Xpath Injection in Buffalo with Bearer Tokens

XPath Injection is a server-side injection class where untrusted input is concatenated into an XPath expression, allowing an attacker to alter the logic of the query. In the Buffalo framework, this typically arises when building XPath queries in Go code using string concatenation or formatting rather than parameterized expressions. Bearer Tokens are commonly used for API authentication in Buffalo applications, often passed via the Authorization header as Bearer <token>. When a Buffalo application uses these tokens as part of an XPath lookup—such as selecting a user or role based on a token value—the risk emerges if the token or a derived value is directly interpolated into the XPath string.

Consider a scenario where an endpoint validates an incoming Bearer Token by querying an XML or in-memory data store using XPath. If the token is appended to the query without escaping or parameterization, an attacker can supply a token like ' or 1=1 or 'a'='a, which modifies the XPath predicate logic and potentially returns unauthorized data. This becomes a path traversal or privilege escalation issue when the query selects sensitive nodes (e.g., //users/role) and the attacker can change the predicate to always be true. The combination is particularly dangerous because Bearer Tokens are often treated as opaque values by developers, leading to a false sense of safety while they remain subject to injection when used in string-based XPath construction.

Buffalo applications that parse incoming headers and embed them directly into XPath expressions expose the unauthenticated attack surface. Even without authentication, an attacker can probe endpoints that use XPath over Bearer Token–like inputs, especially if the application reflects or logs the token in error messages. Because XPath does not have a standard parameterized query interface in Go’s standard library, developers must manually separate data from logic, for example by using dedicated XML parsing libraries that support compiled expressions or by avoiding XPath construction from untrusted input entirely. The OWASP API Top 10 category for Injection and Broken Object Level Authorization (BOLA) applies here, and findings from middleBrick scans often highlight such XPath-related issues when scanning Buffalo APIs that use token-derived values in query construction.

Remediation centers on never concatenating token values or any user-controlled data into XPath expressions. Instead, use library features that separate data from query structure, or avoid XPath for token-based lookups entirely. Below are concrete, idiomatic Go examples for a Buffalo application.

1) Avoid XPath for token validation; use structured mapping

If the Bearer Token is used to look up user roles or permissions, store mappings in a map or database keyed by the token (or by a token subject/claim) rather than querying XML with XPath.

package actions

import (
    "github.com/gobuffalo/buffalo"
    "net/http"
)

// Safe token-to-role lookup using a map (or database in production)
var roleByToken = map[string]string{
    "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9": "admin",
    "abc123def456":                              "user",
}

func ValidateToken(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    // Expect "Bearer <token>"
    const prefix = "Bearer "
    if len(auth) < len(prefix) || auth[:len(prefix)] != prefix {
        return c.Render(http.StatusUnauthorized, r.JSON(map[string]string{"error": "invalid_auth"}))
    }
    token := auth[len(prefix):]
    role, ok := roleByToken[token]
    if !ok {
        return c.Render(http.StatusForbidden, r.JSON(map[string]string{"error": "insufficient_scope"}))
    }
    // Use role safely downstream; no XPath involved
    c.Set("role", role)
    return c.Next()
}

2) If XPath is required, use compiled expressions and strict parameterization

When XPath is necessary (e.g., querying an XML document), use Go’s encoding/xml or a library that supports compiled XPath expressions with variable binding, and never interpolate strings.

package xpathutil

import (
    "bytes"
    "encoding/xml"
    "fmt"
    "strings"
)

// SafeXMLSelect retrieves a node value using a parameterized approach.
// It avoids XPath Injection by not concatenating token values into the query.
func SafeXMLSelect(data []byte, token string) (string, error) {
    // Example: find <user token="..."> and return role
    // Instead of building an XPath like //user[@token='...']/role via string concat,
    // we decode into a structured type.
    type User struct {
        XMLName xml.Name `xml:"user"`
        Token   string   `xml:"token,attr"`
        Role    string   `xml:"role"`
    }
    var users struct {
        XMLName xml.Name `xml:"users"`
        Users   []User   `xml:"user"`
    }
    if err := xml.Unmarshal(data, &users); err != nil {
        return "", fmt.Errorf("failed to unmarshal XML: %w", err)
    }
    for _, u := range users.Users {
        if u.Token == token {
            return u.Role, nil
        }
    }
    return "", fmt.Errorf("token not found")
}

// If you must use XPath-like queries, use a library that supports
// compiled expressions. Example using github.com/antchfx/xpath:
// expr, err := xpath.Compile(`//user[@token=$token]/role`)
// if err != nil { return "", err }
// iter := expr.Evaluate(xmlquery.NewTokenNode(root)).(xpath.NodeIterator)
// for iter.Next() { node := iter.Current() ... }

3) Validate and sanitize Authorization header format only

Ensure the header follows the Bearer scheme without attempting to use its value in XPath logic. Reject malformed inputs early.

package middleware

import (
    "net/http"
    "strings"
)

func BearerAuthMiddleware(next http.Handler) http.Handler {
    return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
        auth := r.Header.Get("Authorization")
        if auth == "" {
            http.Error(w, `{"error": "authorization_required"}`, http.StatusUnauthorized)
            return
        }
        parts := strings.Split(auth, " ")
        if len(parts) != 2 || strings.ToLower(parts[0]) != "bearer" {
            http.Error(w, `{"error": "invalid_auth_format"}`, http.StatusUnauthorized)
            return
        }
        // Pass token to context for later safe lookup; do not use in XPath
        ctx := context.WithValue(r.Context(), "token", parts[1])
        next.ServeHTTP(w, r.WithContext(ctx))
    })
}

4) General secure coding practices

• Never log raw Authorization headers in production; if logging is required, redact the token value.
• Use structured data (JSON, protobuf) instead of XML where possible to avoid XPath-style injection classes.
• Apply input validation on the Authorization header format and reject unexpected schemes.

These fixes ensure that Bearer Token handling in Buffalo does not introduce XPath Injection by design, aligning with secure coding practices and reducing exposure under frameworks like OWASP API Top 10 and relevant compliance mappings.