CVE-2026-58270 in server
Summary
by MITRE • 09/22/2026
Sync-in Server is an open-source platform for file storage, sharing, collaboration, and syncing. Prior to version 2.4.0, the sync diff endpoint compiles a user-supplied string into a `RegExp` with no complexity validation. A catastrophic-backtracking pattern (e.g. `^(a+)+b`) blocks the Node.js event loop, making the entire server unresponsive to all users until the container is restarted. Version 2.4.0 patches the issue.
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Analysis
by VulDB Data Team • 09/22/2026
Sync-in Server operates as an open-source platform designed for file storage, sharing, collaboration, and synchronization across distributed environments. The architecture relies heavily on efficient processing of data differences to maintain consistency between client devices and the central server. Within this system, a critical security flaw was identified in the sync diff endpoint prior to version 2.4.0. This specific component is responsible for calculating changes required to synchronize file states, but it contains a significant implementation error regarding input validation when processing regular expressions.
The technical root cause of this vulnerability lies in how the server handles user-supplied strings within the sync diff logic. Specifically, the application compiles these inputs directly into JavaScript Regular Expressions without implementing any complexity checks or safeguards against pathological patterns. This lack of validation allows an attacker to craft a malicious input string that triggers catastrophic backtracking when processed by the Node.js regular expression engine. A classic example of such a pattern is `^(a+)+b`, which forces the regex engine to explore an exponential number of possible matches before determining failure, thereby consuming excessive CPU resources and blocking execution flow.
The operational impact of this flaw is severe due to the single-threaded nature of the Node.js event loop. When a catastrophic backtracking attack is executed against the vulnerable endpoint, it monopolizes the main thread for an extended period. This effectively freezes the entire server instance, rendering it unresponsive to all legitimate users and requests. The denial of service persists until the affected container or process is manually restarted by system administrators, leading to significant downtime and potential data availability issues for organizations relying on the platform for critical file operations.
This vulnerability aligns with CWE-400, which describes Uncontrolled Resource Consumption, as well as CWE-1321, specifically related to Improperly Controlled Modification of Object Prototype Attributes or Regular Expression Complexity. In terms of offensive security frameworks, this flaw facilitates a Denial of Service attack under the MITRE ATT&CK technique T1499, where an adversary overwhelms system resources to disrupt service availability. The exploitation does not require authentication in many configurations if the endpoint is publicly accessible, making it particularly dangerous for exposed deployments.
To mitigate this risk, organizations running Sync-in Server versions prior to 2.4.0 must upgrade immediately to version 2.4.0 or later, where the developers have implemented proper validation and complexity checks before compiling user inputs into regular expressions. For environments that cannot be upgraded instantly, network-level controls such as rate limiting on the sync diff endpoint can help mitigate the impact by restricting the frequency of requests from a single source. Additionally, deploying Web Application Firewalls with rules designed to detect regex-based denial-of-service patterns may provide an additional layer of defense against exploitation attempts while permanent remediation is pursued.