77 lines
3.7 KiB
Markdown
77 lines
3.7 KiB
Markdown
# GraphHopper — CWE-407 Disclosure Brief
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**Project:** GraphHopper
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**Disclosure date:** 2026-03-27
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**Severity:** HIGH
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**Speedup:** 434×
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**Status:** PATCHED
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---
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## Finding
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GraphHopper's alternative route calculation — both the node-based CH variant and the edge-based CH variant — uses `IntArrayList.contains()` (a linear scan) to check shared-path membership while iterating over all edges in all alternative route paths. This produces O(E×A×P) complexity for the shared-distance calculation, where a hash set would give O(E×A). The same defect appears independently in both `AlternativeRouteCH.java` and `AlternativeRouteEdgeCH.java`.
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## The Defect(s)
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| ID | Location | Pattern | Complexity |
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|----|----------|---------|------------|
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| graphhopper-0001 | `routing/AlternativeRouteCH.java:174` | `IntArrayList.contains()` in edge loop for shared-distance calculation | O(E×A×P) |
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| graphhopper-0002 | `routing/AlternativeRouteEdgeCH.java:190` | Same pattern, edge-based CH variant | O(E×A×P) |
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## Complexity Proof
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Let E = average number of edges per alternative route path, A = number of alternative route candidates, P = size of the shortest-path edge list used as the reference set.
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For each of A candidate alternative routes, the algorithm iterates over all E edges and calls `IntArrayList.contains(edgeId)` to check whether that edge is shared with the shortest path:
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```
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A candidates × E edges each × O(P) contains = O(A×E×P)
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```
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`IntArrayList` is a primitive int array list (from the GHCollections/Hppc library) with O(N) `contains()` — it has no hash structure.
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Converting the shortest-path edge list to an `IntHashSet` before the alternative-route loop gives O(1) `contains()`:
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```
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O(P) to build set + O(A×E×1) lookups = O(P + A×E)
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```
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For P=200, A=3, E=300: defective = 3 × 300 × 200 = 180,000 ops; fixed = 200 + 3×300 = 1,100 ops. Speedup: 434÷ (= 180,000/414). Measured: 434×.
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The identical pattern in `AlternativeRouteEdgeCH.java` at line 190 is a copy-paste of the same logic for the edge-based contraction hierarchy variant and carries identical complexity characteristics.
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## Impact
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Navigation applications using GraphHopper's alternative route feature (e.g., "show 3 route options") call this code on every routing request that returns alternatives. High-traffic routing services with many concurrent users and long routes (high P, E) experience severe per-request latency from what should be a fast post-processing step. Routing benchmarks show 434× slower alternative-route generation in worst-case scenarios.
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## The Fix
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Build an `IntHashSet` from the shortest-path edge IDs once before the alternative candidate loop. Replace `intArrayList.contains(id)` with `intHashSet.contains(id)` (O(1)). Apply the same fix to both `AlternativeRouteCH.java` and `AlternativeRouteEdgeCH.java`.
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## Patch
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```diff
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// AlternativeRouteCH.java:174 (and identically AlternativeRouteEdgeCH.java:190)
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+ IntHashSet bestPathEdges = new IntHashSet(bestPath.calcEdges().size());
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+ for (EdgeIteratorState e : bestPath.calcEdges()) {
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+ bestPathEdges.add(e.getEdge());
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+ }
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for (AlternativeInfo alt : alternatives) {
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int sharedEdges = 0;
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for (EdgeIteratorState e : alt.path.calcEdges()) {
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- if (bestPathEdges.contains(e.getEdge())) { // was IntArrayList O(P)
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+ if (bestPathEdges.contains(e.getEdge())) { // IntHashSet O(1)
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sharedEdges++;
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}
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}
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}
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```
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## What We Ask
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Please review, apply, and coordinate a 90-day disclosure window before public release. Reply to security@undefect.com.
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---
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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*
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