AUGUST 4, 2026
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Case File

CVE-2021-21393

MEDIUM · CVSS 5.3 EPSS 1.60% Public Exploit

Source: NVD + CISA KEV + EPSS (historical backfill) · Published 2021-04-12 · Last synced 2026-08-04

CyberRota Analysis

AI analysis pending.

Public Exploit Signal

A public exploit, PoC, GitHub repository or Metasploit reference was detected for this CVE.

Note: these links are listed for security research and verification purposes only.

CVE
CVE-2021-21393
Severity
MEDIUM
CVSS
5.3
EPSS
1.60%
GitHub

Original NVD Description

Synapse is a Matrix reference homeserver written in python (pypi package matrix-synapse). Matrix is an ecosystem for open federated Instant Messaging and VoIP. In Synapse before version 1.28.0 Synapse is missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion. Note that the groups feature is not part of the Matrix specification and the chosen maximum lengths are arbitrary. Not all clients might abide by them. Refer to referenced GitHub security advisory for additional details including workarounds.