Header Injection in Fiber with Hmac Signatures

Header Injection in the Fiber web framework occurs when user-controlled input is reflected into HTTP headers without validation or sanitization. This becomes particularly risky when Hmac Signatures are used for request authentication and integrity because an attacker can manipulate headers that influence how the signature is computed or verified. For example, if a header such as x-request-id, x-forwarded-proto, or a custom routing header is used as part of the signed string, injecting newline or carriage return characters can enable header splitting. This can cause the server to interpret injected headers as separate header lines, potentially bypassing intended validation logic or changing which headers are included in the Hmac computation.

When Hmac Signatures are generated over selected headers and a request includes injected headers, the runtime behavior may diverge from the developer's expectations. The signature verification might succeed for a modified request path that includes attacker-controlled headers, leading to authentication bypass or privilege escalation. Even if the application does not directly include injected headers in the signature base string, reflected headers can influence routing, middleware decisions, or logging, creating indirect security impacts. In a black-box scan, middleBrick tests for header splitting by injecting sequences such as \r\nX-Override: true and observing whether the request is still processed as intended or whether downstream components react to the injected data.

Because Hmac Signatures rely on a deterministic set of headers and payload, any change in which headers are present or their values can alter the computed digest. If the server includes all incoming headers (including injected ones) when recomputing the Hmac, an attacker may be able to forge a valid signature by carefully controlling both the header and the signature value. MiddleBrick’s LLM/AI Security checks include prompt injection and jailbreak probes, but header injection is validated through crafted HTTP requests that attempt to split headers and observe parsing behavior. The scanner also checks for missing input validation on header values and ensures that signature verification logic does not implicitly trust reflected headers.

In practice, this vulnerability manifests when frameworks or libraries normalize headers inconsistently between the application code and the verification step. For instance, Fiber may preserve header case differently depending on the underlying http.Server behavior, and if the signing function uses a different normalization rule, the computed Hmac will not match, either causing false rejections or, in some edge cases, allowing an attacker to exploit discrepancies. The risk is compounded when multiple services share a common Hmac scheme and one service is more permissive in what it accepts. middleBrick evaluates these inconsistencies by sending requests with mixed-case headers, duplicate headers, and injected line breaks to ensure that the runtime behavior aligns with the documented signing process.

Because this is a black-box scan, middleBrick does not inspect source code or runtime internals. Instead, it focuses on observable outcomes: whether injected headers change the response status, headers, or behavior; whether Hmac verification accepts modified requests; and whether sensitive data or error messages are exposed through crafted inputs. These tests are part of the 12 security checks that run in parallel, including Authentication, Input Validation, and Property Authorization. The scanner provides prioritized findings with severity ratings and remediation guidance, helping teams understand how header manipulation can undermine Hmac-based schemes even when strong cryptographic primitives are in place.

To remediate header injection risks in Fiber when using Hmac Signatures, enforce strict header validation and canonicalization before computing or verifying the signature. Do not directly concatenate user-supplied header values into the signed string. Instead, define a fixed set of header names that are allowed to participate in signing, normalize their names to a canonical case (for example, lowercasing), and reject any headers containing newline or carriage return characters. The following example shows a secure approach using the Fiber framework and the standard library’s Hmac implementation.

// Define allowed headers that may participate in signing
const allowedSigningHeaders = ['x-content-sha256', 'x-date', '(request-target)'];

// Normalize header name to lowercase for canonicalization
function canonicalHeaderName(name string) string {
    return strings.ToLower(name)
}

// Validate header value: reject control characters
func isValidHeaderValue(value string) bool {
    return !strings.ContainsAny(value, "\r\n")
}

// Compute Hmac over selected, normalized headers and body
func computeRequestSignature(headers http.Header, body []byte) string {
    var baseString strings.Builder
    for _, h := range allowedSigningHeaders {
        if values := headers[h]; len(values) > 0 {
            if !isValidHeaderValue(values[0]) {
                // reject request with invalid characters
                return ""
            }
            baseString.WriteString(canonicalHeaderName(h))
            baseString.WriteString(":")
            baseString.WriteString(strings.TrimSpace(values[0]))
            baseString.WriteString("\n")
        }
    }
    baseString.WriteString(hex.EncodeToString(body))
    mac := hmac.New(sha256.New, []byte(secretKey))
    mac.Write([]byte(baseString.String()))
    return hex.EncodeToString(mac.Sum(nil))
}

// In a Fiber handler, verify before processing
app.Post("/api/resource", func(c *fiber.Ctx) error {
    receivedSignature := c.Get("X-Request-Signature")
    computedSignature := computeRequestSignature(c.Request().Header, c.Body())
    if computedSignature == "" || !hmac.Equal([]byte(receivedSignature), []byte(computedSignature)) {
        return c.Status(fiber.StatusUnauthorized).JSON(fiber.Map{"error": "invalid signature"})
    }
    // continue with authenticated logic
    return c.JSON(fiber.Map{"status": "ok"})
})

This pattern ensures that only explicitly allowed headers are included in the Hmac computation, that their values are free of injection characters, and that header names are normalized to avoid case-based discrepancies. It also avoids including all incoming headers in the signed base, which reduces the attack surface. In a production deployment, rotate secretKey securely and consider adding a timestamp or nonce to mitigate replay attacks. middleBrick’s Pro plan includes continuous monitoring and can be configured to alert you if security scores drop, helping you verify that such fixes remain effective after deployments.

Additionally, integrate this check into your CI/CD pipeline using the GitHub Action to fail builds if risk scores degrade. The CLI tool can be invoked as part of automated tests to validate that header injection attempts are properly rejected. For AI-assisted development, the MCP Server enables you to scan APIs directly from your coding assistant, surfacing potential issues as you write signing logic. These measures complement the core remediation by ensuring that Hmac-based authentication remains robust against header manipulation in Fiber and similar frameworks.