crowdsecurity/crowdsec · error

failed to generate challenge JS: %w

Error message

failed to generate challenge JS: %w

What it means

GetChallengePage serves the challenge JS from the runtime cache. If the cached challenge code is empty, it attempts a synchronous generateAndCacheChallengeJS; if that generation fails, this error wraps the cause and no challenge page can be served.

Source

Thrown at pkg/appsec/challenge/challenge.go:547

		c.preWarmCancel()
	}

	return c.r.Close(ctx)
}

// GetChallengePage renders the challenge HTML page with the given PoW difficulty.
// If difficulty is 0, the default difficulty is used.
func (c *ChallengeRuntime) GetChallengePage(ctx context.Context, userAgent string, difficulty int) (string, error) {
	_ = userAgent

	if difficulty <= 0 {
		difficulty = c.powDifficulty
	}

	challengeCode := c.getChallengeCode()
	if challengeCode == "" {
		if err := c.generateAndCacheChallengeJS(ctx); err != nil {
			return "", fmt.Errorf("failed to generate challenge JS: %w", err)
		}
		challengeCode = c.getChallengeCode()
		if challengeCode == "" {
			return "", errors.New("challenge JS cache is empty")
		}
	}

	// Issuance is stateless (only submission is stateful — see the single-use
	// burn in ValidateChallengeResponse). `r` seeds the per-challenge secret
	// `s = HMAC(K_epoch, r)` the client derives from the obfuscated dynamic
	// module, so `s` never appears in plain HTML; the PoW MAC binds the salt to
	// `r`+ts so a client can't pick a favorable salt.
	ts := fmt.Sprintf("%d", time.Now().UnixNano())
	r, err := generateChallengeNonce()
	if err != nil {
		return "", err
	}
	powSalt, err := generatePowPrefix()

View on GitHub (pinned to 909b515798)

Solutions

1. Rebuild/restart so the baked-in initial bundle is present (`go generate`, `make build`).
2. Check the wrapped error: fix the underlying generation failure (context deadline, crypto, template).
3. Increase request/startup timeouts so the ~5s synchronous obfuscation can finish.
4. Verify the pre-warmer is running (skipPreWarm=false) so the cache is never empty.

Example fix

// before: challenge served with an already-half-expired request ctx
page, err := rt.GetChallengePage(req.Context(), opts)
// after: use a detached ctx with a generation budget
page, err := rt.GetChallengePage(context.WithTimeout(context.Background(), 30*time.Second), opts)

page, err := rt.GetChallengePage(ctx, opts)
if err != nil {
    var genErr error
    if errors.As(err, &genErr) && strings.Contains(err.Error(), "challenge JS") {
        // one bounded retry with a fresh, longer-lived context
    }
    http.Error(w, "challenge unavailable", http.StatusServiceUnavailable)
}

When it happens

Trigger: Calling GetChallengePage when getChallengeCode() returns "" (cache empty — e.g. constructor fell back and generation never succeeded, or cache was cleared) and generateAndCacheChallengeJS(ctx) errors (ctx cancelled, obfuscation failure).

Common situations: Request arriving before any bundle was ever generated; expired/cleared cache with a request context too short for the ~5s obfuscation; upstream generation failures after a bad rebuild.

Related errors

• build dynamic key module: %w
• ErrChallengeFields
• ErrChallengeTicket
• unable to get challenge page: %w
• unable to seal allowlist cookie: %w

AI-assisted analysis of crowdsecurity/crowdsec@909b515798 (2026-09-06). Data as JSON: /api/errors/3a00292b7397cb2e. Report an issue: GitHub.