Unicode Normalization in Echo Go with Bearer Tokens

Unicode normalization inconsistencies can create security risks when API endpoints in Echo Go compare bearer tokens without first normalizing input. In Go, the same logical string can have multiple binary representations due to combining characters, compatibility forms, and canonical equivalence. For example, the character é can be represented as a single code point U+00E9 or as a base e (U+0065) followed by a combining acute accent (U+0301). If your Echo Go API accepts bearer tokens in headers and does not normalize these values before comparison or storage, an attacker may supply a visually identical but differently encoded token that bypasses validation or enumeration logic.

Consider an Echo Go route that protects a resource using a bearer token check without normalization:

func verifyTokenFromHeader(c echo.Context) (string, error) {
    auth := c.Request().Header.Get("Authorization")
    // Expects: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9...
    prefix := "Bearer "
    if !strings.HasPrefix(auth, prefix) {
        return "", errors.New("invalid authorization header")
    }
    token := strings.TrimPrefix(auth, prefix)
    // Unsafe: direct comparison without normalization
    if token != expectedToken {
        return "", errors.New("invalid token")
    }
    return token, nil
}

If expectedToken is stored in its canonical NFC form, an attacker could provide the same logical token in NFD (or NFKC/NFKD). Because the comparison is byte-wise, the check may fail or, worse, an attacker might leverage subtle mismatches to probe valid tokens through timing differences or error-message leakage. This becomes critical when tokens are used as opaque identifiers that grant access to user-specific endpoints, as inconsistent normalization can expose authorization boundaries (BOLA/IDOR) by allowing access to another user’s resource if token comparisons are not deterministic.

In the context of security scanning, middleBrick runs 12 security checks in parallel, including Input Validation and Authorization, to detect such normalization-related inconsistencies during unauthenticated scans. It does not fix the logic, but its findings include remediation guidance to ensure tokens are compared in a canonical form. The scanner also checks OpenAPI/Swagger specs (2.0, 3.0, 3.1) with full $ref resolution, cross-referencing spec definitions with runtime behavior to highlight places where authentication surfaces may be inconsistently handled.

Additional risk can arise if token handling logic interacts with other components such as logging or caching. For example, failing to normalize before hashing or storing tokens may lead to duplicate entries for the same logical token, complicating revocation and audit. MiddleBrick’s checks for Data Exposure and Encryption help surface these weaknesses by analyzing how tokens are handled across the API surface. Its unique LLM/AI Security checks do not apply here, as this scenario focuses on standard bearer token handling rather than prompt injection or jailbreak testing.

To summarize, the combination of Unicode normalization variance and bearer token validation in Echo Go can undermine access control if tokens are compared without canonicalization. Attackers may exploit visually identical representations to bypass checks or infer token validity, making normalization a necessary part of secure token handling.

To remediate Unicode normalization issues with bearer tokens in Echo Go, normalize both the incoming token and the stored reference to a canonical form before comparison. Use the golang.org/x/text/unicode/norm package to apply NFC (Normalization Form Canonical Composition), which is widely recommended for security-sensitive comparisons. The following example demonstrates a secure approach:

import (
    "errors"
    "strings"
    "golang.org/x/text/unicode/norm"
    "github.com/labstack/echo/v4"
)

var expectedToken = "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9" // stored in NFC

func verifyTokenNormalized(c echo.Context) (string, error) {
    auth := c.Request().Header.Get("Authorization")
    prefix := "Bearer "
    if !strings.HasPrefix(auth, prefix) {
        return "", errors.New("invalid authorization header")
    }
    raw := strings.TrimPrefix(auth, prefix)
    // Normalize incoming token to NFC before comparison
    incoming := norm.NFC.String(raw)
    // Normalize expected token as well to ensure consistency
    expected := norm.NFC.String(expectedToken)
    if subtleCompare(incoming, expected) {
        return incoming, nil
    }
    return "", errors.New("invalid token")
}

// subtleCompare performs a constant-time comparison to avoid timing leaks
func subtleCompare(a, b string) bool {
    return subtle.ConstantTimeCompare([]byte(a), []byte(b)) == 1
}

Key points in this remediation:

• Normalize both sides to NFC using norm.NFC.String to ensure canonical binary representation.
• Use a constant-time comparison function such as subtle.ConstantTimeCompare to prevent timing attacks that could reveal token validity.
• Store the expected token in its normalized form in configuration or a secure store to avoid repeated normalization at runtime and to ensure consistency.

If your API relies on opaque tokens issued by an identity provider, ensure that the provider’s token representation is normalized consistently on your side. MiddleBrick’s CLI tool can be used to scan your Echo Go endpoints from the terminal with middlebrick scan <url>, producing JSON or text output that highlights the affected endpoints and maps findings to frameworks such as OWASP API Top 10 and SOC2. For teams that need ongoing assurance, the Pro plan adds continuous monitoring and CI/CD integration, including GitHub Action PR gates that can fail builds if risk scores exceed your configured thresholds.

Finally, when integrating into CI/CD or using the Web Dashboard to track scores over time, prioritize remediation for findings related to Authentication and Input Validation. This reduces the likelihood of bypasses due to encoding mismatches and ensures that bearer token handling remains robust across updates.