CVE-2021-21404 : Exploit Details and Defense Strategies

Syncthing, a continuous file synchronization program, before version 1.15.0 is prone to crashing due to a vulnerability in the relay server 'strelaysrv'. An attacker can cause the server to crash by sending a relay message with a negative length field, leading to service disruption. This flaw is considered high severity with a CVSS base score of 7.5.

Understanding CVE-2021-21404

This CVE describes a vulnerability in Syncthing that allows an attacker to crash the relay server by sending a malicious relay message.

What is CVE-2021-21404?

CVE-2021-21404 is a vulnerability in Syncthing that affects versions prior to 1.15.0. It results in a crash of the relay server 'strelaysrv' when a relay message with a negative length field is received.

The Impact of CVE-2021-21404

The impact of this vulnerability is high, with a CVSS base score of 7.5. It can lead to service disruption and availability impact due to the relay server crashing.

Technical Details of CVE-2021-21404

The vulnerability lies in the relay server component of Syncthing, which can crash when receiving a malformed relay message. This flaw allows an attacker to disrupt the service by exploiting this vulnerability.

Vulnerability Description

The vulnerability arises from improper handling of relay messages with a negative length field, leading to a crash in the relay server 'strelaysrv'.

Affected Systems and Versions

Syncthing versions prior to 1.15.0 are affected by this vulnerability. Users are advised to update to version 1.15.0 or later to mitigate the risk.

Exploitation Mechanism

An attacker can exploit this vulnerability by sending a malformed relay message with a negative length field to the relay server, causing it to crash and disrupt service.

Mitigation and Prevention

To address CVE-2021-21404, users and administrators are recommended to take immediate steps and adopt long-term security practices to enhance system protection.

Immediate Steps to Take

Update Syncthing to version 1.15.0 or later to fix the vulnerability.
Monitor network traffic for any suspicious activity that may indicate an exploit attempt.

Long-Term Security Practices

Regularly update the software and keep abreast of security advisories to protect against emerging threats.
Implement network security measures to restrict access and prevent unauthorized exploitation.

Patching and Updates

Syncthing has released version 1.15.0, which contains a fix for this vulnerability. Users should promptly update their installations to this version or the latest release available.