zae-limiter: DynamoDB hot partition throttling enables per-entity Denial of Service
Date: 2026-02
Affected: pip/zae-limiter
Attack vector: dos
Category: vulnerability-disclosure
Severity: Medium
CVE: CVE-2026-27695
CWE: CWE-770
CVSS: 4.3 (CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L)

## Summary All rate limit buckets for a single entity share the same DynamoDB partition key (`namespace/ENTITY#{id}`). A high-traffic entity can exceed DynamoDB's per-partition throughput limits (~1,000 WCU/sec), causing throttling that degrades service for that entity — and potentially co-located entities in the same partition. ## Details Each `acquire()` call performs a `TransactWriteItems` (or `UpdateItem` in speculative mode) against items sharing the same partition key. For cascade entities, this doubles to 2-4 writes per request (child + parent). At sustained rates above ~500 req/sec for a single entity, DynamoDB's adaptive capacity may not redistribute fast enough, causing `ProvisionedThroughputExceededException`. The library has no built-in mitigation: - No partition key sharding/salting - No write coalescing or batching - No client-side admission control before hitting DynamoDB - `RateLimiterUnavailable` is raised but the caller has already been delayed ## Impact - **Availability**: High-traffic entities experience elevated latency and rejected requests beyond what their rate limits specify - **Fairness**: Other entities sharing the same DynamoDB partition may experience collateral throttling - **Multi-tenant risk**: In a shared LLM proxy scenario, one tenant's burst traffic could degrade service for others ## Reproduction 1. Create an entity with high rate limits (e.g., 100,000 rpm) 2. Send sustained traffic at 1,000+ req/sec to a single entity 3. Observe Dy


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