bug: session-state poisoning — FNV-1a-authenticated dictionary, unbounded tables

[minagishl]

Bug Description

SessionEncoder maintains stateful tables (keys, strings, shapes, field-enums, dictionaries, dictionary profiles, schemas). Two related problems:

1. Dictionary integrity is FNV-1a (dictionary_payload_hash, src/protocol.rs:3811), keyed by data the attacker also supplies. FNV-1a is not a cryptographic MAC; an attacker who can write a dictionary entry can forge any payload that produces the same hash, or simply omit the integrity step entirely since both inputs are attacker-controlled. There is no authentication of the dictionary contents against any out-of-band secret.
2. All session tables grow unbounded. There is no per-table cap on number of registered keys, strings, shapes, field-enums, dictionaries, or observations. An attacker streaming unique-per-message keys/strings against a long-lived SessionEncoder causes linear memory growth until OOM.

Steps to Reproduce

For (1):

1. Open a connection that uses a shared SessionEncoder between peers.
2. Send a RegisterDictionary payload, then send a normal message that references that dictionary id with content of the attacker's choosing.
3. Receiver decodes the attacker's content as if it were authenticated.

For (2):

1. Open a long-lived session.
2. Stream millions of small messages each containing a unique string in String{Literal} mode (or unique map keys).
3. Watch process RSS grow linearly.

Expected Behavior

For (1): Dictionaries should either be authenticated with a keyed MAC tied to the session, or treated as opaque advisory data that does not affect application semantics. At minimum, document the trust model so consumers understand FNV-1a is not security.

For (2): Add configurable per-table caps (max_keys, max_strings, max_shapes, max_observations, etc.) on SessionOptions, with sensible defaults. Reject Register* operations that exceed the cap.

Actual Behavior

Dictionary integrity is FNV-1a only; tables grow unboundedly.

Environment

• Crate: recurram, session/state module

Additional Context

Severity: High — long-lived OOM (slow-burn DoS) and stateful confusion primitive.