CVE-2026-91776: jackson-databind retains every unknown raw type ID

### Summary With `@JsonTypeInfo(use = Id.NAME, defaultImpl = ...)`, every distinct unknown raw type ID selects the same fallback deserializer but is retained as a separate key in `TypeDeserializerBase._deserializers`. An attacker who can repeatedly supply new unknown type IDs can grow this process-lifetime cache without a configured bound. ### Details The affected path is `TypeDeserializerBase._findDeserializer()`. After an unknown name-based type ID resolves to the configured fallback/default implementation, jackson-databind caches the result under the attacker-provided raw `typeId`. Although all such IDs select the same fallback deserializer, each new string remains a distinct cache key. The behavior is runtime-confirmed in jackson-databind 2.22.1 and 3.2.1. Current 2.22 and 3.2 source branches retained the unbounded `_deserializers` map and per-raw-ID cache write when rechecked. The earlier affected floor has not been established, patched versions are: 2.18.11, 2.21.7, 2.22.3, 3.1.7 and 3.2.3. The vulnerable application must enable name-based polymorphism with a `defaultImpl` or equivalent fallback, accept attacker-influenced type IDs, and reuse a long-lived mapper/type deserializer across requests. Suggested correction: avoid caching each unknown raw ID when every such ID resolves to the same fallback, use a fallback sentinel, or use an explicitly bounded concurrency-safe cache. A regression should contrast many distinct unknown IDs with repetitions of one unknown ID across requests. ### PoC Configure a polymorphic base type with `@JsonTypeInfo(use = JsonTypeInfo.Id.NAME, defaultImpl = Fallback.class)` and deserialize inputs containing unknown type names through the same mapper. Inspect `TypeDeserializerBase._deserializers` after the run. On affected 2.x and 3.x versions, 10,000 distinct unknown raw type IDs produce 10,000 retained cache entries even though every input selects the same fallback deserializer. A matched control that repeats one unknown ID 10,000 times produces one retained entry. This isolates attacker-controlled key cardinality from ordinary request count. ### Impact Where the stated polymorphic fallback configuration is exposed to attacker-influenced type IDs, distinct inputs cause incremental process-lifetime memory retention and eventual availability pressure or denial of service. This is not claimed as a single-request allocation spike, and no fixed bytes-per-ID or time-to-out-of-memory value is asserted. No confidentiality, integrity, or code-execution impact is claimed. Requested credit: Daniel Birtwhistle