Xpath Injection in Fiber with Jwt Tokens

XPath Injection occurs when an attacker can influence an XPath expression used to query an XML document, enabling data extraction or bypass of access controls. In a Fiber application, this risk can intersect with JWT handling when token payloads or claims are placed into XML structures or when XPath is used to validate or extract data from token-related XML sources. Because XPath lacks parameterized queries, concatenating user-influenced values—such as roles or usernames extracted from a JWT—into an XPath expression allows attackers to alter logic, retrieve unintended nodes, or bypass authentication checks.

Consider a scenario where a Fiber service decodes a JWT, extracts a user role, and uses that role to build an XPath query against an XML document that stores permissions. If the role value is directly interpolated without sanitization or strict validation, an attacker can supply a crafted JWT with a role like " or "1"="1 to shift the predicate logic, potentially granting elevated access or exposing sensitive nodes. middleBrick testing across the 12 checks will flag this as an Injection-related finding with severity mapped to the data exposure risk when JWT-derived data reaches XML processing paths.

Another realistic pattern is an unauthenticated endpoint that exposes a public XML metadata document and accepts a token-derived parameter to filter results. If an attacker probes the endpoint with manipulated JWTs containing malicious XPath fragments, they can probe the XML structure and enumerate usernames or configuration details. Because the scan tests authentication surfaces and input validation in parallel, findings related to XPath Injection alongside JWT usage will highlight both the input handling gap and the missing controls around token-derived data entering XML queries.

middleBrick’s LLM/AI Security checks do not directly test XPath Injection, but the scanner’s input validation and property authorization tests exercise endpoints that process JWT claims and XML inputs, revealing combinations where token data reaches unsafe XPath evaluations. By correlating runtime behavior with spec definitions, the tool can surface these cross-category risks in the prioritized findings, providing remediation guidance that focuses on strict input validation, whitelisting allowed values, and avoiding dynamic XPath construction from JWT-derived content.

To mitigate XPath Injection when using JWTs in Fiber, ensure JWT claims are never directly interpolated into XPath expressions. Instead, validate and sanitize all data derived from tokens against strict allowlists, and use parameterized approaches or structured access patterns that avoid dynamic string construction. Below are concrete code examples for a Fiber route that decodes a JWT, extracts a role, and safely checks permissions without exposing XPath Injection risk.

First, install required packages and set up a secure JWT verification flow:

// main.go
package main

import (
	"context"
	"encoding/xml"
	"log"
	"net/http"

	"github.com/gofiber/fiber/v2"
	"github.com/gofiber/fiber/v2/middleware/jwt"
)

type PermissionsXML struct {
	XMLName xml.Name `xml:"permissions"`
	Roles   []Role   `xml:"role"`
}

type Role struct {
	Name string `xml:"name,attr"`
	Read bool   `xml:"read,attr"`
}

func main() {
	app := fiber.New()
	app.Use(jwt.New(jwt.Config{
		SigningKey: jwt.SigningKey{Key: []byte("your_secret_key")},
	}))

	app.Get("/resource", func(c *fiber.Ctx) error {
		user := c.Locals("user").(*jwt.Token)
		claims := user.Claims.(jwt.MapClaims)
		role := claims["role"]

		// Safe: use a map to avoid XPath entirely; if you must use XML, parse and match by exact allowed values
		perms, err := getPermissionsByRole(role)
		if err != nil {
			return c.Status(http.StatusForbidden).SendString("invalid role")
		}
		return c.JSON(perms)
	})

	log.Fatal(app.Listen(":3000"))
}

func getPermissionsByRole(role interface{}) (map[string]bool, error) {
	// Define an allowlist of roles; this prevents injection via token claims
	allowed := map[string]map[string]bool{
		"admin": {"read": true, "write": true},
		"user":  {"read": true, "write": false},
	}
	r, ok := role.(string)
	if !ok {
		return nil, "invalid role type"
	}
	if perms, ok := allowed[r]; ok {
		return perms, nil
	}
	return nil, "role not allowed"
}

If you must work with XML, parse it into structures and filter by exact allowed values rather than building XPath strings from JWT data:

// safexml.go
package main

import (
	"encoding/xml"
	"errors"
)

var permissionsDoc = `
<permissions>
  <role name="admin" read="true" write="true"/>
  <role name="user" read="true" write="false"/>
</permissions>
`

type PermissionsXML struct {
	XMLName xml.Name `xml:"permissions"`
	Roles   []Role   `xml:"role"`
}

type Role struct {
	Name  string `xml:"name,attr"`
	Read  bool   `xml:"read,attr"`
	Write bool   `xml:"write,attr"`
}

func getPermissionsSafe(role string) (map[string]bool, error) {
	var doc PermissionsXML
	if err := xml.Unmarshal([]byte(permissionsDoc), &doc); err != nil {
		return nil, err
	}
	for _, r := range doc.Roles {
		if r.Name == role {
			return map[string]bool{
				"read":  r.Read,
				"write": r.Write,
			}, nil
		}
	}
	return nil, errors.New("role not found")
}

Key remediation practices:

• Never concatenate JWT claims into XPath strings.
• Use allowlists to validate roles and claims from JWTs before any downstream use.
• Prefer structured data (JSON, Go structs) over XML when possible; if XML is required, parse and index by exact values rather than constructing dynamic queries.
• Apply strict input validation at the edge (Fiber middleware) so that only expected claim formats reach business logic.