crowdsecurity/crowdsec · error

ErrCookieSignature

ErrCookieSignature

Error message

%w: %w

What it means

AES-GCM authentication of the cookie ciphertext failed while opening a v0 cookie envelope: the ciphertext (or AAD) does not match the master cookie key. ErrCookieSignature wraps the crypto error; causes are key rotation to a different secret, a tampered cookie, or corruption. The cookie is treated as invalid and the client re-challenged.

Source

Thrown at pkg/appsec/challenge/crypto.go:239

	if err != nil {
		return nil, fmt.Errorf("failed to create cipher: %w", err)
	}

	gcm, err := cipher.NewGCM(block)
	if err != nil {
		return nil, fmt.Errorf("failed to create GCM: %w", err)
	}

	nonceSize := gcm.NonceSize()
	if len(body) < nonceSize {
		return nil, fmt.Errorf("%w: ciphertext too short", ErrCookieMalformed)
	}

	nonce, ciphertext := body[:nonceSize], body[nonceSize:]

	plaintext, err := gcm.Open(nil, nonce, ciphertext, aad)
	if err != nil {
		return nil, fmt.Errorf("%w: %w", ErrCookieSignature, err)
	}

	if len(plaintext) < cookiePlaintextFixedHeaderLen {
		return nil, fmt.Errorf("%w: plaintext shorter than fixed header", ErrCookieMalformed)
	}

	notAfter := int64(binary.BigEndian.Uint64(plaintext[:8]))
	flags := plaintext[8]
	reasonLen := int(binary.BigEndian.Uint16(plaintext[9:11]))

	if reasonLen > MaxAllowlistReasonLen {
		return nil, fmt.Errorf("%w: reason_len=%d", ErrCookieMalformed, reasonLen)
	}

	if len(plaintext) < cookiePlaintextFixedHeaderLen+reasonLen {
		return nil, fmt.Errorf("%w: plaintext shorter than declared reason_len", ErrCookieMalformed)
	}

View on GitHub (pinned to 909b515798)

Solutions

1. Check cookie-key consistency across replicas — different master keys make all cookies fail GCM open
2. No action for isolated occurrences (tampered cookies are the attacker's problem)
3. If all cookies fail after a redeploy, restore the previous secret or wait for cookie TTL expiry

When it happens

Trigger: Thrown at pkg/appsec/challenge/crypto.go:239 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/49888cf2968dbdec. Report an issue: GitHub.