Prompt Injection in Buffalo with Hmac Signatures

In Buffalo applications that integrate LLM endpoints, using Hmac Signatures for request authentication can inadvertently expose prompt injection surfaces if the signature is computed over user-influenced headers or query parameters that an attacker can control. Buffalo is a Go web framework that encourages straightforward request handling; when developers forward incoming requests to an LLM service, they may sign only a subset of the HTTP metadata (e.g., a selected header or query parameter) with an Hmac key. If an attacker can manipulate the signed inputs—such as a user-controlled query string or header—the attacker can craft requests that produce valid signatures while smuggling injected instructions intended for the LLM system prompt or jailbreak attempts.

The vulnerability chain typically unfolds as follows: the Buffalo handler reads parameters from the request, constructs a canonical string for signing (often a concatenation of selected headers, query values, and sometimes a timestamp), computes an Hmac-SHA256 signature, and forwards the request to the LLM endpoint including the signature in a header. Because the signature is tied to user-influenced data, an attacker who knows or guesses the signing logic can forge a valid signature. The forged request may then reach the LLM with injected content embedded in a header or query parameter that the application treats as non-privileged. The LLM may interpret that injected content as a directive, leading to system prompt leakage, instruction override, or data exfiltration. This is especially risky when the Buffalo app also performs minimal input validation on the signed fields, allowing an attacker to bypass intended constraints by encoding malicious instructions in permissible parameter formats.

Consider a Buffalo handler that signs the X-Model-Version header and the prompt query parameter, then forwards both to an unauthenticated LLM endpoint. An attacker can probe the endpoint by iterating over variations of the query parameter while maintaining a valid Hmac signature if the signing process does not bind to a strict allowlist of permitted values. The LLM may treat the manipulated prompt value as part of the system prompt or as a tool-use instruction, enabling prompt injection techniques such as DAN jailbreak or data exfiltration. Because the signature validates only integrity and not semantic safety, the attack appears legitimate to the backend. The LLM security checks that middleBrick applies—such as system prompt leakage detection and active prompt injection testing with sequential probes—can surface these weaknesses by observing anomalous LLM behavior when signed parameters are varied.

Additionally, if the Buffalo application reuses the same Hmac key across multiple endpoints or combines it with weak canonicalization (e.g., unordered query parameters or inconsistent header casing), the attack surface expands. An attacker might exploit differences in how the Buffalo handler and the LLM client serialize data, leading to signature malleability that permits injection without invalidating the signature. This interplay between framework-level routing, selective signing, and powerful LLM agents increases the likelihood of successful jailbreak or instruction override. middleBrick’s LLM/AI Security checks, including unauthorized endpoint detection and output scanning for executable code, help identify whether manipulated signed inputs lead to unsafe agent behaviors or leaked system instructions.

To mitigate prompt injection risks when using Hmac Signatures in Buffalo, you must ensure that the signature covers a strict, server-defined allowlist of parameters and that user input never influences the LLM instructions or prompts directly. The following code examples demonstrate a hardened approach that signs only server-controlled metadata and passes user input as a safely escaped payload that the LLM treats as data, not as directive.

First, define a canonical signing function that explicitly selects headers and excludes any user-influenced fields from the signature base. This prevents attackers from forging valid signatures by altering query parameters or headers that the LLM might interpret as instructions.

// hmac_signing.go
package handlers

import (
    "crypto/hmac"
    "crypto/sha256"
    "encoding/hex"
    "net/http"
    "sort"
    "strings"
)

// serverSignedHeaders is an allowlist of headers we include in the signature.
// No user-controlled header is included.
var serverSignedHeaders = []string{"X-API-Version", "X-Timestamp"}

// computeHmacSignature returns a hex-encoded HMAC-SHA256 over selected headers.
func computeHmacSignature(r *http.Request, secret string) string {
    var parts []string
    for _, h := range serverSignedHeaders {
        if v := r.Header.Get(h); v != "" {
            parts = append(parts, h+":"+v)
        }
    }
    // Sort to ensure deterministic canonical form
    sort.Strings(parts)
    canonical := strings.Join(parts, "||")
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    return hex.EncodeToString(mac.Sum(nil))
}

Next, in your Buffalo handler, verify the signature before forwarding the request. Keep user input out of the signature base and treat it as opaque data for the LLM. Use strict schema validation for any query or form fields to prevent injection via allowed parameter formats.

// llm_proxy.go
package handlers

import (
    "net/http"
    "strings"
)

type LLMRequest struct {
    Prompt string `json:"prompt" validate:"required,max=2000"`
}

func ProxyToLLM(secret string) http.HandlerFunc {
    return func(w http.ResponseWriter, r *http.Request) {
        expected := computeHmacSignature(r, secret)
        provided := r.Header.Get("X-Request-Signature")
        if !hmac.Equal([]byte(expected), []byte(provided)) {
            http.Error(w, "invalid signature", http.StatusUnauthorized)
            return
        }

        var req LLMRequest
        if err := parseJSONBody(r, &req); err != nil {
            http.Error(w, "bad request", http.StatusBadRequest)
            return
        }
        // Validate input strictly; do not include raw user input in signature
        if err := validate.Struct(req); err != nil {
            http.Error(w, "validation failed", http.StatusBadRequest)
            return
        }

        // Build LLM request with user input as data, not as directive
        llmReq := map[string]interface{}{
            "input":  req.Prompt, // treated as data by the model
            "system": "You are a helpful assistant. Follow instructions literally.",
        }
        // Forward llmReq to the LLM endpoint, including X-Request-Signature: expected
        // Do NOT concatenate user input into the signature base
    }
}

Additionally, adopt these practices to reduce prompt injection risk: pin the LLM model version via a server-controlled header, enforce a strict allowlist for query parameters, and avoid including timestamps or nonces in the signature if they do not affect server-side authorization. Configure your Buffalo app to reject requests with unexpected content types or malformed encodings before they reach the signing logic. These measures ensure that Hmac Signatures provide integrity for authorized metadata without enabling attackers to smuggle instructions through signed fields.

Finally, integrate middleBrick’s CLI tool to scan your Buffalo endpoints from the terminal—run middlebrick scan <url> to validate that your Hmac implementation does not leak system prompts or allow instruction override. For CI/CD, add the GitHub Action to fail builds if risk scores drop below your threshold, and consider the Pro plan for continuous monitoring of these endpoints to detect regressions introduced by changes in request handling logic.