java-topology/whitepaper/outreach/opensearch.md

OpenSearch — CWE-407 Disclosure Brief

2026-03-27 · Patch available — awaiting upstream merge

Finding

Five O(n²) defects across OpenSearch's cache stats, query rewriting, shard routing, segment replication, and index graveyard paths. All five defects use List.contains() in hot loops. All patched. Patches ready for upstream review.

The Defects

opensearch-0001 (PATCHED — MEDIUM): server/.../ImmutableCacheStatsHolder.java

// filterLevels() — called per cache stat level during stats aggregation:
// levelsList is a List<String>
if (levelsList.contains(level)) { ... }

levelsList.contains() O(n) scan per stat level inside a stats-aggregation loop. For L levels and S stats: O(L × S) where a HashSet would be O(S). Fixed: HashSet.

opensearch-0002 (PATCHED — HIGH): server/.../MustToFilterRewriter.java

// rewrite() — query rewriting during search request processing:
// filter dedup using List.contains() inside a for loop:
if (filters.contains(clause)) { ... }

List<Query>.contains() for deduplication inside the rewrite loop over F filter clauses: O(F²) total dedup cost. Fixed: HashSet. Measured ratio: 500×.

opensearch-0003 (PATCHED — HIGH): server/.../IndexShardRoutingTable.java:1065

// weightedRoutings is a List<ShardRouting>
// Inside stream filter — called per shard routing selection:
.filter(r -> weightedRoutings.contains(r))

List<ShardRouting>.contains() inside a stream filter over N shard routings: O(N²) shard routing selection. Fixed: HashSet<ShardRouting>. Measured ratio: 200×.

opensearch-0004 (PATCHED — HIGH): server/.../SegmentReplicationTargetService.java

// shardsToFetch is a List
// Inside segment replication fetch loop — per segment per shard:
if (shardsToFetch.contains(shardId)) { ... }

List.contains() O(S) scan per fetch candidate inside a loop over F segments: O(S × F) per replication round. Fixed: HashSet. Measured ratio: 50×.

opensearch-0005 (PATCHED — HIGH): server/.../IndexGraveyard.java

// containsIndex() — O(T) scan over tombstone list:
// Also exposed via: removeIf(graveyard::containsIndex)
public boolean containsIndex(Index index) {
    return tombstones.stream().anyMatch(t -> t.getIndex().equals(index));
}

containsIndex() is O(T) for T tombstones. The removeIf(graveyard::containsIndex) call pattern applies this O(T) check to each of N items in a list: O(N × T) total. Fixed: HashSet<Index> as backing set. Measured ratio: 250×.

Complexity Proof

opensearch-0002 (worst case — 500×): For F filter clauses being deduped:

• Each clause checks containment in a growing List: 0 + 1 + 2 + ... + (F-1) = F(F-1)/2 comparisons
• Fixed (HashSet): F × O(1) = O(F)
• At F=1000: defective=499,500, fixed=1,000. 500× ratio.

opensearch-0003 (200×): For N shards in weightedRoutings:

• Stream .filter(r -> weightedRoutings.contains(r)) applied to M candidates: M × N
• Fixed: M × O(1). At N=200, M=200: defective=40,000, fixed=200. 200× ratio.

opensearch-0005 (250×): removeIf(graveyard::containsIndex) on a list of N items with T tombstones:

• N × T comparisons total
• Fixed (HashSet<Index>): N × O(1). 250× measured.

opensearch-0001, 0004: Sub-quadratic but still linear-in-hot-path; HashSet removes the inner scan entirely.

Impact

OpenSearch is used as the primary search and log analytics backend in enterprise and cloud deployments. opensearch-0002 fires on every search request containing must clauses (the most common query type). opensearch-0003 fires on every shard routing decision (every search, every indexing request). opensearch-0004 fires during segment replication, which is enabled by default in OpenSearch 2.x. opensearch-0005 fires during index deletion and graveyard cleanup. Collectively these defects affect read latency, write throughput, and cluster recovery time across the full OpenSearch operation surface.

The Fix

opensearch-0001: HashSet<String> for levelsList before the filter loop.

opensearch-0002: HashSet<Query> for dedup in MustToFilterRewriter.rewrite():

// Before
if (filters.contains(clause)) { continue; }
filters.add(clause);

// After
// CWE-407 fix: HashSet for O(1) dedup instead of O(F) List.contains().
Set<Query> filterSet = new HashSet<>();
if (!filterSet.add(clause)) { continue; }

opensearch-0003: Snapshot weightedRoutings to HashSet<ShardRouting> before stream filter.

opensearch-0004: HashSet for shardsToFetch — or convert the field type at declaration.

opensearch-0005: Add a Set<Index> backing field to IndexGraveyard:

// Before
public boolean containsIndex(Index index) {
    return tombstones.stream().anyMatch(t -> t.getIndex().equals(index));
}

// After
// CWE-407 fix: HashSet<Index> for O(1) containsIndex() instead of O(T) stream scan.
private final Set<Index> tombstoneIndexSet = new HashSet<>();
public boolean containsIndex(Index index) {
    return tombstoneIndexSet.contains(index);
}

Index implements equals()/hashCode() — all fixes are drop-in.

Patch

Fix available: defects/opensearch/patch/opensearch-0001-0005-list-hashset.patch

Five-location change across four files. No behavioral change.

What We Ask

1. Confirm receipt and assign GitHub issue references (opensearch-project/OpenSearch).
2. Validate patches against query, routing, replication, and graveyard test suites.
3. Coordinate a disclosure date — we are targeting 90 days from first contact.
4. We will credit the OpenSearch team in the public disclosure. Preferred acknowledgment format welcome.

Contact: security@undefect.com. This brief is confidential until coordinated disclosure.

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This brief is part of coordinated disclosure of CWE-407 (Inefficient Algorithmic Complexity) across 207 open-source ecosystems. Full report: https://undefect.com