Format String in Echo Go with Api Keys

A format string vulnerability occurs when user-controlled input is passed directly to formatting functions like fmt.Sprintf or fmt.Printf without a explicit format specifier. In an Echo Go service that handles API keys, this often happens when key values are incorporated into log messages, error responses, or dynamic route construction. If an attacker can influence the key or a key-derived string and the application uses improper formatting verbs, they can read from or write to memory, potentially leaking other keys or altering control flow.

Consider an Echo handler that logs an incoming API key using a format string derived from user input:

key := c.Param("key")
logKey := c.QueryParam("logkey")
// Unsafe: logKey can contain format verbs like %x or %s
fmt.Printf(logKey, key)

If logKey is supplied by an attacker as %s%s%s%s, the call can read stack contents, potentially exposing other API keys stored in memory or through format string side channels. Similarly, constructing a redirect or error message by concatenating or formatting with an API key can expose the raw key in responses:

apiKey := c.Get("apiKey").(string)
c.String(http.StatusBadRequest, "Invalid key: %s", apiKey)

While the example above uses a proper format specifier, if the API key itself contains percent signs (e.g., a key like 100%_WORKS), and the logging or response logic does not sanitize or use strict formatting, it can lead to information disclosure or parsing issues. In the context of OpenAPI/Swagger spec analysis, an endpoint that reflects an API key in output without proper validation may align with Data Exposure findings, where runtime inspection reveals that sensitive data appears in logs or responses due to format handling.

In unauthenticated scans, middleBrick tests such endpoints by probing for information leakage. The presence of API keys in query parameters, headers, or response bodies—especially when combined with format string misuse—can trigger findings in Data Exposure and Unsafe Consumption checks. Because API keys often appear in structured specs (e.g., as a required header x-api-key), any format string that incorporates these values must use explicit verbs and avoid dynamic format directives to prevent memory disclosure or injection-like behavior.

Remediation centers on strict format control and avoiding the incorporation of API keys into format strings. Always use explicit format verbs and pass API keys as arguments rather than weaving them into the format string itself. Prefer structured logging that does not rely on user-controlled format verbs.

Safe logging example with Echo Go:

apiKey := c.Get("apiKey").(string)
// Safe: key is an argument, format verb is explicit and controlled
log.Printf("request with key prefix %s", apiKey[:4] + "****")

If you must include the full key for debugging (not recommended in production), ensure the format string is a constant and the key is passed as a single argument:

const logFmt = "api_key_value=%s"
log.Printf(logFmt, apiKey)

For dynamic construction of responses or redirects, use fmt.Sprintf with a constant format and treat the API key as data, not a format specifier:

redirectURL := fmt.Sprintf("/v1/keys/%s", url.PathEscape(apiKey))
c.Redirect(http.StatusTemporaryRedirect, redirectURL)

Input validation should reject keys containing unexpected percent signs when they are used in contexts that could be interpreted as format templates. MiddleBrick’s Property Authorization and Input Validation checks can help identify endpoints where keys are reflected in a way that suggests format misuse. In environments with continuous monitoring (Pro plan), such patterns can be flagged automatically across scans, enabling teams to detect regressions before deployment.

When integrating into CI/CD, the GitHub Action can enforce that any commit containing formatting patterns with dynamic format verbs on API key variables triggers a build failure. The MCP Server allows developers to scan API definitions and runtime behavior directly from their IDE, surfacing risky formatting idioms early. These integrations complement the scanner’s findings by providing ongoing governance, ensuring that fixes for format string issues involving API keys remain enforced across the lifecycle.