Attack Chain

1. Initial Access & Authentication: An attacker obtains valid credentials for an authenticated user account with write permissions to at least one data record within the deepstream server.
2. Identify Writeable Record: The attacker identifies a specific data record within the deepstream server that their authenticated user account has permissions to modify.
3. Craft Malicious Payload: The attacker constructs a data modification message, such as a SET operation, targeting the identified record. This message includes a data path or key containing the string __proto__, constructor, or prototype (e.g., {"someField.__proto__.isAdmin": true} or {"data": {"__proto__": {"isAdmin": true}}}).
4. Send Malicious Message: The crafted message is sent to the vulnerable deepstream server through its API or client interface. This might occur via an HTTP request body or, in some cases, via URI components.
5. Prototype Pollution Trigger: The deepstream server processes the incoming message. Due to the prototype pollution vulnerability (CVE-2026-49252), the server improperly handles the special prototype path component within the message, leading to the modification of the Object.prototype or a similar base object prototype in the server's JavaScript environment.
6. Privilege Escalation: The attacker leverages the polluted prototype to inject or modify properties (e.g., isAdmin flags, internal configuration settings, or references to executable functions) that are subsequently used by other parts of the server application.
7. Impact Fulfillment: Through this manipulation, the attacker successfully elevates their own privileges within the deepstream server, potentially gaining administrative control, accessing sensitive data, or executing arbitrary commands within the server's context.

Impact

The successful exploitation of CVE-2026-49252 can lead to severe consequences, primarily privilege escalation. Any authenticated user with write permissions to any record can elevate their privileges, potentially gaining administrative access over the deepstream server. This could result in complete compromise of the data managed by deepstream, unauthorized access to sensitive information, arbitrary code execution on the server, and disruption of services. While no specific victim counts are provided, all deepstream server instances running vulnerable versions (up to 10.0.4) are at risk, necessitating immediate action to prevent broad compromise.

Recommendation

• Upgrade the @deepstream/server package to version 10.0.5 immediately to patch CVE-2026-49252.
• Implement the recommended workaround by filtering all incoming messages containing the strings __proto__, constructor, or prototype in their path before they reach the deepstream server's message pipeline.
• Deploy the Sigma rule below to your SIEM to detect attempts to exploit this vulnerability against webserver logs by monitoring URI paths and query parameters.