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SynapseApplication · Matrix

CVE-2024-53863

CRITICAL · 9.1 CVSS v3.1 Published 2024-12-03
Fix available
A fix is available. Upgrade to 1.120.1 or later.
See remediation →
100/100
Remediation priority · Urgent
Remotely reachable No privileges Zero-click

Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.

NVD · unedited
Synapse is an open-source Matrix homeserver. 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. Synapse 1.120.1 addresses the issue by restricting thumbnail generation to images in the following widely used formats: PNG, JPEG, GIF, and WebP. This vulnerability is fixed in 1.120.1.

Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.

dbcve analysis · high confidence

Synapse before 1.120.1 allows thumbnail generation for uncommon image formats when dynamic_thumbnails is enabled or via specially crafted requests. This triggers external tools like Ghostscript to process these formats, expanding the attack surface in a historically vulnerable area (image processing). Version 1.120.1 restricts thumbnail generation to only PNG, JPEG, GIF, and WebP.

MitigationUpgrade Synapse to version 1.120.1 or later. If immediate upgrade is not feasible, disable the dynamic_thumbnails configuration option as a temporary mitigation.

Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.

Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.

NVD · CPE data
SynapseApplication
Affected:< 1.120.1

CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.

From the vector
Attack vector
Network
Complexity
Low
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
High
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:N

Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.

dbcve checks

Work through these to decide whether this CVE applies to you.

  1. Determine installed Synapse version
    Run 'python -m synapse.app.homeserver --version' or check the package manager (e.g., 'dpkg -l matrix-synapse' or 'pip show synapse'), or inspect the version in the Synapse logs on startup
    Affected if The installed version is before 1.120.1 (e.g., 1.119.0, 1.118.0, etc.)
  2. Locate Synapse configuration file
    The default location is homeserver.yaml in the Synapse config directory (often /etc/matrix-synapse/ or ~/synapse/). Check for custom path via command-line args or environment variable SYNAPSE_CONFIG_DIR
    Affected if Configuration file exists and is readable
  3. Check dynamic_thumbnails setting
    In the configuration file, search for 'dynamic_thumbnails' setting. It is typically under the 'thumbnail' or 'images' section. If present, note its value (true/false). If absent, the default is false
    Affected if dynamic_thumbnails is set to true OR if version is vulnerable AND the server accepts image uploads that trigger thumbnail generation
  4. Verify thumbnail generation is accessible
    Confirm that the Synapse server is configured to accept and process room avatar or uploaded images. This is enabled by default if the server handles any rooms with image content. Check that 'downloads' or 'upload' endpoints are not explicitly disabled
    Affected if The server accepts image uploads or serves room/user avatars, which enables thumbnail processing for those images

You are affected if Synapse version is before 1.120.1 AND the server processes image uploads (especially with dynamic_thumbnails enabled), allowing specially crafted requests for uncommon image formats to trigger thumbnail generation via external tools.

Generated from the published advisory. Verify against your own configuration.

Check your environment

Paste your version and any relevant configuration and it will be compared against the affected criteria above. Do not include secrets or credentials.

AI-assisted, checked against the advisory. Informational, not a guarantee.

Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.

dbcve · scoped
Upgrade available Upgrade to 1.120.1 or later
Fixed in 1.120.1
Interim mitigation

Upgrade Synapse to version 1.120.1 or later. If immediate upgrade is not feasible, disable the dynamic_thumbnails configuration option as a temporary mitigation.

Recommended fix High confidence

Synapse 1.120.1

  1. Check current Synapse version using `python -c import synapse; print(synapse.__version__)` or your package manager
  2. Upgrade Synapse to version 1.120.1 or later using your deployment method (pip, package manager, or container)
  3. Verify the upgrade was successful by checking the installed version
  4. Ensure dynamic_thumbnails configuration remains as desired (the fix now restricts thumbnail generation to PNG, JPEG, GIF, and WebP formats only)

Generated from the published advisory — verify against the referenced sources before acting.

Fix this in Synapse Scoped from the published advisory
  • Consultation2.0 h
  • Implementation2.0 h
  • Testing2.0 h
  • Review / QA1.0 h
7.0 hours of engineering $1,240
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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2024-53863 in production — separate from our analysis above.

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • Verified mitigations, workarounds, and config changes
  • Version or environment caveats, and links to real fixes
  • No weaponised exploit code, or anything meant to cause harm
  • No spam, self-promotion, credentials, or personal data