Xml External Entities in Buffalo with Basic Auth

Buffalo is a popular Go web framework for building rapid web applications. When it parses XML payloads—typically from HTTP request bodies or uploaded files—it may use standard Go XML decoders that support external entity references. If an application does not explicitly disable external entity processing, an attacker can supply a malicious XML document that references an external entity, such as a local file or a network resource. This is an XML External Entity (XXE) injection issue, cataloged in the OWASP API Security Top 10 and relevant to API endpoints that accept XML.

When Basic Authentication is used, the request includes an Authorization header like Authorization: Basic base64(username:password). The presence of Basic Auth does not inherently prevent XXE; it only provides a transport-layer identity check. An attacker can first obtain or guess valid credentials, or rely on an endpoint that accepts unauthenticated requests but still processes XML with external entities. Once authenticated (or even without authentication if the endpoint is exposed), the attacker can send crafted XML that causes the server to read sensitive files (e.g., /etc/passwd), trigger SSRF against internal services, or leak internal configuration through DOCTYPE declarations. Because Buffalo applications often expose RESTful routes that accept XML, failing to secure the XML parser means that even protected routes using Basic Auth can be compromised via XXE if the parser is not hardened.

The risk is compounded when the API also supports OpenAPI/Swagger specs with external references or when debugging endpoints echo request details. An attacker may probe for unauthenticated LLM endpoints or inspect error messages returned by Buffalo’s default handlers; malformed XML can lead to verbose errors that disclose file paths or internal hostnames. While XXE is a server-side parsing issue, not a flaw in Buffalo itself, the framework’s conventions—like using middleware for authentication and relying on standard library XML unmarshaling—can inadvertently expose this vector if developers do not explicitly disable external entities. Tools like middleBrick can detect such misconfigures during a black-box scan, surfacing the XXE finding with severity and remediation guidance alongside the authentication method used.

Remediation focuses on two layers: hardening the XML parser and ensuring authentication does not create a false sense of security. For the XML layer, disable external entities and DTDs entirely. For the authentication layer, treat Basic Auth as a transport safeguard and never rely on it to prevent injection attacks. Below are concrete examples for a Buffalo application.

Hardening XML parsing in Buffalo

Create a custom XML decoder that explicitly disables external entities. In Go, this is done by setting EntityResolver to a function that rejects external references and by disabling DTD parsing through the xml.Decoder options available via the underlying encoding/xml package.

package actions

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

    "github.com/gobuffalo/buffalo"
)

// secureXMLDecoder returns a decoder that rejects external entities and DTDs.
func secureXMLDecoder(r io.Reader) *xml.Decoder {
    dec := xml.NewDecoder(r)
    dec.Entity = func(ctx context.Context, e xml.Entity) (xml.EntityReader, error) {
        // Reject all external entities by returning an error.
        return nil, xml.ErrUnsupportedEntity
    }
    // In standard library XML decoders, DTD processing is limited; ensure no external DTDs are fetched.
    // Avoid setting dec.CharsetReader to custom readers that could bypass restrictions.
    return dec
}

// UserProfile is an example model that might receive XML input.
type UserProfile struct {
    XMLName xml.Name `xml:"profile"`
    Name    string   `xml:"name"`
    Email   string   `xml:"email"`
}

// ProfileHandler demonstrates a secured Buffalo route expecting XML.
func ProfileHandler(c buffalo.Context) error {
    // Enforce authentication separately before parsing body.
    user, pass, ok := c.Request().BasicAuth()
    if !ok || !isValidUserPass(user, pass) {
        return c.Error(http.StatusUnauthorized, errors.New("invalid credentials"))
    }

    dec := secureXMLDecoder(c.Request().Body)
    var profile UserProfile
    if err := dec.Decode(&profile); err != nil {
        return c.Error(http.StatusBadRequest, errors.Wrap(err, "invalid XML"))
    }

    // Proceed with business logic using profile.Name and profile.Email.
    _ = profile
    return c.Render(200, r.HTML("profiles/show.html"))
}

// isValidUserPass is a placeholder for your actual user store lookup.
func isValidUserPass(user, pass string) bool {
    // In production, use constant-time comparison and a secure user store.
    return user == "admin" && pass == "s3cur3P@ss"
}

Middleware and route protection

Use Buffalo middleware to enforce authentication on sensitive routes and to reject requests with malformed headers before they reach business logic. Combine this with input validation to ensure that XML content types are explicitly required and that unexpected content types are rejected early.

package actions

import (
    "net/http"

    "github.com/gobuffalo/buffalo"
)

// AuthMiddleware ensures requests include valid Basic Auth.
func AuthMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        user, pass, ok := c.Request().BasicAuth()
        if !ok || !isValidUserPass(user, pass) {
            return c.Error(http.StatusUnauthorized, errors.New("auth required"))
        }
        return next(c)
    }
}

// ApplyAuth secures the profile route.
func ApplyAuth(app *buffalo.App) {
    app.GET("/profiles/{id}", ProfileHandler, AuthMiddleware)
}

Additional secure practices

• Do not echo raw XML error details to clients; use generic error messages to avoid information leakage.
• Validate and sanitize all input fields even after successful XML parsing.
• If you consume OpenAPI specs, ensure external references are not used to pull in malicious schemas; middleBrick can scan your endpoints to highlight such risks alongside authentication methods.

By combining a secured XML parser with disciplined authentication checks, Buffalo applications can mitigate XXE while still using Basic Auth for initial identity verification. Remember that middleBrick’s scans can verify these protections in the wild, providing findings with severity and remediation guidance to help maintain a strong security posture.