Stack Overflow in Echo Go with Bearer Tokens

A common pattern in Echo Go services is to extract a Bearer token from the Authorization header and pass it to downstream services or use it for authorization checks. When this pattern interacts with resource-level authorization — such as fetching a user resource by ID from a database — it can lead to a BOLA/IDOR condition that is amplified by a Stack Overflow style payload. An attacker can send an extremely large numeric ID (e.g., a value exceeding 64-bit integer limits or crafted to exhaust memory during parsing) in the route /users/:id. If Echo Go does not validate the ID type and size before using it in a database query, the oversized payload can cause a Stack Overflow during request parsing or parameter binding, potentially crashing the worker or leading to unexpected behavior that bypasses authorization checks. Meanwhile, the Bearer token is still accepted as valid, so authentication succeeds while the authorization layer destabilizes due to the malformed input, allowing an unauthenticated or low-privilege context to trigger a crash or bypass. This combination turns a malformed input into an availability or logic bypass vector, because the service fails to validate input size and type before processing. The use of Bearer tokens does not mitigate this; if anything, an authenticated request with a valid token but malicious ID can make the failure harder to detect in logs because the request appears legitimate.

Consider an OpenAPI spec that defines a path /api/users/{userId} with a securitySchemes of type http and scheme bearer. A runtime handler in Echo Go might look like this insecure example:

func getUser(c echo.Context) error {
    userID := c.Param("userId")
    // Insecure: no validation of userID size or format
    var user User
    if err := db.Where("id = ?", userID).First(&user).Error; err != nil {
        return c.JSON(http.StatusInternalServerError, echo.Map{"error": "failed"})
    }
    return c.JSON(http.StatusOK, user)
}

If an attacker sends a path such as /api/users/99999999999999999999999999999999, the string-to-integer conversion during ORM use or JSON serialization can overflow internal stacks or buffers, leading to a Stack Overflow crash. Because the Bearer token is accepted, authentication passes, but the service becomes unreliable. The scanner detects this as an instability and highlights it alongside the exposed authorization boundary, mapping findings to the BOLA/IDOR and Input Validation checks.

Remediation focuses on strict input validation, type-safe parsing, and avoiding unsafe string-to-number conversions. Always validate and constrain IDs before using them in queries, and ensure that Bearer token validation remains independent of parameter handling. Do not rely on the token to imply safe input.

Secure Echo Go handler example with input validation and proper error handling:

func getUserSecure(c echo.Context) error {
    userIDStr := c.Param("userId")
    // Validate format and length before conversion
    if !isValidID(userIDStr) {
        return c.JSON(http.StatusBadRequest, echo.Map{"error": "invalid user id"})
    }
    userID, err := strconv.ParseInt(userIDStr, 10, 64)
    if err != nil {
        return c.JSON(http.StatusBadRequest, echo.Map{"error": "invalid user id"})
    }
    var user User
    if err := db.Where("id = ?", userID).First(&user).Error; err != nil {
        return c.JSON(http.StatusInternalServerError, echo.Map{"error": "failed"})
    }
    // Optional: ensure the requesting user has permission to view this user
    return c.JSON(http.StatusOK, user)
}

func isValidID(s string) bool {
    // Allow only digits, limit length to prevent overflow
    if len(s) < 1 || len(s) > 19 {
        return false
    }
    for _, r := range s {
        if r < '0' || r > '9' {
            return false
        }
    }
    return true
}

When using Bearer tokens, validate them explicitly via middleware and ensure token validation does not short-circuit parameter validation. For example, add an auth middleware that verifies the token and sets a user context, but still allow the handler to reject malformed IDs:

func AuthMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        auth := c.Request().Header.Get("Authorization")
        if auth == "" {
            return c.JSON(http.StatusUnauthorized, echo.Map{"error": "authorization header required"})
        }
        const prefix = "Bearer "
        if !strings.HasPrefix(auth, prefix) {
            return c.JSON(http.StatusUnauthorized, echo.Map{"error": "invalid authorization header format"})
        }
        token := strings.TrimPrefix(auth, prefix)
        if !isValidBearerToken(token) {
            return c.JSON(http.StatusUnauthorized, echo.Map{"error": "invalid token"})
        }
        // Set user claims in context for downstream use
        c.Set("user", extractClaims(token))
        return next(c)
    }
}

func isValidBearerToken(token string) bool {
    // Implement token validation (e.g., JWT verification)
    return len(token) > 10
}

These examples demonstrate concrete fixes: validate ID format and size, use int64 with bounded parsing, and keep Bearer token validation separate from business logic. This reduces the risk of instability from malformed inputs and clarifies authorization boundaries, which scanners will map to improved findings in BOLA/IDOR and Input Validation categories.