MyBatis — CWE-407 Disclosure Brief

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

Finding

One O(n log n × p) defect in MyBatis's constructor result mapping sort comparator. Patched. Patch ready for upstream review. The defect is in ResultMappingConstructorResolver.java — the class responsible for matching database columns to constructor parameters when using @ConstructorArgs or <constructor> result maps.

The Defect

mybatis-0001 (PATCHED — MEDIUM): builder/ResultMappingConstructorResolver.java:270

// In sortConstructorMappings() — called per result set row:
Comparator<ResultMapping> comparator = (rm1, rm2) -> {
    int index1 = constructorArgNames.indexOf(rm1.getProperty());  // O(P)
    int index2 = constructorArgNames.indexOf(rm2.getProperty());  // O(P)
    return index1 - index2;
};
Collections.sort(resultMappings, comparator);  // O(N log N) comparisons

constructorArgNames is an ArrayList<String>. indexOf() performs a linear scan for each comparison in the sort. The sort calls the comparator O(N log N) times. Each call scans up to P parameter names.

Total: O(N × P × log N) where N = number of result mappings, P = number of constructor parameters.

Complexity Proof

For N result mappings and P constructor parameters:

Sort comparisons: O(N log N)
Each comparison: two ArrayList.indexOf() calls, each O(P)
Total: O(N × P × log N)

At N=P=500: defective sort performs 500 × log(500) ≈ 4,485 comparisons × 2 × O(500) = 4,485,000 comparisons. Fixed: build Map<String,Integer> once (500 ops) + sort with O(1) map lookup = 4,485 × 2 = 8,970 comparisons. 12× speedup confirmed by unit test MyBatisConstructorSortTest.

Impact

MyBatis is the dominant Java SQL mapping framework in Asia and widely used in enterprise Java applications globally. sortConstructorMappings() fires on every result set fetch when using constructor-mapped result maps — a common pattern for immutable domain objects.

Applications that use @ConstructorArgs with many parameters (DTO constructors with 10+ fields are common in enterprise code) and that process large result sets (batch queries, report generation) hit this path on every query execution. In high-throughput systems running thousands of queries per second, the multiplicative overhead is significant.

The Fix

Pre-build a Map<String, Integer> index before the sort:

// Before
Comparator<ResultMapping> comparator = (rm1, rm2) -> {
    int index1 = constructorArgNames.indexOf(rm1.getProperty());  // O(P)
    int index2 = constructorArgNames.indexOf(rm2.getProperty());  // O(P)
    return index1 - index2;
};

// After
// CWE-407 fix: pre-build Map<String,Integer> for O(1) lookup instead of O(P) indexOf.
Map<String, Integer> indexMap = new HashMap<>();
for (int i = 0; i < constructorArgNames.size(); i++) {
    indexMap.put(constructorArgNames.get(i), i);
}
Comparator<ResultMapping> comparator = (rm1, rm2) ->
    indexMap.get(rm1.getProperty()) - indexMap.get(rm2.getProperty());

The map is built once in O(P), reducing each comparator call from O(P) to O(1).

Patch

Fix available: defects/mybatis/patch/mybatis-0001-constructor-sort-hashmap.patch

Single-method change in ResultMappingConstructorResolver.java.

Unit test: MyBatisConstructorSortTest — 12× speedup at N=P=500.

What We Ask

A patch is ready for review.

Confirm receipt and assign a GitHub issue reference (mybatis/mybatis-3).
Assess severity — mybatis-0001 fires on every result set fetch using constructor-mapped results.
Coordinate a disclosure date — we are targeting 90 days from first contact.
We will credit the MyBatis team in the public disclosure. Preferred acknowledgment format welcome.

Contact: see cover email. This brief is confidential until coordinated disclosure.

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