# Queryable Encryption ESC/ECOC metadata compaction and leakage tradeoffs > Source-anchored research for Queryable Encryption ESC/ECOC metadata compaction and leakage tradeoffs. Run status: COMPLETED. Parent: [MongoDB Security Architecture](https://llms-explorer.com/tree/mongodb-security-architecture/) · 3 facets · 16 facts · page: https://llms-explorer.com/tree/queryable-encryption-esc-ecoc-metadata-compaction-and-leakage-tradeoffs/ ## Measurements and reference values - MongoDB documents manual metadata compaction, recommends running it when metadata exceeds 1 GB, and exposes compactStructuredEncryptionData with per-field compactionTokens and a mongosh wrapper. (medium confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/reference/limitations.md) ## Problems, failure modes and limitations - MongoDB recommends manual metadata compaction above 1 GB; treat that operational threshold together with the scheduling leakage warning. (medium confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/reference/limitations.md) - MongoDB documents omission of compactStructuredEncryptionData and other encrypted-collection operations from the slow query log and system.profile; this reduces diagnostic visibility and is not a claim that every log channel is protected. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/reference/limitations.md) - SERVER-128430 describes concurrent writes racing with ECOC rename/recreation during cleanup or compaction. On sharded clusters, implicit ECOC creation can fail inside a distributed transaction. The ticket was updated October 5, 2026; its listed versions and resolution do not prove every later build is affected. (low confidence) (verified as of 2026-10-05) — [source](https://jira.mongodb.org/browse/SERVER-128430) - MongoDB excludes persistent adversaries and adversaries possessing both database snapshots and accompanying query transcripts/logs from Queryable Encryption security guarantees. Its documented redaction omits compactStructuredEncryptionData from the slow query log and profiler on encrypted collections. (low confidence) (verified as of 2026-10-14) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/reference/limitations.md) ## Facts and statements - QE maintains ESC and ECOC metadata collections alongside the encrypted collection; Go driver integration tests explicitly check their creation. (medium confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - For equality-indexed fields, each insert or update adds one ESC write and one ECOC write per indexed field. Before compaction, each metadata collection stores one document per indexed field/value occurrence. (low confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - Metadata compaction empties ECOC and reduces ESC by removing redundant metadata documents; compactStructuredEncryptionData is the dedicated command. (medium confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - MongoDB warns that compaction can, in the worst case, reveal how many unique field/value pairs were inserted since the previous compaction. (low confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - The documented scheduling rule for removing at least t ESC documents is (encfields × docinserts) − valinserts ≥ t, with valinserts counting unique field/value pairs since the last compaction. (low confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - The documented workflow runs compaction manually through an encryption-configured mongosh client; the command accepts field-to-compaction-token mappings. (medium confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md) - The ETH Zurich analysis published in June 2023 studied the MongoDB 6.0 public-preview design, including an ECC collection absent from the current two-collection description. Its compaction attack assumes access to an operation-log snapshot after compaction and a similarly distributed auxiliary plaintext dataset; statistical attack evaluation used simulated leakage. These are historical results, not verification against current releases. (low confidence) — [source](https://ethz.ch/content/dam/ethz/special-interest/infk/inst-infsec/appliedcrypto/research/Security-Analysis-of-MongoDB-Queryable-Encryption.pdf) - MongoDB excludes persistent adversaries and attackers with snapshots plus query transcripts/logs from QE security guarantees. The ETH attack illustrates why log-inclusive compromise needs separate assessment. (medium confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/reference/limitations.md) - MongoDB engineering issue SERVER-92399 describes range-compaction padding documents added to ESC to reduce leakage of real anchor documents. It reports an unsigned-underflow problem in unit-test padding calculations; this record does not establish a deployed production exploit. (low confidence) — [source](https://jira.mongodb.org/browse/SERVER-92399) - SWiSSSE proposes evaluating leakage across both encrypted indexes and encrypted documents. Its research constructions combine response-volume bucketization, delayed pseudorandom write-backs, and indistinguishable searches and updates; these are research techniques, not documented MongoDB compaction options. (low confidence) — [source](https://research.ibm.com/publications/swissse-system-wide-security-for-searchable-symmetric-encryption) - MongoDB creates enxcol_..esc and enxcol_..ecoc alongside an encrypted collection. These metadata structures and __safeContent__ must not be manually altered. (low confidence) — [source](https://www.mongodb.com/docs/manual/core/queryable-encryption/fundamentals/manage-collections.md)