Ssrf in Buffalo with Hmac Signatures

Server-Side Request Forgery (SSRF) in Buffalo combined with HMAC signatures can occur when an application constructs requests to internal or third-party endpoints using a signed query parameter or header without proper validation of the target URL. In Buffalo, this typically happens when a developer uses url_for or similar helpers to generate a full URL, then adds an HMAC signature to prove authenticity, but fails to validate or sanitize the destination host or path. An attacker can supply a malicious URL pointing to internal services (e.g., http://169.254.169.254/latest/meta-data/iam/security-credentials/), and if the server-side logic still computes and sends an HMAC signature for that attacker-controlled URL, the request is executed with the server’s identity.

The vulnerability is specific to the interaction between URL generation, signature inclusion, and lack of endpoint allowlisting. For example, if your Buffalo app builds a redirect or backend call like externalHost + "/api/data?signature=" + hmacSign(url), and externalHost is user-supplied without strict validation, an SSRF path opens. The HMAC does not protect against SSRF; it only ensures integrity for a given URL, but if the URL itself is attacker-controlled, the signature will still be valid and the forged request will reach an internal endpoint. This can lead to metadata exposure, internal network port scanning, or injection into internal protocols that are not HTTP(S), such as `redis://` or `file://`, depending on how the HTTP client in Buffalo is configured.

Real-world patterns include using an unsigned or weakly guarded parameter to specify a target service (e.g., service_url) while a separate, trusted parameter carries the HMAC. If the HMAC is computed over only a subset of the request components (e.g., path and query, but not the host), an attacker can pivot to an internal host while keeping the signature valid. Additionally, Buffalo applications that parse OpenAPI specs or route via custom middleware might inadvertently expose internal URLs in ways that bypass intended access controls, and an HMAC-signed request to those internal URLs can amplify the impact.

To remediate SSRF when using HMAC signatures in Buffalo, enforce strict allowlisting of hosts and paths before computing or verifying the signature, and ensure the signature covers all components that must be trusted. Do not allow user input to directly dictate the request target that will be signed and executed. Below are concrete code examples demonstrating a secure approach.

First, define a fixed set of allowed hosts and optionally paths. Compute the HMAC only over the path and query, using a server-side secret, and attach it to requests that you yourself initiate to trusted endpoints — not to arbitrary user-supplied URLs.

-- Example: secure URL builder in Buffalo (using Go-like pseudo-code for clarity)
local function buildSignedRequest(userSuppliedPath string) (string, string, error)
    -- 1. Validate and normalize the path against an allowlist
    local allowedPaths = {"/api/data", "/api/status"}
    if not tableContains(allowedPaths, userSuppliedPath) then
        return "", "", errors.New("invalid path")
    end

    -- 2. Use a fixed, trusted host
    local host = "https://api.trusted.example.com"
    local url = host .. userSuppliedPath

    -- 3. Compute HMAC over method + path + query (no user-controlled host)
    local secret = os.getenv("HMAC_SECRET")
    local payload = "GET|" .. userSuppliedPath .. "|"
    local signature = hmac_sha256(secret, payload)

    return url, signature, nil
end

-- Example: verifying a signed request on the server side before use
local function verifySignedRequest(path string, receivedSig string) bool
    local secret = os.getenv("HMAC_SECRET")
    local expectedPayload = "GET|" .. path .. "|"
    local expectedSig = hmac_sha256(secret, expectedPayload)
    return secureCompare(expectedSig, receivedSig)
end

-- Example: issuing a request to a trusted internal service with HMAC
local function callInternalService(path string)
    local url, sig, err = buildSignedRequest(path)
    if err ~= nil then
        return err
    end
    local client = http.newClient()
    local req = client.get(url)
    req:setHeader("X-API-Signature", sig)
    local resp, err = req:do()
    if err ~= nil then
        return err
    end
    -- handle response
end

In Buffalo, integrate these checks within your application logic or middleware so that any outbound HTTP client call validates the target against an allowlist and only then computes or verifies the HMAC. Avoid computing HMAC over a full URL that includes a user-supplied host. If you use the CLI (middlebrick scan <url>) or GitHub Action to monitor your API surface, ensure that internal endpoints are not exposed via user-controlled parameters; the scanner will flag SSRF findings that can help you identify risky patterns.

For continuous assurance, consider the Pro plan to enable continuous monitoring and CI/CD integration so that changes which introduce permissive URL handling or weak HMAC usage fail the build. The MCP Server can also surface these issues during development in compatible IDEs. Remember, HMACs ensure integrity and authenticity for a known good endpoint, but they do not mitigate SSRF caused by accepting attacker-controlled targets.