instructure/canvas-lms · info

[AUTH] Cookie data (present) failed to unmarshal…

Error message

[AUTH] Cookie data (present) failed to unmarshal. Inactivity timeout or invalid digest.

What it means

Canvas's enhanced cookie store decrypts/verifies and unmarshals session cookie data. If a cookie is present but unmarshals to nil — digest invalid, serialized payload corrupt, or session expired by inactivity — it logs this warning instead of raising, and the user gets a fresh empty session.

Solutions

1. Benign for a single user — they just log in again; confirm new sessions work
2. If widespread: ensure session secrets/signing keys are consistent across all app servers (no mixed old/new keys mid-rollout)
3. Check for cookie truncation by proxies (keep session payload small)
4. Verify no recent change to cookie store serializer/version and that inactivity timeout config matches expectations

# Caller-side: treat nil session as anonymous
session_data = EnhancedCookieStore.unmarshal(cookie_data)
user = session_data.present? ? load_user(session_data) : nil

def valid_session?(data)
  data.is_a?(Hash) && data.key?('session_id')
end

# The store already swallows errors; callers should just check for nil
session = store.unmarshal(data)
if session.nil?
  # start a fresh anonymous session and require re-login
end

When it happens

Trigger: A present, non-empty cookie's data fails Marshal.load or digest verification in unmarshal — inactivity-timeout expiry, stale cookie after a signing-secret change, or truncated/corrupted cookie data.

Common situations: Deploy that changed session store secrets or serialization; idle user past inactivity timeout reusing an old cookie; proxies truncating oversized cookies; cookie tampering; format changes across upgrades.

Understand the failure class

Background: "failed to unmarshal" / json.Unmarshal errors: why parsing a response into a Go struct fails and how to fix it — this error's family across 23 libraries.

• Timeouts: ETIMEDOUT, deadlines, and hung requests — what actually expires when a request times out.

Related errors

• Attachment verifier token expired: #
• Attachment verifier token id mismatch. token id: #
• Authentication required to view other users' module progress
• can't build pseudonym_credentials except on just-generated…
• Cannot generate a services JWT without a 'sub' entry

AI-assisted analysis of instructure/canvas-lms@1c9f0bb801 (2026-09-15). Data as JSON: /api/errors/d70c259349242ab2. Report an issue: GitHub.

Appendix: source

Thrown at app/middleware/enhanced_cookie_store.rb:36

# with this program. If not, see <http://www.gnu.org/licenses/>.

class EnhancedCookieStore < ActionDispatch::Session::EncryptedCookieStore
  def unmarshal(data, options = {})
    unmarshalled_data = nil
    begin
      unmarshalled_data = super
    rescue ArgumentError => e
      # if the data being provided is not formatted in such a way that
      # we can extract appropriately sized segments from it,
      # then this is an auth problem (bad cookie), not a real
      # exception.  We'll return nil as though the cookie
      # was unauthorized (and it is), and log the failure, but not explode because
      # handling this as some 4xx is more accurate than a 500.
      Canvas::Errors.capture_exception(:cookie_store, e, :info)
      return nil
    end
    if unmarshalled_data.nil? && data.present?
      Rails.logger.warn("[AUTH] Cookie data (present) failed to unmarshal. Inactivity timeout or invalid digest.")
    end
    unmarshalled_data
  end
end

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