Xml External Entities in Buffalo with Jwt Tokens

XML External Entity (XXE) attacks occur when an application processes XML input that references external entities in a way that causes the parser to disclose local files, trigger SSRF, or consume excessive resources. The Buffalo web framework for Go does not include a built-in XML parser in its core routing or middleware, but applications using Buffalo often add XML handling for APIs or webhooks. When such XML processing is combined with JWT tokens—used for authentication and authorization—the interaction can expose sensitive logic or data if the JWT is accepted as XML or if XML parsing is inadvertently applied to authentication payloads.

Consider a scenario where an endpoint accepts both XML payloads containing external entity declarations and JWT tokens in headers or query parameters. A developer might parse an XML body to extract user details while also validating a JWT for access control. If the XML parser is configured to resolve external entities, an attacker can supply a malicious XML document in the request body that references a file path present on the server, such as /etc/passwd, while also including a JWT that identifies a privileged user. Because the JWT may be trusted implicitly by the application, the server processes the XML with the privileges implied by the token, potentially reading sensitive files or triggering SSRF via internal endpoints. The JWT itself does not cause the XXE, but its presence can escalate impact by bypassing identity checks or by being included in logs and error messages returned as part of the entity expansion response.

Real-world risk increases when the application uses JWTs for unauthenticated or weakly authenticated endpoints, such as public webhook URLs. For example, an unauthenticated LLM endpoint or an exported OpenAPI spec might reveal routes that accept XML, and an attacker can probe these while supplying crafted JWTs that contain elevated scopes or roles. If the XML parser resolves DOCTYPE declarations pointing to internal URLs, the server may make internal requests that the JWT context implicitly authorizes. This combination can lead to account takeover or data exposure when findings from scans—such as those provided by middleBrick—highlight missing input validation and insecure handling of XML entities alongside token usage. middleBrick’s checks for Input Validation, Data Exposure, and SSRF help surface these risks by correlating runtime behavior with the presence of JWT handling patterns.

Using concrete code, a vulnerable Buffalo handler might look like this, where XML parsing occurs without disabling external entities and a JWT is validated but not properly scoped for XML context:

// Example vulnerable handler in Buffalo
func ProcessXML(c buffalo.Context) error {
    tokenString := c.Request().Header.Get("Authorization")
    // Assume ValidateJWT returns claims without strict audience/role checks
    claims, err := ValidateJWT(tokenString)
    if err != nil {
        return c.Error(401, errors.New("invalid token"))
    }
    // Dangerous: XML parser with external entity resolution enabled
    decoder := xml.NewDecoder(c.Request().Body)
    decoder.Entity = xml.HTMLEntity
    var payload interface{}
    if err := decoder.Decode(&payload); err != nil {
        return c.Error(500, errors.New("parse error"))
    }
    // Use claims and payload...
    return nil
}

In this pattern, even though JWT validation occurs, the XML decoder remains capable of external entity resolution. An attacker can send a Content-Type of application/xml with a DOCTYPE that reads /etc/passwd, and the server may process it under the permissions suggested by the JWT. middleBrick’s scans would flag the missing input validation and SSRF risks, emphasizing that JWT context should not be assumed safe when processing untrusted XML.

Remediation focuses on disabling external entity resolution in any XML processing and ensuring JWT usage does not leak sensitive information or bypass validation. When Buffalo applications handle XML, developers should use a parser that explicitly forbids external entities and DTDs. For JWTs, enforce strict validation, avoid including sensitive data in payloads, and scope token usage to the minimal required context, particularly when XML or other complex inputs are involved.

To fix XXE in a Buffalo handler, replace the generic XML decoder with a secure parser configuration that disables external entities. Below is a hardened example that processes XML safely while validating JWTs with audience and role checks:

// Secure handler in Buffalo
func ProcessXMLSecure(c buffalo.Context) error {
    tokenString := c.Request().Header.Get("Authorization")
    claims, err := ValidateJWTStrict(tokenString)
    if err != nil {
        return c.Error(401, errors.New("invalid token"))
    }
    // Ensure the JWT has required scopes for this operation
    if !hasScope(claims, "xml:process") {
        return c.Error(403, errors.New("insufficient scope"))
    }
    // Use a secure XML decoder that disables external entities
    body := io.LimitReader(c.Request().Body, 10<<20) // 10 MB limit
    decoder := xml.NewDecoder(body)
    decoder.CharsetReader = charset.NewReaderLabel
    // Explicitly disable external entity resolution
    decoder.Entity = nil
    decoder.Strict = true
    var payload interface{}
    if err := decoder.Decode(&payload); err != nil {
        return c.Error(400, errors.New("invalid XML"))
    }
    // Process payload with claims context...
    return nil
}

Complementary measures include avoiding JWTs for unauthenticated endpoints, using middleware to reject XML Content-Type where not needed, and applying input validation rules that mirror middleBrick’s findings for Input Validation and Property Authorization. For continuous assurance, integrate the CLI tool with middlebrick scan <url> to detect insecure XML handling in staging, and leverage the GitHub Action to fail builds if risk scores degrade. The Pro plan’s continuous monitoring can schedule regular scans, ensuring that new code paths involving JWTs and XML remain protected without manual review.
Additional secure patterns include using strongly typed structs instead of generic interfaces when unmarshaling XML, enforcing strict Content-Type checks, and ensuring that any logging of JWTs or XML errors redacts tokens and sensitive fields. By combining these code-level controls with periodic scans, developers reduce the chance that XXE vectors interact with JWT-trusted contexts in ways that escalate risk.