python-igraph — CWE-407 Disclosure Brief

Project: python-igraph Disclosure date: 2026-03-27 Severity: MEDIUM Speedup: 47× Status: PATCHED

Finding

python-igraph's CohesiveBlocks.max_cohesion() method calls list.index() — an O(V) linear scan — inside a double loop over blocks B and vertices V. The list.index() call converts a vertex ID to its position in a list on every iteration, a lookup that can be reduced to O(1) by pre-building an inverse mapping dict.

The Defect(s)

ID	Location	Pattern	Complexity
igraph-0001	`igraph/clustering.py`	`list.index()` O(V) inside O(B×V) loop in `CohesiveBlocks.max_cohesion()`	O(B×V²)

Complexity Proof

Let V = number of vertices in the graph, B = number of cohesive blocks.

max_cohesion() iterates over all B blocks, and for each block iterates over all V vertices to determine cohesion levels. Inside this O(B×V) loop, it calls some_list.index(vertex) to find the position of the vertex in a reference list:

for block in blocks:              # B iterations
    for vertex in range(V):       # V iterations
        pos = vertex_list.index(vertex)   # O(V) scan

Total: O(B × V × V) = O(B×V²)

Pre-building an inverse dict vertex_to_index = {v: i for i, v in enumerate(vertex_list)} reduces each lookup to O(1) amortized:

Total with dict: O(V) build + O(B×V×1) = O(V + B×V) = O(B×V)

For a graph with V=1000 vertices and B=50 blocks: defective = 50 × 10⁶ = 50M ops; fixed = 50 × 1000 = 50K ops. Measured speedup: 47×.

Impact

Graph analysts using cohesive block decomposition on social networks, biological interaction graphs, or infrastructure dependency graphs with thousands of vertices experience disproportionate slowdown in max_cohesion() queries. This function is called in post-processing and visualization workflows where interactive latency is expected.

The Fix

Pre-build a {vertex: index} dict before the loop. Replace vertex_list.index(vertex) with vertex_to_index[vertex].

Patch

  def max_cohesion(self):
+     vertex_to_index = {v: i for i, v in enumerate(self._vertices)}
      result = [0] * len(self._graph.vs)
      for block_idx, block in enumerate(self._blocks):
          for vertex in block:
-             pos = self._vertices.index(vertex)
+             pos = vertex_to_index[vertex]
              if self._cohesion[block_idx] > result[pos]:
                  result[pos] = self._cohesion[block_idx]
      return result

What We Ask

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

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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