CVE-2026-27695: zae-limiter Denial of Service Vulnerability

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-76rv-2r9v-c5m6",
  "modified" : "2026-02-25T22:31:10Z",
  "published" : "2026-02-25T22:31:10Z",
  "aliases" : [ "CVE-2026-27695" ],
  "summary" : "zae-limiter: DynamoDB hot partition throttling enables per-entity Denial of Service",
  "details" : "## Summary\n\nAll 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.\n\n## Details\n\nEach `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`.\n\nThe library has no built-in mitigation:\n- No partition key sharding/salting\n- No write coalescing or batching\n- No client-side admission control before hitting DynamoDB\n- `RateLimiterUnavailable` is raised but the caller has already been delayed\n\n## Impact\n\n- **Availability**: High-traffic entities experience elevated latency and rejected requests beyond what their rate limits specify\n- **Fairness**: Other entities sharing the same DynamoDB partition may experience collateral throttling\n- **Multi-tenant risk**: In a shared LLM proxy scenario, one tenant's burst traffic could degrade service for others\n\n## Reproduction\n\n1. Create an entity with high rate limits (e.g., 100,000 rpm)\n2. Send sustained traffic at 1,000+ req/sec to a single entity\n3. Observe DynamoDB `ThrottledRequests` CloudWatch metric increasing\n4. Observe `acquire()` latency spikes and `RateLimiterUnavailable` exceptions\n\n## Remediation Design: Pre-Shard Buckets\n\n- Move buckets to `PK={ns}/BUCKET#{entity}#{resource}#{shard}, SK=#STATE` — one partition per (entity, resource, shard)\n- Auto-inject `wcu:1000` reserved limit on every bucket — tracks DynamoDB partition write pressure in-band (name may change during implementation)\n- Shard doubling (1→2→4→8) triggered by client on `wcu` exhaustion or proactively by aggregator\n- Shard 0 at suffix `#0` is source of truth for `shard_count`. Aggregator propagates to other shards\n- Original limits stored on bucket, effective limits derived: `original / shard_count`. Infrastructure limits (`wcu`) not divided\n- Shard selection: random/round-robin. On application limit exhaustion, retry on another shard (max 2 retries)\n- Lazy shard creation on first access\n- Bucket discovery via GSI3 (KEYS_ONLY) + BatchGetItem. GSI2 for resource aggregation unchanged\n- Cascade: parent unaware, protected by own `wcu`\n- Aggregator: parse new PK format, key by shard_id, effective limits for refill, filter `wcu` from snapshots\n- Clean break migration: schema version bump, old buckets ignored, new buckets created on first access\n- **$0.625/M preserved on hot path**",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:L"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "zae-limiter"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "0.10.1"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 0.10.0"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/zeroae/zae-limiter/security/advisories/GHSA-76rv-2r9v-c5m6"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2026-27695"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/zeroae/zae-limiter/commit/481ce44d818d66e31d8837bc48519660ce4c267f"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/zeroae/zae-limiter"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/zeroae/zae-limiter/releases/tag/v0.10.1"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-770" ],
    "severity" : "MODERATE",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-02-25T22:31:10Z",
    "nvd_published_at" : "2026-02-25T15:20:52Z"
  }
}