Xml Bomb in Buffalo (Go)

An XML bomb (also known as a billion laughs attack) leverages entity expansion in XML parsers to consume excessive memory and CPU. When using Go with the Buffalo web framework, developers often parse XML payloads as part of integrations (for example, consuming RSS feeds or processing legacy SOAP APIs). If the XML parser is not configured to limit entity expansion and depth, a malicious payload can cause the application to spike CPU usage and memory consumption, degrading performance or causing crashes.

Buffalo does not provide its own XML parsing middleware by default, but it is common to use Go standard library packages such as encoding/xml or third-party libraries within Buffalo handlers. For example, a handler that decodes an incoming XML request without constraints may inadvertently allow an XML bomb:

func XmlHandler(c buffalo.Context) error {
    decoder := xml.NewDecoder(c.Request().Body)
    var payload interface{}
    if err := decoder.Decode(&payload); err != nil {
        return c.Render(400, r.JSON(Error{Message: err.Error()}))
    }
    return c.Render(200, r.JSON(payload))
}

In this pattern, the decoder does not limit entity expansions or nesting depth. An attacker can send a small XML document that expands to gigabytes of data, exhausting server resources. Because Buffalo encourages rapid development and easy routing, it is important to apply secure parsing practices explicitly when working with XML input to avoid unintentionally exposing this attack surface.

Additionally, an XML bomb can be coupled with other attack patterns such as external entity injection (XXE) when DTDs are processed. Even though the primary goal here is resource exhaustion, the presence of an XML bomb endpoint in a Buffalo application can amplify the impact of related vulnerabilities by making the service more susceptible to denial-of-service conditions.

To mitigate XML bombs in Buffalo applications, control parser behavior by limiting entity expansion and nesting. The standard library’s xml.Decoder supports setting Entity and Strict fields, and you can wrap the decoder to enforce depth limits. Below is a secure handler example that disables external entities and restricts nesting depth:

func SecureXmlHandler(c buffalo.Context) error {
    const maxDepth = 32
    decoder := xml.NewDecoder(io.LimitReader(c.Request().Body, 10<<20)) // 10 MB limit
    decoder.Entity = xml.HTMLEntity // minimal predefined entities only
    decoder.Strict = false
    depth := 0
    decoder.CharsetReader = func(charset string, input io.Reader) (io.Reader, error) {
        return input, nil
    }
    var payload interface{}
    // Custom token reading to enforce depth
    for t, err := decoder.Token(); err != io.EOF; t, err = decoder.Token() {
        if err != nil {
            return c.Render(400, r.JSON(Error{Message: err.Error()}))
        }
        switch tok := t.(type) {
        case xml.StartElement:
            depth++
            if depth > maxDepth {
                return c.Render(400, r.JSON(Error{Message: "XML depth limit exceeded"}))
            }
        case xml.EndElement:
            depth--
        }
    }
    if err := decoder.Decode(&payload); err != nil {
        return c.Render(400, r.JSON(Error{Message: err.Error()}))
    }
    return c.Render(200, r.JSON(payload))
}

You can also use a third-party library such as github.com/clbanning/mxj/v2 with controlled options if you prefer a higher-level API. Another effective approach is to reject XML when JSON or alternative formats are acceptable, which reduces complexity and risk. For Buffalo applications that must accept XML, adding a request size limit and applying these parser hardening steps significantly reduces the likelihood of successful XML bomb attacks.

Finally, consider scanning your Buffalo API with middleBrick to detect endpoints that process XML and evaluate whether XML parsing is necessary. middleBrick’s scans run 5–15 seconds and include checks related to Input Validation and Unsafe Consumption, which can highlight risky parsing patterns and provide remediation guidance without requiring agents or credentials.