Xpath Injection in Buffalo with Hmac Signatures

XPath Injection occurs when untrusted data is concatenated into an XPath expression without proper sanitization or parameterization, allowing an attacker to alter the query logic. In Buffalo, a Go web framework, this typically arises when building XML or SOAP requests that include security tokens implemented via Hmac Signatures. If the signature value or related parameters (such as timestamp or nonce) are reflected into an XPath context—often through XML external entity (XXE) payloads or data-driven XML parsing—the attacker can escape the intended node selection and read arbitrary XML content, bypassing authentication or extracting sensitive configuration.

Consider a Buffalo application that uses Hmac Signatures to sign request parameters and then validates the signature by looking up the associated user or certificate stored in an XML document. If the application constructs an XPath expression by string concatenation, for example selecting a node by username directly interpolated from user input, an attacker can supply a payload like ' or 1=1 or 'a'='a to manipulate the predicate. Because the Hmac Signature may be verified before or after the XPath step, an attacker can leverage a crafted XML payload to retrieve sibling nodes or the entire document, leading to Data Exposure. The presence of Hmac Signatures does not inherently prevent XPath Injection; if the signature is computed over a canonical representation but the runtime XPath uses unescaped input, the security control can be bypassed. This maps to the OWASP API Top 10 category of Broken Object Level Authorization (BOLA) and Input Validation weaknesses, and can be surfaced by middleBrick’s XPath-related checks under Property Authorization and Input Validation.

In practice, a Buffalo handler that parses an XML body and selects nodes based on user-supplied identifiers is at risk. For instance, an attacker may send an XML payload with embedded malicious entity references to exploit external entity expansion (XXE) and exfiltrate files such as /etc/passwd when the XPath engine resolves the external reference. If the handler also verifies an Hmac Signature provided in a header, the signature may validate successfully due to a weak canonicalization choice, while the XPath traversal remains vulnerable. middleBrick’s checks for Data Exposure and Input Validation, run in parallel with its LLM/AI Security probes, can identify such patterns in the OpenAPI spec and runtime behavior, highlighting where untrusted data reaches XPath-like selection logic even when Hmac Signatures are present.

To remediate XPath Injection in Buffalo while preserving Hmac Signatures, ensure that all XPath construction uses parameterized or compiled expressions and that input is validated before being used in XML parsing or XPath evaluation. Avoid string concatenation for XPath selectors; instead, use XPath expressions with placeholders and bind variables via a library that supports safe evaluation. Hmac Signatures should be verified on a canonical representation of the request, and sensitive data should not be retrieved via dynamic XPath that depends on attacker-controlled values.

Below are concrete code examples for a Buffalo handler that signs requests with Hmac and safely processes XML without exposing XPath Injection.

Secure Hmac Signature Verification and XPath Usage in Buffalo

package actions

import (
	"crypto/hmac"
	"crypto/sha256"
	"encoding/base64"
	"encoding/xml"
	"net/http"
	"strings"

	"github.com/gobuffalo/buffalo"
)

// VerifyHmac checks the Hmac Signature header against a canonical string.
func VerifyHmac(payload, receivedSig, secret string) bool {
	h := hmac.New(sha256.New, []byte(secret))
	h.Write([]byte(payload))
	expected := base64.StdEncoding.EncodeToString(h.Sum(nil))
	return hmac.Equal([]byte(expected), []byte(receivedSig))
}

// CanonicalizeRequest creates a canonical representation for signing.
func CanonicalizeRequest(params map[string]string) string {
	var b strings.Builder
	keys := []string{"timestamp", "nonce", "action"}
	for _, k := range keys {
		if v, ok := params[k]; ok {
			b.WriteString(k)
			b.WriteString("=")
			b.WriteString(v)
			b.WriteString("&")
		}
	}
	return strings.TrimSuffix(b.String(), "&")
}

// SafeXMLHandler demonstrates secure handling with Hmac and safe XPath alternatives.
type UserCredentials struct {
	XMLName xml.Name `xml:"credentials>user"`
	Name    string   `xml:"name,attr"`
	Token   string   `xml:"token"`
}

type CredentialsList struct {
	XMLName xml.Name    `xml:"credentials"`
	Users   []UserCredentials `xml:"user"`
}

// LookupUserToken retrieves a token without constructing dynamic XPath from input.
func LookupUserToken(users []UserCredentials, targetName string) (string, bool) {
	for _, u := range users {
		if u.Name == targetName {
			return u.Token, true
		}
	}
	return "", false
}

// Handler using Buffalo context.
func SecureSignInHandler(c buffalo.Context) error {
	// Assume XML body is parsed into CredentialsList via xml.Unmarshal.
	var creds CredentialsList
	if err := xml.Unmarshal([]byte(c.Request().PostFormValue("xml_body")), &creds); err != nil {
		return c.Error(http.StatusBadRequest, err)
	}

	username := c.Params().Get("username")
	signature := c.Request().Header.Get("X-Hmac-Signature")

	// Build canonical payload for Hmac verification.
	params := map[string]string{
		"timestamp": c.Params().Get("timestamp"),
		"nonce":     c.Params().Get("nonce"),
		"action":    "signin",
	}
	canonical := CanonicalizeRequest(params)
	if !VerifyHmac(canonical, signature, "your-secret-key") {
		return c.Error(http.StatusUnauthorized, errors.New("invalid signature"))
	}

	// Safe lookup: no XPath built from username; iterate in memory.
	token, found := LookupUserToken(creds.Users, username)
	if !found {
		return c.Error(http.StatusNotFound, errors.New("user not found"))
	}

	// Proceed with token usage, ensuring no further XPath usage on user input.
	_ = token
	return c.Render(http.StatusOK, r.JSON(map[string]string{"token": token}))
}

If you must use XPath on XML data, use a compiled, namespace-aware XPath object with constant expressions and bind values via a type-safe API rather than string interpolation. Never include raw request parameters in the expression string. middleBrick’s scans can help detect remaining risks by analyzing your OpenAPI spec and runtime behavior for improper data flows into XML parsing or XPath-like operations.