CVE-2021-21236

Doyensec Vulnerability Advisory

• Regular Expression Denial of Service (REDoS) in cairosvg
• Affected Product: CairoSVG v2.0.0+
• Vendor: https://github.com/Kozea
• Severity: Medium
• Vulnerability Class: Denial of Service
• Author(s): Ben Caller (Doyensec)

When processing SVG files, the python package CairoSVG uses two regular expressions which are vulnerable to Regular Expression Denial of Service (REDoS).
If an attacker provides a malicious SVG, it can make cairosvg get stuck processing the file for a very long time.

Technical description

The vulnerable regular expressions are

https://github.com/Kozea/CairoSVG/blob/9c4a982b9a021280ad90e89707eacc1d114e4ac4/cairosvg/colors.py#L190-L191

The section between 'rgb(' and the final ')' contains multiple overlapping groups.

Since all three infinitely repeating groups accept spaces, a long string of spaces causes catastrophic backtracking when it is not followed by a closing parenthesis.

The complexity is cubic, so doubling the length of the malicious string of spaces makes processing take 8 times as long.

Reproduction steps

Create a malicious SVG of the form:

<svg width="1" height="1"><rect fill="rgb(                     ;"/></svg>

with the following code:

'<svg width="1" height="1"><rect fill="rgb(' + (' ' * 3456) + ';"/></svg>'

Note that there is no closing parenthesis before the semi-colon.

Run cairosvg e.g.:

cairosvg cairo-redos.svg -o x.png

and notice that it hangs at 100% CPU. Increasing the number of spaces increases the processing time with cubic complexity.

Disclosure timeline

• 2020-12-30: Vulnerability disclosed via email to CourtBouillon

Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.

CVE-2021-21236 has a CVSS score of 7.5 (High). The vector is network-reachable, no privileges required, and no user interaction. A CVSS score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether this affects your application depends on whether the vulnerable code is present and reachable in your environment. A fixed version is available (2.5.1); upgrading removes the vulnerable code path.

Kodem intelligence

Fix the regexes to avoid overlapping parts. Perhaps remove the [ \n\r\t]* groups from the regex, and use .strip() on the returned capture group.

1. What is CVE-2021-21236? CVE-2021-21236 is a high-severity uncontrolled resource consumption vulnerability in CairoSVG (pip), affecting versions < 2.5.1. It is fixed in 2.5.1. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
2. How severe is CVE-2021-21236? CVE-2021-21236 has a CVSS score of 7.5 (High). This score reflects the worst-case severity of the vulnerability, not your specific exposure. Whether it represents real risk in your environment depends on whether the vulnerable code is present and reachable.
3. Which versions of CairoSVG are affected by CVE-2021-21236? CairoSVG (pip) versions < 2.5.1 is affected.
4. Is there a fix for CVE-2021-21236? Yes. CVE-2021-21236 is fixed in 2.5.1. Upgrade to this version or later.
5. Is CVE-2021-21236 exploitable, and should I be worried? Whether CVE-2021-21236 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
6. What actually determines whether CVE-2021-21236 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
7. How do I fix CVE-2021-21236? Upgrade CairoSVG to 2.5.1 or later.