CVE-2021-29471Network attack vector

Service interruption via wildcard regex backtracking

Published May 11, 2021 · Updated Aug 3, 2024

Uncontrolled resource consumption in Matrix.org Synapse before 1.33.2 allows remote authenticated users to interrupt service via push rules. The push-rule evaluator converts each wildcard into a separate regex repetition, causing excessive backtracking on moderate-length events. A user must install a crafted event_match rule and cause an event to be processed; evaluation consumes CPU and delays homeserver work.

CVSS severity3.7
Low
EPSS probability1.65%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Uncontrolled resource consumption in Matrix.org Synapse before 1.33.2 allows remote authenticated users to interrupt service via push rules. The push-rule evaluator converts each wildcard into a separate regex repetition, causing excessive backtracking on moderate-length events. A user must install a crafted event_match rule and cause an event to be processed; evaluation consumes CPU and delays homeserver work.

The record

CVE
CVE-2021-29471
Published
May 11, 2021
Updated
Aug 3, 2024
Vendor
Matrix.org
Product
Synapse
Classifications
CWE-400, CWE-331, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 11, 2021CVE publishedPublication date reported by the CVE source.
  2. Aug 3, 2024Record updatedLatest update available in the CVE record.

Exploitability

Present is not the same as exploitable

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Is a vulnerable build present?

Compare these published version ranges with your installed build and any vendor patches.

  1. Affected versionversion=< 1.33.2

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Matrix.org · Synapseversion=< 1.33.2

Published CVSS scores

5.0NISTAV:N/AC:L/Au:N/C:N/I:N/A:P
5.3NISTCVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:L
3.7GitHub, Inc.CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

CVSS describes severity. EPSS estimates exploitation probability.

Attacks

What attackers are doing with it

Daily unique IPs observed by Shadowserver honeypots for known exploited vulnerabilities (KEVs). Missing observations do not establish an absence of attacks.

Daily unique IPsNo honeypot observations are available for this CVE in the selected window.

No observations available

Sep 10, 2026Sep 16, 2026
Latest reporting daySep 16, 2026
Latest daily unique IPsUnavailable
Prior 30-day averageUnavailable
SourceShadowserver honeypots (KEV)
Vectornetwork
Privilegesauthenticated
Known exploitationUnconfirmed
Public exploitUnconfirmed

Weakness, pattern, technique

CWE-400Uncontrolled Resource Consumption
CWE-331Insufficient Entropy
T1499.004Endpoint Denial of Service: Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

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Technologies

Your stack

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Your stack

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