Xpath Injection in Echo Go with Bearer Tokens

XPath Injection occurs when untrusted input is concatenated into an XPath expression without proper escaping or parameterization, allowing an attacker to alter the query logic. In Echo Go, this risk is amplified when Bearer Tokens are handled as user-influenced data within authentication-sensitive XPath queries, such as selecting a user or token record from an XML or in-memory dataset based on token values.

Consider an example where an Echo Go service parses an XML document containing token metadata and uses an XPath expression to locate a node by a token supplied in an Authorization header. If the token value is directly interpolated into the XPath string, an attacker can modify the token to change the query path. For instance, a token like abc123 might be used in an expression like //token[@value='abc123']. An attacker could supply ' or 1=1 or 'a'='a, producing //token[@value='' or 1=1 or 'a'='a'], which can bypass intended scoping and return unintended nodes, potentially exposing or modifying sensitive token-related data.

This combination is particularly dangerous because Bearer Tokens are often high-value targets; compromising or enumerating token entries can aid in privilege escalation or data exposure. In Echo Go, if the XPath is built using string formatting or concatenation—such as fmt.Sprintf("//token[@value='%s']", tokenValue)—the application fails to neutralize special XPath characters like quotes, enabling path traversal or predicate manipulation. Even when the token is extracted from a header, middleware, or context, treating it as trusted input for XPath creates an injection vector.

Moreover, if the XML structure includes nested elements or uses namespaces, an attacker can craft token values that traverse unintended branches or reveal other nodes via union or wildcard expressions. Because XPath lacks native parameterized query support in many Go libraries, developers must manually escape quotes and normalize input, which is often overlooked. The risk is further increased when token values are logged or reflected, as injection payloads can obfuscate malicious activity in audit trails.

XPath Injection in this context aligns with OWASP API Top 10 categories such as Broken Object Level Authorization and Injection, and it can map to real-world attack patterns like those seen in CVE-related XML External Entity (XXE) or path traversal scenarios when XPath is used to locate resources. Since middleBrick tests input validation and authentication checks in parallel, it can detect whether token-derived XPath expressions are improperly constructed, highlighting the need for strict input sanitization and query parameterization.

To remediate XPath Injection when using Bearer Tokens in Echo Go, avoid string interpolation for XPath construction entirely. Instead, use defensive input handling and, where possible, switch to parameterized or compiled query approaches. Below are concrete code examples demonstrating secure handling.

1. Avoid XPath with token-derived paths; use map-based lookups

Restructure data access to avoid XPath for token-based selection. For example, load token metadata into a Go map keyed by token value, eliminating XPath injection risk:

// Safe alternative: use a map instead of XPath
type TokenMeta struct {
    Scope  string
    UserID string
}

var tokenStore = map[string]TokenMeta{
    "abc123": {Scope: "read", UserID: "u100"},
    "def456": {Scope: "write", UserID: "u200"},
}

func handler(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    token := strings.TrimPrefix(auth, "Bearer ")
    meta, exists := tokenStore[token]
    if !exists {
        return echo.NewHTTPError(http.StatusUnauthorized, "invalid token")
    }
    // Use meta.Scope, meta.UserID safely
    return c.JSON(map[string]interface{}{
        "scope": meta.Scope,
        "user":  meta.UserID,
    })
}

2. If XPath is required, escape input and validate format

If your use case requires XPath (for example, querying an XML document), escape single quotes by doubling them and reject tokens containing problematic characters. Do not rely on simple escaping alone; prefer compile-time queries or external libraries that support parameterized XPath.

// Example with input sanitization for XPath (use cautiously)
func sanitizeXPathToken(token string) (string, error) {
    if strings.ContainsAny(token, "'") {
        return "", errors.New("invalid token: contains quotes")
    }
    // Further validation: allow only alphanumeric and safe chars
    if matched, _ := regexp.MatchString(`^[A-Za-z0-9\-._~]+$`, token); !matched {
        return "", errors.New("invalid token format")
    }
    return token, nil
}

func handler(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    token := strings.TrimPrefix(auth, "Bearer ")
    safeToken, err := sanitizeXPathToken(token)
    if err != nil {
        return echo.NewHTTPError(http.StatusBadRequest, err.Error())
    }
    // Only use safeToken in XPath if absolutely necessary
    expr := fmt.Sprintf("//token[@value='%s']", safeToken)
    // Evaluate expr against XML document...
    return c.JSON(map[string]string{
        "xpath": expr,
    })
}

3. Use structured XML parsing instead of dynamic XPath

Parse XML with defined structs and select fields by token value using Go logic rather than building XPath expressions. This approach is robust and avoids injection:

// Define XML structure
type Tokens struct {
    XMLName xml.Name `xml:"tokens"`
    List    []Token  `xml:"token"`
}

type Token struct {
    Value  string `xml:"value,attr"`
    Scope  string `xml:"scope"`
    UserID string `xml:"userID"`
}

func handler(c echo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    token := strings.TrimPrefix(auth, "Bearer ")

    var tokens Tokens
    if err := xml.Unmarshal(data, &tokens); err != nil {
        return echo.NewHTTPError(http.StatusInternalServerError, "failed to parse")
    }

    for _, t := range tokens.List {
        if t.Value == token {
            return c.JSON(map[string]string{
                "scope": t.Scope,
                "user":  t.UserID,
            })
        }
    }
    return echo.NewHTTPError(http.StatusUnauthorized, "not found")
}

4. Leverage middleBrick for detection and guidance

Use the middleBrick CLI to scan your endpoints and identify XPath-related findings. Run middlebrick scan <url> to obtain a security risk score and prioritized remediation guidance. If you integrate scanning into your workflow, the Pro plan’s continuous monitoring and GitHub Action can help prevent regressions by failing builds when risk scores exceed your threshold. The MCP Server also allows AI coding assistants to trigger scans directly from your IDE, supporting secure development practices.