medium4.3

CVE-2026-27695

zae-limiter: DynamoDB hot partition throttling enables per-entity Denial of Service

Impact

## 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 DynamoDB `ThrottledRequests` CloudWatch metric increasing 4. Observe `acquire()` latency spikes and `RateLimiterUnavailable` exceptions ## Remediation Design: Pre-Shard Buckets - Move buckets to `PK={ns}/BUCKET#{entity}#{resource}#{shard}, SK=#STATE` — one partition per (entity, resource, shard) - Auto-inject `wcu:1000` reserved limit on every bucket — tracks DynamoDB partition write pressure in-band (name may change during implementation) - Shard doubling (1→2→4→8) triggered by client on `wcu` exhaustion or proactively by aggregator - Shard 0 at suffix `#0` is source of truth for `shard_count`. Aggregator propagates to other shards - Original limits stored on bucket, effective limits derived: `original / shard_count`. Infrastructure limits (`wcu`) not divided - Shard selection: random/round-robin. On application limit exhaustion, retry on another shard (max 2 retries) - Lazy shard creation on first access - Bucket discovery via GSI3 (KEYS_ONLY) + BatchGetItem. GSI2 for resource aggregation unchanged - Cascade: parent unaware, protected by own `wcu` - Aggregator: parse new PK format, key by shard_id, effective limits for refill, filter `wcu` from snapshots - Clean break migration: schema version bump, old buckets ignored, new buckets created on first access - **$0.625/M preserved on hot path**

Affected packages

Ecosystem	Package	Affected	Fixed in
pip	`zae-limiter`	<= 0.10.0	0.10.1

Affected products

Vendor	Product	Affected versions
Zeroae	Zae-Limiter	before 0.10.1

Weakness classification

CWE-770

Impact

Affected packages

Affected products

Weakness classification

References