Xxe Oob in Buffalo (Go)

XXE (XML External Entity) attacks occur when an application parses XML input that references external entities. In Buffalo, a Go web framework, this risk arises when XML payloads are processed without disabling external entity resolution. Go’s standard encoding/xml parser resolves external DTDs by default, allowing an attacker-controlled XML body to trigger out-of-band (OOB) interactions such as SSRF or file reads. When a Buffalo application exposes an XML endpoint—often through REST APIs consuming untrusted XML—an attacker can embed entity declarations and system identifiers to force the server to make outbound requests or access local files.

OOB techniques rely on the server initiating network connections to exfiltrate data via protocols like HTTP or file://. With Buffalo, if the application uses xml.NewDecoder directly on an io.ReadCloser from the request body, the parser follows external entity references and can reach attacker-chosen endpoints. This exposes internal services and bypasses network-level restrictions that would otherwise prevent direct access. Because Buffalo applications often bind to public interfaces and handle diverse payloads, misconfigured XML parsing becomes a viable path for unauthorized data exposure or SSRF.

Using middleBrick’s scan API provides visibility into such XML-specific weaknesses. By submitting the Buffalo service URL, the scanner runs checks including Input Validation and SSRF in parallel, identifying unsafe XML configurations without requiring credentials. The report maps findings to frameworks like OWASP API Top 10 and provides remediation guidance, helping teams understand how XXE OOB manifests in their routing and handler code.

To mitigate XXE OOB in Buffalo, restrict XML parser behavior by disabling external entities and DTDs. Use a custom xml.Decoder with a resolver that rejects external references and avoid passing raw request bodies directly into XML parsing logic. Below are concrete Go examples tailored for Buffalo handlers.

Secure XML parsing with disabled external entities

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

    "github.com/gobuffalo/buffalo"
)

func safeXMLHandler(c buffalo.Context) error {
    // Limit the size of the incoming body to prevent resource exhaustion
    lr := &io.LimitedReader{R: c.Request().Body, N: 10 << 20} // 10 MiB
    defer c.Request().Body.Close()

    decoder := xml.NewDecoder(lr)
    // Disable external entity resolution in Go's XML parser
    decoder.Entity = xml.HTMLEntity
    decoder.CharsetReader = func(charset string, input io.Reader) (io.Reader, error) {
        return input, nil
    }

    var payload struct {
        Field string `xml:"field"`
    }
    if err := decoder.Decode(&payload); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": err.Error()}))
    }

    // Process payload.Field safely
    return c.Render(200, r.JSON(map[string]string{"received": payload.Field}))
}

The key steps are:

• Use an io.LimitedReader to cap request body size and mitigate DoS.
• Set decoder.Entity = xml.HTMLEntity to prevent resolution of general entities, which blocks external DTD expansion.
• Avoid xml.Unmarshal with untrusted data; prefer streaming decoding with strict limits.

Alternative: Use a strict, non-external parser

import (
    "encoding/json"
    "github.com/gobuffalo/buffalo"
)

// If you control the API contract, prefer JSON over XML to avoid XML-specific risks.
func jsonHandler(c buffalo.Context) error {
    var body map[string]interface{}
    if err := json.NewDecoder(c.Request().Body).Decode(&body); err != nil {
        return c.Render(400, r.JSON(map[string]string{"error": "invalid JSON"}))
    }
    return c.Render(200, r.JSON(body))
}

For Buffalo applications, middleBrick’s GitHub Action can be integrated into CI/CD to enforce that new endpoints do not introduce unsafe XML handling. The action fails builds if scans detect high-severity findings, complementing code-level fixes with pipeline gates.