crowdsecurity/crowdsec · error

invalid ticket in challenge response

Error message

invalid ticket in challenge response

What it means

Raised in ValidateChallengeResponse when the ticket/nonce material (the 'r' field) fails ticket validation — e.g. the ticket is expired, unknown to this runtime, or malformed. Signals a stale or replayed challenge session rather than a client-side computation failure.

Source

Thrown at pkg/appsec/challenge/challenge.go:640

	clientNonce := vars.Get("n")
	clientPowSalt := vars.Get("p")
	clientPowMAC := vars.Get("m")
	clientDifficultyStr := vars.Get("d")
	clientPath := vars.Get("u")

	if encryptedFingerprint == "" || clientR == "" || clientTS == "" || clientSig == "" || clientNonce == "" || clientPowSalt == "" || clientPowMAC == "" || clientDifficultyStr == "" || clientPath == "" {
		return nil, FingerprintData{}, 0, errors.New("missing required fields in challenge response")
	}

	// Override the request path so the WAF sees the original URL the client
	request.URL.Path = clientPath

	// The difficulty the client claims it solved. It is untrusted until the PoW
	// MAC (which binds it) is verified in verifyChallenge below. Bound to the
	// valid PoW range so a malformed value can't reach the PoW/seal logic.
	clientDifficulty, err := strconv.Atoi(clientDifficultyStr)
	if err != nil || clientDifficulty < PowDifficultyDisabled || clientDifficulty > PowDifficultyImpossible {
		return nil, FingerprintData{}, 0, errors.New("invalid ticket in challenge response")
	}

	// Server-issued `r` is a 16-byte nonce in hex (generateChallengeNonce):
	// exactly 32 hex chars. Reject other shapes early so a K_epoch holder can't
	// bloat the spent-set with oversized keys, and to keep the key space canonical.
	if _, err := hex.DecodeString(clientR); err != nil || len(clientR) != 32 {
		return nil, FingerprintData{}, 0, errors.New("invalid ticket in challenge response")
	}

	// Verify freshness + PoW-salt/difficulty authenticity and recover the
	// per-epoch sign key (stateless). Key knowledge is proven by `sig` below.
	signKey, ok := c.verifyChallenge(clientR, clientTS, clientPowSalt, clientPowMAC, clientDifficulty)
	if !ok {
		return nil, FingerprintData{}, 0, errors.New("invalid ticket in challenge response")
	}

	// Impossible difficulty is a deliberate hard-block: never accept anything.
	// clientDifficulty is now MAC-authenticated, so this enforces the difficulty

View on GitHub (pinned to 909b515798)

Solutions

1. Issue a fresh challenge to the client when the ticket is stale
2. Verify clock skew between crowdsec nodes is minimal so freshly issued tickets aren't rejected as expired

When it happens

Trigger: Thrown at pkg/appsec/challenge/challenge.go:640 when the library encounters an invalid state.

Common situations: See trigger scenarios.

AI-assisted analysis of crowdsecurity/crowdsec@909b515798 (2026-09-06). Data as JSON: /api/errors/da4f10afc0b9f74f. Report an issue: GitHub.