Xml External Entities in Buffalo with Cockroachdb

XML External Entity (XXE) injection occurs when an application processes XML input containing references to external entities that are resolved in a way that exposes local files, triggers remote requests, or leads to SSRF. In the Buffalo web framework, XML payloads may be accepted when developers use Go XML parsing libraries such as encoding/xml without disabling external entity resolution. When such an endpoint interacts with CockroachDB — a distributed SQL database commonly used in production backends — the impact can be amplified because the database connection strings, schema definitions, or exported data may be exposed through file reads or outbound HTTP calls triggered by the XXE.

Consider a Buffalo handler that reads an XML configuration upload and unmarshals it into a struct. If the XML parser is not configured to ignore external entities, an attacker-supplied DOCTYPE can force the parser to read files like /etc/cockroachdb/certs/ca.pem or application environment files that contain CockroachDB connection parameters. Because CockroachDB often requires TLS certificates and specific connection strings, reading these files can reveal hostnames, usernames, and certificate paths that facilitate further exploitation, such as SSRF against the database’s HTTP status endpoint or unauthorized SQL execution via a compromised driver configuration.

During a middleBrick scan of a Buffalo application that accepts XML and communicates with CockroachDB, the unauthenticated black-box checks include input validation and data exposure tests. The scanner submits crafted XML with external entity definitions and checks whether the parser resolves them, whether files are returned, and whether any outbound HTTP calls occur. If the application uses a standard xml.NewDecoder without Decoder.Entity restrictions, the scan will flag the endpoint and map the finding to relevant compliance frameworks such as OWASP API Top 10 and PCI-DSS, noting that sensitive CockroachDB-related artifacts may be disclosed through improperly handled XML.

Additionally, if the parsed XML is later used to construct database queries or configuration, the exposed data can lead to privilege escalation or data exposure. For example, a leaked CockroachDB username with read permissions might allow an attacker to enumerate other databases, while a leaked certificate path can aid in setting up man-in-the-middle scenarios. The scanner’s data exposure checks look for indicators such as file reads during XML processing and any indication that sensitive configuration material related to CockroachDB is being returned in responses or logged.

Because Buffalo encourages rapid development, developers may skip explicit parser hardening when handling XML. The framework does not enforce secure defaults for XML unmarshaling, so it is the developer’s responsibility to disable external entities. A typical vulnerable pattern involves using xml.Unmarshal directly on user-supplied byte slices. middleBrick’s findings in such cases include clear remediation guidance: switch to a parser configuration that disables DTDs and external entities, and validate all XML against a strict schema before processing.

To prevent XXE in a Buffalo application that interacts with CockroachDB, ensure that any XML parsing explicitly disables external entities and DTDs. Below are concrete code examples for a Buffalo handler that safely processes XML configuration without exposing sensitive database credentials or certificate paths.

First, define a secure XML decoder helper that disables external entities:

import (
    "encoding/xml"
    "io"
)

func newSecureXMLDecoder(r io.Reader) *xml.Decoder {
    dec := xml.NewDecoder(r)
    // Disable DTD and external entity resolution
    dec.Entity = xml.HTMLEntity
    return dec
}

Use this decoder in your Buffalo action when handling uploaded XML:

func (v UploadXMLAction) Exec(c buffalo.Context) error {
    file, _, err := c.Request().FormFile("config")
    if err != nil {
        return c.Render(400, r.String("missing file"))
    }
    defer file.Close()

    dec := newSecureXMLDecoder(file)
    var cfg map[string]interface{}
    if err := dec.Decode(&cfg); err != nil {
        return c.Render(400, r.Error(err))
    }

    // Validate cfg against expected schema before using with CockroachDB
    // Example: ensure no unexpected external entity references remain
    if _, hasDoctype := cfg["DOCTYPE"]; hasDoctype {
        return c.Render(400, r.String("invalid XML: external entities not allowed"))
    }

    // Safely use cfg to construct CockroachDB connection parameters
    // Avoid directly embedding user input into connection strings
    connStr := buildConnectionString(cfg)
    db, err := gorm.Open(postgres.Open(connStr), &gorm.Config{})
    if err != nil {
        return c.Render(500, r.String("database connection failed"))
    }
    defer db.Close()

    return c.Render(200, r.JSON(map[string]string{"status": "ok"}))
}

In this example, the helper sets dec.Entity = xml.HTMLEntity, which prevents the parser from resolving external DTDs and external entities. Before using configuration values to connect to CockroachDB, validate the structure and reject any unexpected DOCTYPE or entity declarations. Do not construct connection strings by concatenating user-supplied values; instead, map validated fields to known parameters such as host, port, user, and database name.

For production use with CockroachDB, prefer connection parameters stored securely in environment variables or a secrets manager, and reference them programmatically rather than extracting them from user-controlled XML. If you must accept XML configuration, enforce a strict schema (XSD or equivalent) and validate it before processing. middleBrick’s Pro plan can be integrated into your CI/CD pipeline to automatically scan Buffalo endpoints for XXE and other input validation issues, ensuring that any changes to XML handling do not reintroduce risks to your CockroachDB-backed services.