Crlf Injection in Buffalo with Api Keys

Crlf Injection occurs when an attacker can control part of an HTTP header and injects a CRLF sequence (\r\n) to split the header line. In the Buffalo framework, this commonly arises when constructing response headers using untrusted data, such as values derived from or influenced by API keys. API keys are often passed in headers (e.g., Authorization: ApiKey ) or used to look up user or application context. If a developer uses attacker-controlled API key values to build headers without sanitization, an API key containing \r\n can inject additional headers or split a header line, leading to header manipulation.

Consider a scenario where an API key is used to set a custom header in a Buffalo action. If the API key is taken directly from the request and reflected into a header, an attacker can supply a key such as abc123\r\nX-Injected: true. When the application uses this value to set a header, Buffalo may produce a response that includes both the intended header and an injected one. This can enable attacks like session fixation, cross-site scripting via injected headers, or response splitting. The vulnerability is not in Buffalo itself but in the application logic that incorporates untrusted API key data into HTTP headers without validation or encoding.

Another relevant pattern involves routing or parameter interpolation where API keys influence URL construction or header values. For example, if an application uses API keys to select tenant-specific configurations and then sets headers such as X-Tenant-ID based on that key, a CRLF sequence embedded in the key can cause multi-line headers, bypassing intended access controls or obscuring the true response structure. The risk is compounded when responses are cached or logged, as injected headers may be stored and replayed. While Buffalo provides tools for safe header setting, developers must ensure any data derived from API keys is sanitized, restricted, and validated before being used in header construction.

Remediation focuses on preventing untrusted API key data from affecting HTTP headers. In Buffalo, always treat API key values as opaque identifiers and avoid directly interpolating them into headers. Use Buffalo’s header helpers safely and validate inputs before use. Below are concrete code examples demonstrating insecure patterns and their secure alternatives.

Insecure example where an API key is used to set a custom header without validation:

// Avoid: directly using API key value in header
func MyAction(c buffalo.Context) error {
    apiKey := c.Param("api_key") // attacker-controlled
    c.Response().Header().Set("X-Custom-Key", apiKey)
    return c.Render(200, r.String("ok"))
}

Secure alternative: validate and sanitize the API key, and avoid reflecting its raw value into headers. If you must associate metadata, use a controlled mapping:

// Secure: use a mapped value or constant logic instead of raw API key
func MyAction(c buffalo.Context) error {
    apiKey := c.Param("api_key")
    // Validate format (e.g., hex, length) and check against a store if needed
    if !isValidApiKeyFormat(apiKey) {
        return c.Error(400, errors.New("invalid api key"))
    }
    // Use a stable, sanitized header value derived from the key, not the key itself
    c.Response().Header().Set("X-Key-Fingerprint", fingerprint(apiKey))
    return c.Render(200, r.String("ok"))
}

func isValidApiKeyFormat(key string) bool {
    // Example: allow only alphanumeric and limited length
    for _, r := range key {
        if !(r >= 'a' && r <= 'z' || r >= 'A' && r <= 'Z' || r >= '0' && r <= '9') {
            return false
        }
    }
    return len(key) == 32
}

func fingerprint(key string) string {
    // Return a sanitized hash or truncated representation
    return "fp-" + key[:8]
}

When using the middlebrick CLI to assess your Buffalo API, you can run middlebrick scan <url> to detect potential header injection issues among the 12 security checks. For teams managing multiple services, the Pro plan supports continuous monitoring and integrates with GitHub Actions to fail builds if risk scores degrade, helping prevent regressions that could enable CRLF Injection through API key handling.