Header Injection in Fiber with Firestore

Header Injection in a Fiber application that interacts with Google Firestore occurs when untrusted input is used to construct HTTP response headers without validation or sanitization. Because Firestore is often used as a backend datastore for API responses, data retrieved from Firestore or used to query it can become part of header values if the application directly interpolates that data into headers. For example, if a route reads a document field such as displayName from a Firestore document and sets it in a Content-Disposition or custom header without sanitization, an attacker may inject additional headers or break header structure.

In a typical Fiber handler, you might retrieve a document from Firestore and use a field in a response header. If the field contains newline characters or other control characters, an attacker can inject extra headers, leading to HTTP response splitting. This is a classic example of Header Injection, where the combination of an HTTP framework (Fiber), a backend service (Firestore), and unsafe use of data creates the vulnerability. The risk is compounded when the Firestore query uses dynamic values from user-supplied parameters, such as an ID or a filter, which can themselves be manipulated to retrieve unexpected documents containing maliciously crafted data.

Consider a scenario where an endpoint fetches a user document from Firestore based on a path parameter and uses a user-supplied filename for download. If the filename is taken directly from Firestore and placed into the Content-Disposition header without sanitization, an attacker who can influence the stored filename (for example, via a compromised admin account or through another injection vector) can inject newline sequences to add arbitrary headers, such as Set-Cookie or Location. Because the application trusts the Firestore data, the injected header is sent to the client, potentially enabling cross-site scripting, request smuggling, or cache poisoning. This illustrates how Header Injection in Fiber with Firestore emerges not from a flaw in Firestore itself, but from insecure handling of data retrieved from or used with Firestore within HTTP response construction.

An attacker may also exploit Header Injection to manipulate caching behavior or to perform cross-protocol attacks if the injected headers affect how intermediaries treat responses. Because Fiber is a fast HTTP framework, developers may inadvertently pass data from Firestore into headers without considering injection risks, especially when using convenience patterns that directly bind request parameters to Firestore queries and response headers. The vulnerability is particularly dangerous when the API exposes endpoints that return user-controlled data in headers, such as content disposition or custom metadata, without proper input validation, output encoding, or allowlisting.

middleBrick scans API endpoints that interact with Firestore and flags Header Injection risks when response headers are influenced by untrusted data. The scanner performs black-box testing against the unauthenticated attack surface, identifying places where header values may be derived from external sources, including Firestore documents. In a scan that includes OpenAPI/Swagger spec analysis with full $ref resolution, middleBrick cross-references spec definitions with runtime findings to highlight risky header usage patterns. This is valuable because Header Injection can be subtle when Firestore data is involved, and the scanner provides prioritized findings with severity ratings and remediation guidance to help developers address these issues before deployment.

To remediate Header Injection in Fiber when using Firestore, you must validate and sanitize any data that flows into HTTP response headers, especially fields retrieved from Firestore documents. The safest approach is to avoid placing untrusted data into headers entirely. If you must include Firestore data in headers, apply strict allowlisting, normalization, and encoding. Below are concrete code examples that demonstrate secure handling patterns.

Example 1: Safe Content-Disposition header with Firestore data

Instead of directly using a Firestore field like fileName in a header, sanitize it by removing newline characters and restricting to a safe character set. Use a deterministic filename or a hash if possible.

const sanitizedName = strings.ReplaceAll(doc.Data["fileName"].(string), "\n", "")
// Further restrict to alphanumeric, dashes, and dots
if !regexp.MustCompile(`^[a-zA-Z0-9._-]+$`).MatchString(sanitizedName) {
    c.Status(http.StatusBadRequest).Send("Invalid file name")
    return
}
c.Set("Content-Disposition", fmt.Sprintf("attachment; filename=%s", sanitizedName))

Example 2: Parameterized Firestore query with header-safe response

When querying Firestore using user input, use parameterized queries and avoid reflecting untrusted data into headers. If metadata must be returned, place it in the response body and use safe headers.

docRef := client.Collection("files").Doc(id)
snapshot, err := docRef.Get(ctx)
if err != nil || !snapshot.Exists() {
    c.Status(http.StatusNotFound).Send("Not found")
    return
}
// Use only trusted internal fields for headers; never use document data directly
c.Set("X-Document-Id", id) // id is path-based and validated
c.Status(http.StatusOK).JSON(snapshot.Data())

Example 3: Enforce header allowlist in Fiber middleware

Add middleware that inspects outgoing headers and removes or escapes any headers that contain injected newlines or unexpected values, especially when the values originate from Firestore.

func HeaderSecurityMiddleware() fiber.Handler {
    return func(c *fiber.Ctx) error {
        // Inspect response headers before sending
        // In practice, track header values set in handlers and sanitize here
        return c.Next()
    }
}
// Usage in app
app.Use(HeaderSecurityMiddleware())

Example 4: Validate Firestore document fields used in headers

If a Firestore document contains a field intended for header use (e.g., exportFilename), validate it against a strict pattern before use.

exportName, ok := doc.Data()["exportFilename"].(string)
if !ok || regexp.MustCompile(`^[\w\-]+\.csv$`).MatchString(exportName) {
    c.Status(http.StatusBadRequest).Send("Invalid export filename")
    return
}
c.Set("X-Export-Filename", exportName)

These examples emphasize that Firestore data should never be assumed safe for headers. Use allowlists, avoid reflection of raw document fields into headers, and prefer placing sensitive or dynamic data in the response body. middleBrick’s scans can help identify endpoints where Firestore data influences headers and provide prioritized findings with remediation guidance to support these secure coding practices.