CVE-2024-53863 in synapse
Summary
by MITRE • 12/03/2024
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.
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Analysis
by VulDB Data Team • 08/26/2025
The vulnerability identified as CVE-2024-53863 affects Synapse, an open-source Matrix homeserver implementation that serves as a central communication hub for decentralized messaging networks. This security flaw emerges from the software's handling of dynamic thumbnail generation capabilities, which were designed to automatically create preview images for media shared within the Matrix ecosystem. The issue becomes particularly concerning when administrators enable the dynamic_thumbnails option or when the server processes specially crafted requests that exploit the thumbnail generation subsystem. The vulnerability stems from the server's indiscriminate processing of various image formats without proper validation or restriction of input types, creating a potential attack vector that leverages the underlying image processing infrastructure.
The technical flaw manifests through the server's reliance on external tools such as Ghostscript for handling uncommon image formats during thumbnail generation. When a malicious actor submits a crafted request or when dynamic thumbnails are enabled, the system attempts to decode and process image files using various decoding libraries and external utilities. This creates a significant expansion of the attack surface because these external tools often have their own vulnerabilities and complex code paths that can be exploited through crafted inputs. The vulnerability specifically targets the lack of proper input validation and format restriction within the thumbnail processing pipeline, allowing attackers to potentially trigger arbitrary code execution or resource exhaustion through specially crafted image files that invoke external processing tools. This aligns with CWE-20, which describes improper input validation, and represents a classic case of insecure deserialization or processing of untrusted input.
The operational impact of this vulnerability extends beyond simple denial of service scenarios, as the exploitation could potentially lead to full system compromise through external tool invocation. When external tools like Ghostscript are invoked during image processing, they often have extensive functionality that can be abused, including file system access, network connectivity, and memory manipulation capabilities. The risk is particularly elevated because the affected versions of Synapse process images that are rarely used on the open web or within the Matrix ecosystem, meaning that the attack surface expands significantly when these uncommon formats are encountered. Attackers could leverage this vulnerability to perform remote code execution, resource exhaustion, or information disclosure attacks, depending on how the external tools respond to malicious input. This vulnerability directly maps to ATT&CK technique T1059.007 for command and scripting interpreter and T1499.004 for network infiltration, as it enables unauthorized access through the processing of malicious image files.
The mitigation implemented in Synapse version 1.120.1 addresses the core issue by restricting thumbnail generation to only widely used image formats including PNG, JPEG, GIF, and WebP. This approach follows the principle of least privilege and defense in depth by limiting the scope of potentially vulnerable external tool invocations to only formats that are commonly used and well-understood within the Matrix ecosystem. The restriction effectively eliminates the attack surface associated with uncommon image formats that might invoke external tools with complex processing capabilities. Organizations should immediately upgrade to version 1.120.1 or later to remediate this vulnerability, and administrators should review their configuration settings to ensure that dynamic_thumbnails is disabled unless absolutely necessary. Additionally, implementing network segmentation, monitoring for unusual thumbnail generation requests, and conducting regular security assessments of the image processing pipeline can further reduce the risk of exploitation. The fix demonstrates proper security engineering practices by reducing the attack surface through format restriction rather than attempting to patch individual vulnerabilities in external tools, which would be both complex and potentially incomplete.