(body))
    for k, v := range settings {
        combined[k] = v
    }
    for k, v := range body {
        // Optionally validate keys to block proto-like injections
        if k == "__proto__" || k == "constructor" || k == "prototype" {
            return echo.NewHTTPError(http.StatusBadRequest, "invalid key")
        }
        combined[k] = v
    }
    return c.JSON(http.StatusOK, combined)
}

3) Enforce strict schema validation on incoming JSON using libraries that reject unexpected top-level keys or enforce concrete structs:

// Define expected shape
type RequestPayload struct {
    Action string   `json:"action" validate:"required"`
    Target string   `json:"target" validate:"required"`
    Tags   []string `json:"tags"`
}

func handler(c echo.Context) error {
    key := c.Request().Header.Get("X-API-Key")
    settings, err := getSettingsByKey(key)
    if err != nil {
        return echo.NewHTTPError(http.StatusUnauthorized)
    }
    var req RequestPayload
    if err := c.Bind(&req); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest)
    }
    if err := validator.New().Struct(req); err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, err.Error())
    }
    // Use settings and validated req separately
    return c.JSON(http.StatusOK, map[string]interface{}{
        "settings": settings,
        "action":   req.Action,
    })
}

4) Use the CLI tool to scan your endpoints and detect risky patterns; integrate the GitHub Action to fail builds if risk scores degrade; for ongoing assurance, enable continuous monitoring via the Pro plan to catch regressions early.