Summary
Workarounds
- Ensure any image codecs and helper programs, such as Ghostscript, are patched against security vulnerabilities.
- Uninstall unused image decoder libraries and helper programs, such as Ghostscript, from the system environment that Synapse is running in.
- Depending on the installation method, there may be some decoder libraries bundled with Pillow and these cannot be easily uninstalled.
- The official Docker container image does not include Ghostscript.
References
- The Pillow documentation includes a list of supported image formats and which libraries or helper programs are used to decode them.
For more information
If you have any questions or comments about this advisory, please email us at security at element.io.
Impact
In Synapse versions before 1.120.1, enabling the dynamic_thumbnails option or processing a specially crafted request could trigger the decoding and thumbnail generation of uncommon image formats, potentially invoking external tools like Ghostscript for processing.
This significantly expands the attack surface in a historically vulnerable area, presenting a risk that far outweighs the benefit, particularly since these formats are rarely used on the open web or within the Matrix ecosystem.
For a list of image formats, as well as decoding libraries and helper programs used, see the Pillow documentation.
The application accepts file uploads without adequately restricting the file type or content. Typical impact: remote code execution if the uploaded file can be served and executed on the server.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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. Kodem's runtime-powered SCA identifies whether this CVE is reachable in your applications.
Remediation advice
Synapse 1.120.1 addresses the issue by restricting thumbnail generation to images in the following widely used formats: PNG, JPEG, GIF, and WebP.
Frequently Asked Questions
- What is CVE-2024-53863? CVE-2024-53863 is a high-severity unrestricted upload of dangerous file types vulnerability in matrix-synapse (pip), affecting versions < 1.120.1. It is fixed in 1.120.1. The application accepts file uploads without adequately restricting the file type or content.
- Which versions of matrix-synapse are affected by CVE-2024-53863? matrix-synapse (pip) versions < 1.120.1 is affected.
- Is there a fix for CVE-2024-53863? Yes. CVE-2024-53863 is fixed in 1.120.1. Upgrade to this version or later.
- Is CVE-2024-53863 exploitable, and should I be worried? Whether CVE-2024-53863 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
- What actually determines whether CVE-2024-53863 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.
- How do I fix CVE-2024-53863? Upgrade
matrix-synapseto 1.120.1 or later.