Privilege Escalation in Buffalo with Hmac Signatures

In the Buffalo web framework for Go, privilege escalation can occur when HMAC-based signatures are used to protect routes or parameters but are implemented inconsistently or with weak assumptions about identity. HMAC signatures are commonly employed to ensure integrity and authenticity of requests, for example when a client sends an id parameter alongside a signature like id=123&signature=hex. If the server recomputes the HMAC over only a subset of the data (e.g., the id) and skips other mutable or user-controlled inputs, an attacker can change other parameters or context without invalidating the signature, leading to horizontal or vertical privilege escalation.

Consider an endpoint that identifies a user by an ID and uses an HMAC to guard against tampering. A vulnerable pattern computes the signature only over the ID, for instance:

// Vulnerable: signature covers only the id, ignoring role
secret := []byte("weak-secret")
id := ctx.Params.Get("id")
sig := ctx.Params.Get("signature")
expected := hmacSign(id, secret)
if !hmac.Equal([]byte(expected), []byte(sig)) {
    // reject
}
// proceed with id, but role is taken from query or session without verification

Because the role or admin flag is not part of the signed payload, an attacker who knows or guesses another user’s ID can change role in the request (e.g., ?id=123&role=admin) and the HMAC will still validate if the server does not include role in the signature. This is a BOLA/IDOR-like privilege escalation rooted in signature misuse: the signature fails to bind the entire authorization context (ID + role) together.

Another scenario involves replay or insufficient scope binding. If the signature includes only static values (ID and a static timestamp) without a per-request nonce or a tight scope string, an attacker may reuse a valid signed request to perform actions outside the intended permission boundary. For example, a signature that covers id:123|action:read can be replayed to perform action:write if the server does not include the intended action in the signed data. This is especially dangerous when the server infers authorization from session or cookies while trusting the signed parameters, effectively bypassing proper checks.

A concrete attack chain might look like: (1) retrieve a legitimate user’s ID and a valid HMAC for id=1001; (2) modify the request to id=1001&role=admin or to a different user ID that the attacker knows or guesses; (3) forward the request with the original HMAC; (4) if the server recomputes the HMAC only over ID or over a subset that excludes role, the request passes signature verification and privilege escalation occurs. This maps to common API security risks around Broken Object Level Authorization (BOLA) and insecure consumption, where trust is placed in partial signed data rather than a comprehensive, server-side authorization decision.

To map findings to recognized standards, issues like this are typically flagged under OWASP API Top 10 controls related to Broken Object Level Authorization and Security Misconfiguration. Because HMAC usage intersects authentication, integrity, and authorization, implementation errors can undermine all three. middleBrick scans such patterns as part of its Authentication, BOLA/IDOR, and Unsafe Consumption checks, providing prioritized findings with severity and remediation guidance.

Remediation focuses on ensuring the HMAC covers all data used for authorization decisions and that server-side checks are strict and consistent. In Buffalo, compute the HMAC over a canonical string that includes every input that affects authorization: user identifier, role, intended action, and any mutable context. Validate the signature before using any parameter, and avoid mixing trusted server state (e.g., session role) with unsigned client-provided values.

Use a constant-time comparison and a strong secret. Prefer including a short-lived nonce or timestamp to prevent replay. Below are two concrete, syntactically correct examples demonstrating a secure pattern for signing and verifying in Buffalo.

Example 1: Sign a canonical string that includes ID, role, and action

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "strings"
)

func signPayload(parts ...string) string {
    secret := []byte("your-256-bit-secret")
    mac := hmac.New(sha256.New, secret)
    for _, p := range parts {
        mac.Write([]byte(p))
        mac.Write([]byte("|")) // canonical delimiter
    }
    return hex.EncodeToString(mac.Sum(nil))
}

// Build canonical input from validated sources, never raw untrusted query values alone
canonical := signPayload(
    "id:"+userId,
    "role:"+role,
    "action:"+action,
    "ts:"+timestamp,
)
// Send canonical in request, e.g., as query params: id=...&role=...&action=...&ts=...&signature=...

On verification, reconstruct the canonical string from the received parameters using the same order and delimiter, recompute HMAC, and compare using hmac.Equal. Reject the request if any part is missing or mismatched.

func verifyPayload(userId, role, action, timestamp, receivedSig string) bool {
    secret := []byte("your-256-bit-secret")
    expected := signPayload(
        "id:"+userId,
        "role:"+role,
        "action:"+action,
        "ts:"+timestamp,
    )
    return hmac.Equal([]byte(expected), []byte(receivedSig))
}

// In a handler:
userId := ctx.Params.Get("id")
role := ctx.Params.Get("role")
action := ctx.Params.Get("action")
timestamp := ctx.Params.Get("ts")
sig := ctx.Params.Get("signature")

if !verifyPayload(userId, role, action, timestamp, sig) {
    ctx.Error(401, "invalid signature")
    return
}
// Proceed with server-side authorization checks, not solely signature validation

Example 2: Include scope and nonce to prevent replay and tighten authorization binding

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "time"
)

func signWithNonce(scope, nonce string, secret []byte) string {
    mac := hmac.New(sha256.New, secret)
    mac.Write([]byte("scope:" + scope + "|"))
    mac.Write([]byte("nonce:" + nonce + "|"))
    return hex.EncodeToString(mac.Sum(nil))
}

// Server generates a short-lived nonce and scope, includes them in the signed payload
nonce := generateNonce() // e.g., crypto/rand or secure random string
scope := "user:123:action:write"
sig := signWithNonce(scope, nonce, []byte("your-256-bit-secret"))
// Return to client: scope, nonce, signature

// Verification ensures scope and nonce are as expected and within validity window
if !hmac.Equal([]byte(signWithNonce(receivedScope, receivedNonce, secret)), []byte(receivedSig)) {
    // reject
}
if !isWithinTimeWindow(receivedTimestamp) {
    // reject
}
// Perform server-side authorization based on scope, not only on signature

Key remediation practices: always validate and parse parameters before signing/verifying; never trust order inferred only from client; include all authorization-critical fields in the signed string; use server-side checks to enforce permissions rather than relying on signature presence alone; rotate secrets periodically; and prefer using established libraries for HMAC to avoid subtle implementation errors. middleBrick’s scans can surface weak HMAC patterns and missing scope binding as part of its Authentication and BOLA/IDOR checks.