CVE-2020-7693: 
JavaScript vulnerability analysis and mitigation

SockJS before version 0.3.20 contains a Denial of Service (DoS) vulnerability identified as CVE-2020-7693. The vulnerability stems from incorrect handling of Upgrade header with the value websocket, which leads to crashing of containers hosting sockjs applications (Snyk JS, Snyk Java).

The vulnerability occurs when processing websocket upgrade requests. The issue exists in the response handling where res.end() is called instead of res.write(), leading to an Error [ERR_STREAM_WRITE_AFTER_END] condition when attempting to write after the stream has ended. This implementation flaw causes the container to crash when receiving specific websocket upgrade requests (GitHub Issue, GitHub PR). The vulnerability has a CVSS score of 5.3 (Medium) with Network attack vector and Low attack complexity (Snyk JS).

When exploited, this vulnerability can cause denial of service by crashing containers hosting SockJS applications. The attack affects the availability of the service, where resources become either partially available or intermittently unavailable, though there is no direct impact on confidentiality or integrity of the system (Snyk JS).

The vulnerability is remotely exploitable without requiring any privileges or user interaction. A proof-of-concept exploit has been developed and published by Andrew Snow, demonstrating the ability to trigger the vulnerability by sending crafted websocket upgrade requests to the target application (GitHub PoC).

The recommended mitigation is to upgrade SockJS to version 0.3.20 or higher, which contains the fix for this vulnerability. The fix involves changing the response handling to use res.write() instead of res.end() when processing websocket upgrade requests (GitHub PR).