Format String in Cockroachdb

CockroachDB is written in Go, and format string vulnerabilities in Go arise when a developer passes user‑controlled data as the format argument to functions in the fmt package (e.g., fmt.Sprintf, fmt.Fprintf, log.Printf). If the format string contains conversion specifiers like %s, %x or %n, the function will read additional arguments from the stack. Supplying fewer arguments than the format expects can cause the function to read arbitrary memory, leading to information disclosure or a crash.

In CockroachDB, this pattern can appear in logging helpers, error‑message builders, or diagnostic endpoints that echo user input. For example, a handler that builds a debug message like:

func debugHandler(w http.ResponseWriter, r *http.Request) {
    userID := r.URL.Query().Get("id")
    msg := fmt.Sprintf("User %s requested debug info", userID) // safe
    // Vulnerable version:
    // msg := fmt.Sprintf(userID)   // userID used as format string
    http.Error(w, msg, 200)
}

If the developer mistakenly writes fmt.Sprintf(userID) (or passes the user value as the first argument to log.Printf), an attacker can supply a string such as "%x.%x.%x.%x" to leak stack contents, or "%n" to attempt a write‑what‑where condition (though Go’s runtime prevents arbitrary writes, it can still cause panics). CockroachDB’s extensive use of structured logging reduces risk, but any ad‑hoc string building that interpolates raw request parameters into a format string remains a potential vector.

Detecting format string issues in a CockroachDB service relies on observing whether user‑supplied data influences the format argument of a fmt call. Since CockroachDB exposes HTTP/gRPC endpoints for admin tasks, metrics, and debugging, an unauthenticated scanner can probe those endpoints with payloads containing format specifiers and examine the responses for anomalies.

middleBrick’s Input Validation check (one of the 12 parallel scans) automatically sends a variety of test strings—including %x, %s, %n and longer repetitive patterns—to each discovered endpoint. If the reflector returns unexpected data (e.g., hexadecimal stack values, truncated output, or an error indicating a panic), the check flags a potential format string vulnerability.

Practical steps for developers to spot the issue manually:

• Search the codebase for fmt\.(Sprintf|Fprintf|Printf), log\.(Print|Printf|Println), or any fmt.Fprintln where the first argument is a variable that originates from r.URL.Query(), r.FormValue, json.Decode, or similar.
• Check logging wrappers: if a logger is called as logger.Errorf(userInput, ...) where the user input is the format string, flag it.
• Use static analysis tools (e.g., gosimple, staticcheck) with the SA1019 or SA4013 rules that detect user‑controlled format strings.

When middleBrick reports a finding, it includes the exact endpoint, the HTTP method, the payload that triggered the anomaly, and a severity rating based on the observed impact (information leak vs. potential crash).

The fix is to ensure that format strings are constant and never derived from external input. Replace any usage where a variable supplies the format with a constant format string and pass the variable as a regular argument.

Example – unsafe logging:

// Vulnerable
func logRequest(r *http.Request) {
    userAgent := r.Header.Get("User-Agent")
    log.Printf(userAgent)   // userAgent used as format string
}

Fixed version:

// Safe
func logRequest(r *http.Request) {
    userAgent := r.Header.Get("User-Agent")
    log.Printf("User-Agent: %s", userAgent)
}

If the intention is to simply output the user‑provided value, use log.Print(userAgent) or fmt.Fprintln(w, userAgent) without a format verb.

For diagnostic endpoints that echo input for debugging, avoid fmt.Sprintf altogether and return the raw value or a JSON‑encoded representation:

// Unsafe debug endpoint
func debug(w http.ResponseWriter, r *http.Request) {
    val := r.URL.Query().Get("val")
    fmt.Fprintf(w, val)   // dangerous
}

Safe alternative:

// Safe debug endpoint
func debug(w http.ResponseWriter, r *http.Request) {
    val := r.URL.Query().Get("val")
    w.Header().Set("Content-Type", "text/plain")
    fmt.Fprintln(w, val)   // no format verbs
}

When building complex messages, use strings.Builder or fmt.Sprintf with a literal format:

// Safe composition
func buildMsg(id string) string {
    return fmt.Sprintf("ID=%s, timestamp=%d", id, time.Now().Unix())
}

After applying these changes, re‑run middleBrick (via the CLI, GitHub Action, or MCP Server) to confirm that the Input Validation check no longer reports a format string finding. The remediation guidance provided by middleBrick will reference the specific line numbers and suggest the exact pattern to replace.