CairoSVG is an SVG converter based on Cairo, a 2D graphics library. Prior to 2.9.1, rendering an attacker-controlled SVG with a path containing many segments can cause quadratic CPU consumption in cairosvg/path.py. The path tokenizer repeatedly slices and rescans the remaining path data, while draw_markers drains node.vertices with node.vertices.pop(0), causing repeated linear-time work. The svg2png, svg2pdf, and svg2ps APIs reach these operations during ordinary rendering, allowing a sub-megabyte SVG to consume substantial CPU and deny service to a rendering application. This issue is fixed in version 2.9.1.

Affected: PyPI: cairosvg

## Summary

Rendering an untrusted SVG whose `<path d="...">` contains many segments is O(n²) CPU. A single `<path>` under 1 MiB burns tens of seconds. Two independent O(n²) sites in `cairosvg/path.py`:

1. **Tokenizer** — the path-data parser consumes the `d` string with a `while string:` loop that repeatedly slices/re-scans the *remaining* string (each step is O(len remaining)), giving O(n²) over the whole attribute. 2. **draw_markers** — marker handling drains `node.vertices` with `while node.vertices: ... node.vertices.pop(0)`; `list.pop(0)` is O(n), so draining n vertices is O(n²).

Both are hit on a normal render path (`svg2png`/`svg2pdf`), attacker controls only the SVG document.

## PoC (installed cairosvg 2.9.0)

```python import cairosvg d = "M0 0 " + "L1 1 " * 100000 svg = f'<svg xmlns="http://www.w3.org/2000/svg" width="10" height="10"><path d="{d}"/></svg>' cairosvg.svg2png(bytestring=svg.encode()) # ~4.4 s for a 488 KB doc ```

| path segments | SVG size | time | |---|---|---| | 50,000 | 244 KB | 1.14 s | | 100,000 | 488 KB | 4.36 s | | 200,000 | ~960 KB | ~18 s |

Doubling segments ≈ 4× time ⇒ quadratic. Sub-MiB input ⇒ ~18 s CPU; any service rendering user-supplied SVG (thumbnails, avatars, PDF export) is a DoS target.

## Reachability

Public API `svg2png` / `svg2pdf` / `svg2ps` on an untrusted SVG string.

## Suggested fix

Tokenize with a single forward scan / index (or `re.finditer`) instead of re-slicing the remainder; drain `vertices` with an index or…