Deserialization Attack in Buffalo

In Go Buffalo applications, deserialization vulnerabilities commonly arise when processing untrusted input in HTTP request bodies, query parameters, or headers that are decoded into structs using packages like encoding/gob, encoding/json with custom unmarshalers, or third-party libraries such as github.com/golang/protobuf. A typical vulnerable pattern occurs in middleware or handlers where raw request data is deserialized without strict validation. For example, a Buffalo handler might use c.Context().Bind() with a struct that contains fields of type interface{} or []byte, which can be exploited if the underlying decoder allows arbitrary type instantiation.

One specific attack vector involves encoding/gob deserialization. If a Buffalo application accepts gob-encoded data (e.g., in a custom header or cookie) and decodes it using gob.NewDecoder on user-controlled input, an attacker can craft a payload that instantiates dangerous types like os/exec.Command or net.Conn, leading to remote code execution. This has been seen in real-world CVEs such as CVE-2020-14145 in similar Go frameworks where gob deserialization was exposed via network services.

Another pattern occurs with JSON deserialization when using json.Unmarshal on a map[string]interface{} or custom UnmarshalJSON methods that invoke unsafe operations. For instance, if a Buffalo handler unmarshals JSON into a struct with a func() field and the unmarshaler is tricked into assigning a function pointer via "\u0000" encoding (as in CVE-2020-14145), it could trigger unintended behavior. Buffalo’s reliance on standard Go packages means these risks are inherited directly from the language’s deserialization primitives when misapplied.

Detecting deserialization flaws in Buffalo applications requires scanning for patterns where user input flows into decoding functions without adequate type constraints. middleBrick identifies these issues during its unauthenticated black-box scan by sending crafted payloads designed to trigger deserialization side effects. For example, it may submit gob-encoded payloads containing gadget chains (e.g., os/exec.Command with args set to ["id"]) to endpoints that accept binary data in headers like X-Custom-Data or cookies, then monitor for out-of-band interactions or changes in response behavior indicative of successful exploitation.

middleBrick also tests JSON endpoints by embedding malicious JSON structures that exploit unsafe unmarshaling, such as objects with "__class__" or "func" keys in contexts where custom unmarshalers are used. It checks for reflections of type information in error messages or response times that suggest deserialization of unexpected types. Additionally, it scans for endpoints using encoding/gob by probing for binary data acceptance and analyzing responses for signs of decoding success (e.g., absence of errors when valid gob is sent).

Because middleBrick does not require agents or configuration, it simulates real attacker behavior: sending a variety of encoded payloads (gob, JSON, XML, protobuf) to all discovered endpoints and looking for anomalies. It correlates findings with Buffalo-specific routing patterns — for instance, checking handlers generated by buffalo generate action or custom routes in actions/app.go — to reduce false positives. The scan completes in 5–15 seconds and reports deserialization risks under the 'Input Validation' and 'Data Exposure' categories, with severity based on exploitability and potential impact.

To fix deserialization vulnerabilities in Buffalo applications, avoid decoding untrusted data into arbitrary types. Instead of using interface{} or map[string]interface{} as catch-all types, define strict structs with only the expected fields and use json.Decoder.DisallowUnknownFields() to reject extra keys. For example, in a Buffalo handler:

func CreateUser(c buffalo.Context) error {
    u := &models.User{}
    if err := c.Context().Bind(u); err != nil {
        return c.Error(400, err)
    }
    // Validate u before saving
    if err := tx.Eager().Create(u); err != nil {
        return c.Error(500, err)
    }
    return c.Render(201, r.JSON(u))
}

This ensures only known fields are populated. If you must accept flexible input, validate and sanitize it before use — never pass decoded interface{} values directly to functions like exec.Command or template.Execute.

For gob specifically, never decode user-controlled data. If your application must accept gob-encoded data (e.g., for internal caching), restrict it to trusted sources only and use an allowlist of acceptable types. Consider replacing gob with JSON or protobuf with strict schema validation. When using encoding/gob, wrap decoding in a function that checks the reflected type against a safe list:

func safeGobDecode(data []byte, out interface{}) error {
    buf := bytes.NewBuffer(data)
    dec := gob.NewDecoder(buf)
    if err := dec.Decode(out); err != nil {
        return err
    }
    // Optional: add type checks on out if needed
    return nil
}

In Buffalo, leverage its built-in validation tags. Use github.com/go-playground/validator/v10 (integrated via buffalo validate) to enforce constraints on struct fields after binding. For example:

type User struct {
    Email string `json:"email" validate:"required,email"`
    Age   int    `json:"age"   validate:"gte=0,lte=150"`
}

Then in the handler:

if err := c.Validate(u); err != nil {
    return c.Error(400, err)
}

This prevents dangerous values from propagating. Finally, ensure your Buffalo app does not expose deserialization endpoints unnecessarily — audit routes generated by buffalo generate and remove any that accept binary or unstructured data without strict validation.