Unbounded atom creation from unsigned LogoutResponse status codes in esaml allows atom table exhaustion

Allocation of Resources Without Limits or Throttling vulnerability in esaml in dropbox esaml allows an unauthenticated attacker to exhaust the Erlang VM atom table and crash the node via unsigned LogoutResponse messages.

esaml:status_code_map/1 in src/esaml.erl maps any StatusCode value it does not recognize to an atom with list_to_atom/1 on the last colon-separated segment of the URN. esaml_sp:validate_logout_response/2 verifies a signature only when the message carries one, so an unsigned LogoutResponse posted to the single-logout endpoint reaches esaml:decode_logout_response/1 and mints one permanent atom per distinct status value. Atoms are never reclaimed and the VM caps the atom table at about one million entries, so an attacker who posts that many distinct values aborts the node.

This issue affects esaml: from 0.1.0 onward.

1. Input. The single-logout endpoint in esaml_cowboy:validate_logout/2 reads the request body, decodes the SAMLResponse form field with esaml_binding:decode_response/2, and passes a samlp:LogoutResponse document to esaml_sp:validate_logout_response/2.

2. No signature required. validate_logout_response/2 looks for a ds:Signature element and verifies it only when one is present. An unsigned document passes through unchanged to esaml:decode_logout_response/1, which requires only the Version and IssueInstant attributes before mapping the StatusCode value.

3. Interning. esaml:status_code_map/1 matches six known SAML status URNs to fixed atoms. Any other value starting with urn: falls through to a clause that interns the last colon-separated segment with list_to_atom/1. The attacker chooses that segment, so each request with a fresh value mints one permanent atom. The VM aborts when the atom table reaches its limit of about one million entries.

1. Build an unsigned samlp:LogoutResponse document with Version="2.0", any IssueInstant, and a samlp:Status/samlp:StatusCode whose Value is a fresh URN such as urn:x:<unique token>.
2. Deflate and base64-encode it, and post it to the service provider's single-logout endpoint as the SAMLResponse form field with SAMLEncoding set to the DEFLATE URI.
3. esaml_sp:validate_logout_response/2 skips signature verification because none is present, esaml:decode_logout_response/1 maps the status through status_code_map/1, and the atom table grows by one entry per distinct token. The endpoint answers with an error, but the atom is never reclaimed. Repeat with fresh tokens until the VM aborts.

Reachable only where the service provider exposes a single-logout endpoint that hands incoming LogoutResponse messages to esaml_sp:validate_logout_response/2, directly or through esaml_cowboy:validate_logout/2. The consume (ACS) path verifies signatures before decoding and does not reach the status code mapping.