Xml External Entities in Buffalo

Buffalo applications are particularly vulnerable to XML External Entity (XXE) attacks when handling XML-based API requests or configuration files. Buffalo's default XML parsing behavior, inherited from Go's standard library, can inadvertently process external entities if not explicitly configured to reject them.

The most common attack vector occurs when Buffalo applications accept XML payloads for data exchange, especially in enterprise integrations or SOAP-based APIs. An attacker can craft a malicious XML document containing external entity references that trigger the parser to access internal files, make outbound network requests, or even cause denial of service through recursive entity expansion.

Consider this vulnerable Buffalo handler pattern:

func processXML(c buffalo.Context) error {
    var data MyStruct
    if err := c.Bind(&data); err != nil {
        return err
    }
    // Process XML data
    return c.Render(200, r.JSON(data))
}

The c.Bind() method in Buffalo uses Go's default XML decoder, which processes external entities by default. An attacker could send:

<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<data>
  <field>&xxe;</field>
</data>

This would cause the parser to read the contents of /etc/passwd and include it in the parsed structure. In Buffalo applications running on Linux servers, this often reveals sensitive information about system users and configurations.

Another Buffalo-specific scenario involves XML configuration files. Buffalo's pop package for database migrations and configuration can process XML files, and if these files are user-modifiable or come from untrusted sources, they become attack vectors. The same applies to Buffalo's grifts (CLI commands) that might process XML templates or configuration.

Server-side request forgery (SSRF) through XXE is particularly dangerous in Buffalo applications. An attacker can craft XML with external entity references pointing to internal services:

<!ENTITY xxe SYSTEM "http://internal-service:8080/secret">

This can expose internal API endpoints, database connection strings, or other sensitive internal services that should never be accessible from the outside.

Detecting XXE vulnerabilities in Buffalo applications requires examining both the code and runtime behavior. The first step is auditing all XML processing code paths in your Buffalo application.

Using middleBrick's scanner provides immediate visibility into XXE vulnerabilities. middleBrick's black-box scanning approach tests your Buffalo API endpoints without requiring access to source code. The scanner specifically looks for:

• XML parsing endpoints that don't disable external entity processing
• SOAP API endpoints that may be vulnerable to XXE
• Configuration file processing that accepts XML
• XML-based authentication mechanisms
• RSS/Atom feed processing endpoints

To scan a Buffalo application with middleBrick:

middlebrick scan https://your-buffalo-app.com/api/v1/process-xml

The scanner tests for XXE by attempting to access known sensitive files and internal network resources through XML payloads. It reports findings with severity levels and specific remediation guidance.

Code-level detection involves searching for XML processing patterns in your Buffalo codebase. Look for:

grep -r "xml.Unmarshal" . 
# or 
grep -r "c.Bind" . | grep -i xml

Pay special attention to handlers that accept XML content types:

func xmlHandler(c buffalo.Context) error {
    if c.Request().Header.Get("Content-Type") == "application/xml" {
        // This is a potential XXE vector
    }
}

middleBrick's continuous monitoring feature (Pro plan) can automatically rescan your Buffalo APIs on a schedule, alerting you if new endpoints are added that might be vulnerable to XXE attacks.

Securing Buffalo applications against XXE requires both code changes and configuration updates. The most critical fix is configuring Go's XML decoder to disable external entity processing.

Here's the secure approach for Buffalo handlers:

func secureXMLHandler(c buffalo.Context) error {
    decoder := xml.NewDecoder(c.Request().Body)
    // Disable external entity processing
    decoder.Entity = xml.HTMLEntity
    
    var data MyStruct
    if err := decoder.Decode(&data); err != nil {
        return c.Error(400, err)
    }
    
    return c.Render(200, r.JSON(data))
}

For Buffalo applications using the c.Bind() method, create a secure wrapper:

func secureBind(c buffalo.Context, v interface{}) error {
    decoder := xml.NewDecoder(c.Request().Body)
    decoder.Entity = xml.HTMLEntity // Disable external entities
    
    if err := decoder.Decode(v); err != nil {
        return err
    }
    return nil
}

Buffalo's middleware system makes it easy to apply XXE protection globally:

func XXEProtectionMiddleware(next buffalo.Handler) buffalo.Handler {
    return func(c buffalo.Context) error {
        if c.Request().Header.Get("Content-Type") == "application/xml" {
            // Replace the request body with a sanitized version
            c.Request().Body = ioutil.NopCloser(sanitizeXML(c.Request().Body))
        }
        return next(c)
    }
}

func init() {
    app := buffalo.New(buffalo.Options{
        // ... other options
    })
    
    app.Use(XXEProtectionMiddleware)
}

For SOAP APIs commonly used in enterprise Buffalo applications, consider using a library that explicitly disables XXE:

soapServer := soap.NewServer(soap.ServerConfig{
    XMLDecoder: func(r io.Reader) *xml.Decoder {
        decoder := xml.NewDecoder(r)
        decoder.Entity = xml.HTMLEntity
        return decoder
    },
})

Buffalo's configuration files (config/fizz.yml, database.yml) should be reviewed to ensure they don't process XML from untrusted sources. If XML configuration is necessary, validate it against schemas before processing.

The GitHub Action integration for middleBrick can automatically fail builds if XXE vulnerabilities are detected in your Buffalo application:

- name: Run middleBrick Security Scan
  uses: middlebrick/middlebrick-action@v1
  with:
    target: http://localhost:3000
    fail-on-severity: high
    token: ${{ secrets.MIDDLEBRICK_TOKEN }}

This ensures XXE vulnerabilities are caught before deployment to production environments.