Spring4shell in Buffalo with Bearer Tokens

The Spring4shell vulnerability (CVE-2022-22965) involves remote code execution via data binding on the class parameter when using Apache Tomcat behind a reverse proxy. In Buffalo, a Go web framework, applications can integrate with Spring-based backends or expose endpoints that themselves proxy to Spring services, creating a scenario where requests pass through Buffalo to a downstream Spring application. When Bearer Tokens are used for authorization, tokens are often forwarded as headers (e.g., Authorization: Bearer <token>) from Buffalo to the Spring service. If Buffalo passes incoming headers—including the Authorization header—unchecked to the Spring backend, and that backend uses form binding to populate a command object that includes a Class field, the stage is set for Spring4shell exploitation.

Specifically, an attacker can send a crafted request to the Buffalo endpoint with a malicious class parameter (e.g., ?class=java.lang.Runtime) and a spoofed Authorization: Bearer eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9... header. Buffalo may log or forward the token while deserializing or binding query/form parameters to the target Spring application. If the Spring service does not restrict which classes can be bound, remote code execution occurs under the context of the token’s implied permissions. The presence of Bearer Tokens does not mitigate the vulnerability; it can inadvertently propagate the malicious request—including the token—to the backend, enabling post-exploitation actions scoped to the token’s privileges. Because Spring4shell exploits data binding rather than authentication bypass, the token’s validity is orthogonal; the risk is that authenticated requests carrying tokens are more likely to be forwarded to vulnerable Spring microservices within a larger Buffalo-powered API mesh.

In scans run by middleBrick, this combination is flagged under the Authentication and BOLA/IDOR checks when Bearer Tokens are present but insufficient input validation and header-forwarding practices exist. The scanner tests whether the API reflects or forwards the Authorization header without stripping or sanitizing it before passing requests to backend frameworks like Spring. An unauthenticated attack surface that includes endpoints accepting and forwarding headers can reveal Spring4shell exposure, with the Bearer Token acting as a carrier for privilege-context propagation. middleBrick’s LLM/AI Security checks also assess whether system prompts or token handling logic might be exfiltrated when combined with such chained vulnerabilities.

Remediation focuses on preventing the forwarding of untrusted headers and ensuring Buffalo does not inadvertently participate in a chain that delivers malicious requests to Spring services. The primary fix is to strip or reject the Authorization header when Buffalo acts as a proxy, or to validate and sanitize headers before passing them downstream. Below are concrete code examples for both approaches in Buffalo.

Approach 1: Strip Authorization header when proxying

When Buffalo proxies requests to a Spring backend, remove the Authorization header unless it is explicitly required and validated.

package actions

import (
    "net/http"
    "github.com/gobuffalo/buffalo"
)

func ProxyToSpring(c buffalo.Context) error {
    req, err := http.NewRequest(c.Request().Method, "https://spring-backend/api/resource", c.Request().Body)
    if err != nil {
        return err
    }
    // Copy only safe headers; explicitly exclude Authorization
    for name, values := range c.Request().Header {
        lower := http.CanonicalHeaderKey(name)
        if lower == "Authorization" {
            continue // drop Bearer token to prevent forwarding
        }
        for _, v := range values {
            req.Header.Add(name, v)
        }
    }
    client := &http.Client{}
    resp, err := client.Do(req)
    if err != nil {
        return err
    }
    defer resp.Body.Close()
    // handle response...
    return c.Render(200, r.JSON(resp.StatusCode))
}

Approach 2: Validate and allowlist headers

If specific headers including authorization are required, validate the token first and then forward only approved headers.

package actions

import (
    "net/http"
    "strings"
    "github.com/gobuffalo/buffalo"
)

func ValidateAndForward(c buffalo.Context) error {
    auth := c.Request().Header.Get("Authorization")
    if !strings.HasPrefix(auth, "Bearer ") {
        return c.Render(401, r.JSON("unauthorized"))
    }
    token := strings.TrimPrefix(auth, "Bearer ")
    if !isValidToken(token) { // implement token validation (e.g., introspection/jwk)
        return c.Render(403, r.JSON("forbidden"))
    }
    req, err := http.NewRequest(c.Request().Method, "https://spring-backend/api/resource", c.Request().Body)
    if err != nil {
        return err
    }
    req.Header.Set("Authorization", auth) // forward only after validation
    // selectively copy other headers
    req.Header.Set("X-Request-ID", c.Request().Header.Get("X-Request-ID"))
    client := &http.Client{}
    resp, err := client.Do(req)
    if err != nil {
        return err
    }
    defer resp.Body.Close()
    return c.Render(200, r.JSON(resp.StatusCode))
}

func isValidToken(token string) bool {
    // placeholder: call auth server or validate JWT
    return token == "valid-test-token"
}

General mitigations

• Pin Buffalo and its dependencies to versions with known patches for header handling.
• Apply input validation on all query, form, and JSON parameters to reject unexpected fields like class.
• Use middleware to log and audit forwarded headers, ensuring no sensitive tokens are propagated inadvertently.
• In environments where Spring4shell exposure is a concern, conduct regular scans with middleBrick to detect whether your Buffalo endpoints forward headers in a way that could reach vulnerable Spring services. The CLI tool (middlebrick scan <url>) can be integrated into scripts to automate checks.