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CVE-2021-21404— Crash due to malformed relay protocol message

Quick assessment

Affected
syncthing syncthing
Exploitation
Public or AI PoC available; prioritize validation
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Jakob Borg syncthing是Jakob Borg开源的一个应用软件。一个连续的文件同步程序。 Syncthing before version 1.15.0 存在安全漏洞，该漏洞源于可能会因发送带有负长度字段的中继消息而崩溃并退出。

CVSS 7.5 · High EPSS 1.97% · P79
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I. Basic Information for CVE-2021-21404

Vulnerability Information
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Vulnerability Title
Crash due to malformed relay protocol message
Source: CVE Program / CVE List V5
Vulnerability Description
Syncthing is a continuous file synchronization program. In Syncthing before version 1.15.0, the relay server `strelaysrv` can be caused to crash and exit by sending a relay message with a negative length field. Similarly, Syncthing itself can crash for the same reason if given a malformed message from a malicious relay server when attempting to join the relay. Relay joins are essentially random (from a subset of low latency relays) and Syncthing will by default restart when crashing, at which point it's likely to pick another non-malicious relay. This flaw is fixed in version 1.15.0.
Source: CVE Program / CVE List V5
CVSS Information
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
Source: CVE Program / CVE List V5
Vulnerability Type
输入验证不恰当
Source: CVE Program / CVE List V5
Vulnerability Title
Jakob Borg syncthing 输入验证错误漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Jakob Borg syncthing是Jakob Borg开源的一个应用软件。一个连续的文件同步程序。 Syncthing before version 1.15.0 存在安全漏洞，该漏洞源于可能会因发送带有负长度字段的中继消息而崩溃并退出。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
syncthing syncthing < 1.15.0 -

II. Public POCs for CVE-2021-21404

# POC Description Source Link Shenlong Link
AI-Generated POC Premium
Qwen3.6-35B-A3B · 7758 chars
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III. Intelligence Information for CVE-2021-21404

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Patches & Fixes for CVE-2021-21404 (1)

Vendor Advisories for CVE-2021-21404 (1)

Other References for CVE-2021-21404 (2)

IV. Related Vulnerabilities

Same product: syncthing
Same vendor: syncthing
Same weakness: CWE-20

V. Comments for CVE-2021-21404

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