Unicode Normalization in Echo Go with Api Keys

Unicode normalization inconsistencies become significant in Go APIs built with Echo when API keys are involved in path or header processing. An API key that includes Unicode characters, or is compared after normalization of user-supplied input, can lead to authentication bypass or inconsistent authorization decisions. For example, an attacker might provide a key that is canonically equivalent but not byte-for-byte equal to the stored key (e.g., using composed vs decomposed forms), and if the application normalizes only one side, the comparison may incorrectly succeed or fail.

In Echo, routes and middleware often inspect path parameters or headers directly. If an API key is passed via a header such as X-API-Key and the handler normalizes the header value with golang.org/x/text/unicode/norm but does not normalize the stored key, or vice versa, the effective key used for lookup may differ from the expected key. This mismatch can either deny access erroneously or, in some constructions, permit an attacker who supplies a specially crafted, normalized-equivalent key to gain access.

Consider an Echo route that extracts an API key from the URL path to perform authorization. If the route contains a parameter like /v1/resource/{apikey} and the handler normalizes echo.Param("apikey") but stores keys in a precomposed form, an attacker could supply a decomposed variant that normalizes to the same logical key, bypassing exact-byte checks. Similarly, keys stored with non-ASCII characters (rare but possible in opaque test or legacy systems) can lead to inconsistent behavior across different normalization forms across client libraries and servers.

These issues map to authentication and authorization checks covered by middleBrick’s BOLA/IDOR and Authentication scans. When an API key is treated as an identifier whose normalization is inconsistent, it can result in authorization flaws that appear as logic vulnerabilities, where one user’s key unintentionally matches another’s effective normalized representation. The risk is especially pronounced when the API key is used both as a bearer token and as a lookup key in databases or in-memory indexes that may or may not normalize their strings.

To detect such issues, middleBrick runs checks that compare spec-defined security schemes with runtime behavior, including how keys are extracted, normalized, and compared. Findings highlight inconsistencies between declared authentication mechanisms and actual string handling, providing guidance to ensure canonical normalization on both sides of the comparison and to avoid normalization as a substitute for exact-byte matching of secrets.

Remediation centers on ensuring that API key comparison is performed on canonical, normalized forms consistently, and that normalization is not used as a replacement for secure storage and exact matching. Use a single normalization form (NFC is typical) for both stored keys and incoming values, and compare the normalized bytes directly. Avoid using the key for any indirect lookup that might be affected by normalization differences.

Example: normalize both the stored key and the incoming header using the same form, and compare with bytes.Equal to prevent timing attacks. Do not use the normalized key to query a map or database unless the stored keys are also normalized in the same form.

import (
    "bytes"
    "net/http"

    "github.com/labstack/echo/v4"
    "golang.org/x/text/unicode/norm"
)

// Assume storedKeys is a map from normalized key to user/role.
// Keys must be stored in NFC.
var storedKeys = map[string]string{
    "e4c6b2f1d8a74b9c8e3d2a1f0b9c8d7e": "admin",
}

func apiKeyMiddleware(next echo.HandlerFunc) echo.HandlerFunc {
    return func(c echo.Context) error {
        raw := c.Request().Header.Get("X-API-Key")
        if raw == "" {
            return echo.NewHTTPError(http.StatusUnauthorized, "missing key")
        }
        // Normalize to NFC once, for both comparison and lookup.
        normalized := norm.NFC.String(raw)
        expectedRole, ok := storedKeys[normalized]
        if !ok || !bytes.Equal([]byte(normalized), []byte(raw)) {
            // Ensure exact match after normalization to prevent bypass.
            return echo.NewHTTPError(http.StatusUnauthorized, "invalid key")
        }
        c.Set("role", expectedRole)
        return next(c)
    }
}

If your API keys are binary or opaque, avoid any textual normalization entirely. Treat them as byte sequences and compare with a constant-time function. Do not apply Unicode normalization to binary secrets.

When keys are passed in paths, ensure route parameters are not implicitly normalized by frameworks or middleware. Validate and compare the raw parameter with the stored key using the same canonical form, and avoid using the parameter as a database key without explicit normalization at write time.

Using middleBrick’s CLI, you can scan an Echo endpoint to surface inconsistencies between declared authentication and observed handling of API keys:

middlebrick scan https://api.example.com/openapi.json

The dashboard and GitHub Action integrations can be configured to fail builds or raise alerts when findings related to authentication and inconsistent normalization appear, helping maintain secure key handling across deployments.