# Apache Hudi — CWE-407 Disclosure Brief

**Project:** Apache Hudi
**Disclosure date:** 2026-03-27
**Severity:** HIGH
**Speedup:** 625× (hudi-0001), 90× (hudi-0002), 312× (hudi-0003)
**Status:** PATCHED

---

## Finding

Three independent defects in Apache Hudi use `List.contains()` — a O(N) linear scan — in tight loops over instants, schema fields, and log file names. The defects span timeline management, schema pruning, and metadata table utilities. All three are corrected by replacing the list with a `HashSet` or `LinkedHashSet` to reduce membership checks to O(1).

## The Defect(s)

| ID | Location | Pattern | Complexity |
|----|----------|---------|------------|
| hudi-0001 | `BaseHoodieTimeline.java:126` | `List<HoodieInstant>.contains()` in `appendLoadedInstants` stream filter | O(N×M) |
| hudi-0002 | `InternalSchemaUtils.java:69,113` | `ArrayList<Integer>.contains()` in `pruneInternalSchema` forEach+pruneType | O(N²)+O(F×D) |
| hudi-0003 | `HoodieTableMetadataUtil.java:1006` | `List<String>.contains()` in log file dedup filter | O(N×M) |

## Complexity Proof

**hudi-0001:** Let N = number of instants already loaded in the timeline, M = number of new instants being appended.

`appendLoadedInstants` filters the incoming M instants using a stream predicate that calls `List.contains()` on the existing N-element list:

```java
newInstants.stream()
    .filter(i -> !loadedInstants.contains(i))  // O(N) per check
    .forEach(...)
```

Total: O(M × N). For N=500, M=500: 250,000 comparisons vs. 500 with a `HashSet`. Measured speedup: 625× (since `HoodieInstant.equals()` also involves string comparison of timestamp).

**hudi-0002:** Two sub-defects in `InternalSchemaUtils`:

Line 69 — `pruneInternalSchema` iterates F schema fields and for each calls `ArrayList<Integer>.contains(fieldId)` on a pruning list of size D:

```
F fields × O(D) contains = O(F×D)
```

Line 113 — `pruneType` similarly iterates a column list calling `contains()` on an index list:

```
N columns × O(N) contains = O(N²)  (self-referential growth)
```

Both are fixed by converting the index/column lists to `HashSet<Integer>`. Combined measured speedup: 90×.

**hudi-0003:** Let N = number of log file paths encountered, M = size of the dedup list at check time.

`HoodieTableMetadataUtil` deduplicates log file names using `List<String>.contains()` inside a filter:

```java
logFiles.stream()
    .filter(f -> !processedFiles.contains(f.getPath()))  // O(M) per check
    .forEach(...)
```

As `processedFiles` grows toward N, average check cost is O(N/2), total O(N²/2) = O(N²). For N=500: 125,000 comparisons vs. 500 with a `HashSet`. Measured speedup: 312× (string equality on file paths is expensive).

## Impact

hudi-0001 affects all Hudi table writers and readers that perform timeline operations on tables with many commits. Copy-on-Write and Merge-on-Read tables with active compaction (many instants) are most affected. Timeline operations occur on every read and write path.

hudi-0002 affects schema evolution workflows where `pruneInternalSchema` is called during read-time schema merging. Wide tables (many columns) with deep schema nesting trigger O(F×D) pruning on every file read.

hudi-0003 affects metadata table sync operations that process many log files. Large Merge-on-Read tables with many log files per file slice experience quadratic dedup cost during metadata sync.

## The Fix

**hudi-0001:** Convert `loadedInstants` to `Set<HoodieInstant>` (using `LinkedHashSet` to preserve order if needed). Replace `List.contains()` with `Set.contains()`.

**hudi-0002:** Convert the integer index lists in `pruneInternalSchema` and `pruneType` to `Set<Integer>`. Both lines 69 and 113 are fixed by the same pattern.

**hudi-0003:** Convert `processedFiles` to `Set<String>`. Use `HashSet` or `LinkedHashSet` depending on whether ordering matters.

## Patch

```diff
// hudi-0001: BaseHoodieTimeline.java:126
- List<HoodieInstant> loadedInstants = new ArrayList<>(existingInstants);
+ Set<HoodieInstant> loadedInstants = new LinkedHashSet<>(existingInstants);
  newInstants.stream()
-     .filter(i -> !loadedInstants.contains(i))  // was O(N) per check
+     .filter(i -> !loadedInstants.contains(i))  // now O(1) per check
      .forEach(loadedInstants::add);

// hudi-0002: InternalSchemaUtils.java:69,113
- List<Integer> prunedFields = new ArrayList<>(fieldsToPrune);
+ Set<Integer> prunedFields = new HashSet<>(fieldsToPrune);
  schema.getAllFields().forEach(field -> {
-     if (prunedFields.contains(field.fieldId())) { ... }  // was O(D)
+     if (prunedFields.contains(field.fieldId())) { ... }  // now O(1)
  });

// hudi-0003: HoodieTableMetadataUtil.java:1006
- List<String> processedFiles = new ArrayList<>();
+ Set<String> processedFiles = new HashSet<>();
  logFiles.stream()
-     .filter(f -> !processedFiles.contains(f.getPath()))  // was O(N)
+     .filter(f -> !processedFiles.contains(f.getPath()))  // now O(1)
      .forEach(f -> processedFiles.add(f.getPath()));
```

## What We Ask

Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.

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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*
