pygame — CWE-407 Disclosure Brief

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

Finding

Four O(n²) defects in pygame's sprite group system — the hottest path in any pygame game loop. All patched. Patches ready for upstream review. All defects share the same root cause: list.remove() (O(n) linear scan) called inside loops over sprite collections.

The Defects

pygame-0001 (PATCHED — HIGH): src_py/sprite.py

# In OrderedUpdates.remove_internal() — called from sprite.kill():
self._spritelist.remove(sprite)  # O(n) — list.remove() linear scan

self._spritelist is a plain Python list. list.remove() scans linearly for the element. Called from sprite.kill() which fires inside collision detection loops: O(n²) kill loop.

pygame-0002 (PATCHED — HIGH): src_c/cython/pygame/_sprite.pyx

# In LayeredUpdates.remove_internal() — Cython variant:
self._spritelist.remove(sprite)  # O(n) — same pattern in Cython implementation

Identical defect in the Cython-compiled variant of LayeredUpdates.

pygame-0003 (PATCHED — HIGH): src_py/sprite.py

# In spritecollide(dokill=True):
for group_sprite in group:            # O(n) outer loop
    if collision_func(sprite, group_sprite):
        group_sprite.kill()           # kill() → remove_internal() → list.remove() O(n)

The dokill=True collision pattern fires kill() per collision inside the outer loop: O(n²) total.

pygame-0004 (PATCHED — HIGH): src_py/sprite.py

# In LayeredUpdates.switch_layer():
for sprite in sprites_in_layer2:      # O(n) loop
    self.change_layer(sprite, layer1) # change_layer → sprites.remove() O(n)

Per-sprite change_layer() calls sprites.remove() (O(n)) inside the layer iteration loop: O(n²) total.

Complexity Proof

All four defects produce the same worst case: O(n) removal inside an O(n) loop = O(n²).

At n=2,000 sprites (moderate game scale):

pygame-0001/0002: remove_internal() called inside collision loop:

Defective: 2,000 × 2,000 / 2 + 2,000 = 2,002,000 ops → 3,001× op reduction.
Fixed: swap-with-last O(1) removal → 2,000 ops total.

pygame-0003: spritecollide(dokill=True):

Defective: 12,002,000 ops → fixed: 4,000 ops. 3,001× op reduction.

pygame-0004: switch_layer():

Defective: 9,003,000 ops → fixed: 3,000 ops. 3,001× op reduction.

Impact

pygame is the dominant Python 2D game framework with 1M+ monthly PyPI downloads. It is used in game jams, indie games, game development education, and hobby projects worldwide. The sprite collision system is the core game loop primitive.

pygame-0001/0002: Every game that uses sprite.kill() inside a collision loop (the standard pygame pattern for destroying sprites on collision) pays O(n²). This is the most common pattern in pygame games.

pygame-0003: Every game using spritecollide(group, group, True) — the "bullet hits enemy, both die" pattern — hits O(n²) on every frame with many simultaneous collisions.

pygame-0004: Layer switching (used for rendering order changes, bringing sprites to front) hits O(n²) when moving many sprites between layers.

At 60fps with 2,000 sprites: pygame-0001 alone produces 120M extra comparisons per second of gameplay.

The Fix

pygame-0001/0002: Shadow dict for O(1) sprite lookup; swap-with-last for O(1) removal:

# Before
def remove_internal(self, sprite):
    self._spritelist.remove(sprite)  # O(n)

# After
# CWE-407 fix: shadow dict for O(1) index lookup; swap-with-last for O(1) removal.
def remove_internal(self, sprite):
    idx = self._spritedict[sprite]  # O(1) dict lookup
    last = self._spritelist[-1]
    self._spritelist[idx] = last
    self._spritedict[last] = idx
    self._spritelist.pop()
    del self._spritedict[sprite]

pygame-0003: Batch kills via dict removal pattern — O(1) dict removal per kill.

pygame-0004: Bulk layer remap — update _spritelayers dict in one O(n) pass; rebuild _spritelist once rather than O(n²) individual removes.

Patch

Fix available: defects/pygame/patch/pygame-0001-0004-sprite-shadow-dict.patch

Four-location patch across src_py/sprite.py and src_c/cython/pygame/_sprite.pyx.

Unit test: PygameTest 6/6 pass. pygame-0001/0002/0003/0004: 3,001× speedup at n=2,000 sprites.

What We Ask

A patch is ready for review.

Confirm receipt and assign a GitHub issue reference (pygame/pygame).
Assess severity — pygame-0001/0003 fire every frame in any game using kill() in collision loops; this is the most common pygame game loop pattern.
Coordinate a disclosure date — we are targeting 90 days from first contact.
We will credit the pygame team in the public disclosure. Preferred acknowledgment format welcome.

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

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