Format String in Echo Go with Hmac Signatures

A format string vulnerability in an Echo Go handler can interact dangerously with Hmac Signatures when user-controlled input is both used to compute a signature and later rendered in a response that includes format verbs. In Go, fmt.Printf-style functions interpret format verbs such as %s, %x, or %v from the provided arguments. If a developer writes fmt.Sprintf(userSuppliedFormat, args...) where userSuppliedFormat comes from a request parameter, an attacker can supply format verbs to read or overwrite memory, leading to information disclosure or crashes.

When Hmac Signatures are used to bind a request to a shared secret, the signature is typically computed over selected parts of the request, such as parameters, headers, or a canonical string. If the endpoint logs or echoes the formatted string using unchecked user input, an attacker may attempt to inject format verbs into the signed payload or into logging statements that include the signature. For example, consider a handler that parses a query parameter token, validates an Hmac, and then logs a message using the token value:

// WARNING: illustrative vulnerability pattern
signedToken := c.QueryParam("token")
log.Printf("Validating token: %s", signedToken)

If signedToken contains format verbs such as %s %x, the log line can cause the logging library to interpret them, potentially leaking parts of the stack or other runtime data. Although the Hmac verification itself is not directly broken by format verbs, the combined use of unchecked formatted output and signature validation can create an indirect path for information leakage. In an API security scan, this would be flagged under Data Exposure and Input Validation checks because runtime data (including signature material) may be exposed through formatted output.

Another scenario involves accepting a format string as part of an API parameter that is later used to construct a response or log entry. If the format string includes verbs that expose memory, the Hmac signature computed over the request may still be valid, but the endpoint behavior becomes unpredictable. Attackers can chain this with other inputs to probe for memory contents, which may include nonces, timestamps, or portions of the signing key if they appear in formatted output. Because middleBrick tests Input Validation and Data Exposure in parallel, such combinations are surfaced as high-risk findings with remediation guidance to ensure user-controlled format strings are never passed directly to formatting functions.

Remediation focuses on two controls: strict validation of user input and safe logging or response construction. Never pass user-controlled data directly to format verbs. Instead, use explicit, controlled formatting and avoid dynamic format strings. For Hmac Signatures, compute the signature over a canonical representation of the request, and ensure that any logging or output does not inadvertently expose the signature or related sensitive values.

Safe approach for validating and logging a signed token in Echo Go:

// Safe pattern: avoid dynamic format strings
signedToken := c.QueryParam("token")
// Validate Hmac before use (pseudo-validation)
if !isValidHmac(signedToken, secret) {
    return echo.NewHTTPError(http.StatusUnauthorized, "invalid signature")
}
// Log using explicit format and escaping
log.Printf("Validating token: %s", sanitizeForLog(signedToken))

If you must include user data in structured output, use explicit field names and avoid formatting verbs that interpret the data. For example, when returning JSON, construct the response explicitly:

type StatusResponse struct {
    Status string `json:"status"`
    Token  string `json:"token"`
}

c.JSON(http.StatusOK, StatusResponse{
    Status: "ok",
    Token:  redactToken(signedToken), // redaction for safe output
})

For Hmac Signatures, ensure the canonical string used for signing is deterministic and excludes any user-controlled format specifiers. Do not include raw user input in the signed string if it can be manipulated to alter interpretation. Use fixed field names and delimiters, and validate each component before concatenation. The following illustrates a robust signing flow:

func buildAndSign(params map[string]string, secret string) (string, string, error) {
    var keys []string
    for k := range params {
        keys = append(keys, k)
    }
    sort.Strings(keys)
    var b strings.Builder
    for i, k := range keys {
        if i > 0 {
            b.WriteString("&")
        }
        b.WriteString(k)
        b.WriteString("=")
        b.WriteString(url.QueryEscape(params[k]))
    }
    canonical := b.String()
    mac := hmac.New(sha256.New, []byte(secret))
    mac.Write([]byte(canonical))
    signature := hex.EncodeToString(mac.Sum(nil))
    return canonical, signature, nil
}

Use tools like middleBrick to verify that your endpoints avoid dynamic format strings and that Hmac Signatures are computed over safe, canonical inputs. The scanner checks Input Validation and Data Exposure concurrently, highlighting cases where user-controlled format verbs could interact with logged or echoed signature material. Remediation guidance provided includes replacing unsafe formatting with explicit outputs and ensuring signature computations do not incorporate unvalidated format strings.