Xml External Entities in Buffalo with Hmac Signatures

XML External Entity (XXE) injection occurs when an application processes XML input that references external entities, potentially exposing local files, triggering SSRF, or causing denial of service. In Buffalo, this risk can intersect with Hmac Signatures when request validation is performed on XML-parsed data that includes signature parameters or when Hmac verification logic is applied to payloads parsed from untrusted XML.

Buffalo is a Go web framework. When a handler parses XML request bodies (for example, SOAP or custom XML formats) and then validates an Hmac signature extracted from those XML fields, an attacker can supply a malicious XML payload that modifies the parsed values used to compute the Hmac. Because XXE can read files or make internal requests, an attacker may influence the data that participates in Hmac computation, leading to signature bypass or data tampering that the server mistakenly treats as valid.

Consider an endpoint that expects an XML body with a <data> element and an <signature> element parsed from the XML. If the XML parser is vulnerable to XXE, an attacker can inject an external entity that reads a sensitive file and injects its contents into <data>. The server then recomputes the Hmac over the manipulated data and may incorrectly validate the signature if the attacker can also control or guess how the Hmac is derived from the tampered data. Even when the Hmac is computed on the attacker-influenced data, the server might trust the signature and treat the request as authenticated or authorized, leading to security bypass.

Additionally, if your Buffalo application exposes an unauthenticated endpoint that performs XML parsing and Hmac verification, the combination increases the unauthenticated attack surface. An attacker can probe the endpoint with crafted XML containing external entity references to learn internal network details via SSRF or file paths via XXE, while also observing how Hmac validation behaves with manipulated inputs. This can reveal implementation details that aid further attacks, such as discovering whether the Hmac covers specific fields or whether strict canonicalization is enforced.

In practice, the vulnerability is not introduced by Hmac itself, but by unsafe XML parsing combined with trust in data-derived signatures. If the XML parser resolves external entities and the application uses parsed fields in Hmac computation or comparison without strict validation, the integrity guarantee provided by Hmac can be undermined. Defense requires both secure XML handling and disciplined Hmac usage that does not rely on attacker-controlled data.

To mitigate XXE and ensure Hmac integrity in Buffalo, address XML parsing and signature handling separately and together. Use secure XML parsing that disables external entities and DTDs, and design Hmac verification to be resilient against data manipulation by validating and canonicalizing inputs before signing or verification.

First, configure the XML parser to prevent external entity resolution. In Go, this is typically done by customizing the XML decoder or using a library that disables entity expansion. Here is an example of secure XML parsing in a Buffalo action:

import (
    "encoding/xml"
    "io"
    "net/http"

    "github.com/gobuffalo/buffalo"
)

type SignedPayload struct {
    Data     string `xml:"data"`
    Signature string `xml:"signature"`
}

func parseXMLWithoutExternalEntities(r io.Reader) (*SignedPayload, error) {
    dec := xml.NewDecoder(r)
    // Disable external entity resolution in the XML decoder
    dec.Entity = xml.HTMLEntity
    // Optionally set a strict tokenizer behavior to reject DOCTYPE
    var payload SignedPayload
    if err := dec.Decode(&payload); err != nil {
        return nil, err
    }
    return &payload, nil
}

func ValidateHmac(payload *SignedPayload, secret string) bool {
    // Compute Hmac over the canonical data, not over raw attacker-influenced XML strings
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(payload.Data))
    expected := hex.EncodeToString(mac.Sum(nil))
    return hmac.Equal([]byte(expected), []byte(payload.Signature))
}

func MyHandler(c buffalo.Context) error {
    r := c.Request()
    payload, err := parseXMLWithoutExternalEntities(r.Body)
    if err != nil {
        return c.Render(400, r.String("Invalid XML"))
    }
    if !ValidateHmac(payload, "your-256-bit-secret") {
        return c.Render(401, r.String("Invalid signature"))
    }
    // Proceed with trusted payload.Data
    return c.Render(200, r.JSON(payload.Data))
}

Second, ensure Hmac verification does not rely on attacker-controlled fields. If you must include fields from XML, canonicalize and validate them before including them in the Hmac computation. For example, normalize whitespace, enforce strict schemas (XSD or equivalent), and reject unexpected elements or attributes. Here is a more robust approach that separates trusted metadata from signed content:

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

    "github.com/gobuffalo/buffalo"
)

type Envelope struct {
    XMLName xml.Name `xml:"Envelope"`
    Body    struct {
        SignedData string `xml:"signedData"`
        Signature  string `xml:"signature"`
    } `xml:"Body"`
}

func canonicalData(data string) string {
    // Normalize line endings and trim spaces as required by your protocol
    return strings.TrimSpace(data)
}

func VerifyRequest(r *http.Request, secret string) (string, error) {
    dec := xml.NewDecoder(r.Body)
    dec.Entity = xml.HTMLEntity
    var env Envelope
    if err := dec.Decode(&env); err != nil {
        return "", err
    }
    canonical := canonicalData(env.Body.SignedData)
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    expected := hex.EncodeToString(mac.Sum(nil))
    if !hmac.Equal([]byte(expected), []byte(env.Body.Signature)) {
        return "", http.ErrAbortHandler
    }
    return canonical, nil
}

func SecureHandler(c buffalo.Context) error {
    data, err := VerifyRequest(c.Request(), "your-256-bit-secret")
    if err != nil {
        return c.Render(400, r.String("Bad request"))
    }
    return c.Render(200, r.String(data))
}

Third, apply defense-in-depth: disable external entities at the parser level, avoid including sensitive or mutable data in signed payloads, and use strict content-type and charset specifications. If your application supports both XML and JSON, ensure Hmac handling is consistent across formats and that XML-specific risks are not present when JSON is used.

Finally, test your implementation with crafted XXE payloads and Hmac tampering attempts to confirm that external entities are rejected and signatures fail when data is altered. Combine these measures with runtime scanning using tools like middleBrick to detect residual risks in unauthenticated attack surfaces and validate that your Hmac-based protections hold against real-world attack patterns.