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CVE Record

CVE-2021-29471: Denial of service in Matrix Synapse

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 1.33.2 "Push rules" can specify conditions under which they will match, including `event_match`, which matches event content against a pattern including wildcards. Certain patterns can cause very poor performance in the matching engine, leading to a denial-of-service when processing moderate length events. The issue is patched in version 1.33.2. A potential workaround might be to prevent users from making custom push rules, by blocking such requests at a reverse-proxy.

LowCVSS 3.7Not KEV-listedUpdated
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Glexia's TakeAutomated analysislow

Security readout for executives and security teams

Plain-English summary

CVE-2021-29471 can let Matrix Synapse spend excessive processing time evaluating certain push-rule patterns, causing reduced service availability. It affects Synapse before 1.33.2. The business impact is disruption, not data theft or integrity loss, based on the supplied CVSS and advisory details.

Executive priority

Treat this as a routine availability patch unless Synapse is business-critical or already unstable. Prioritize upgrade during the next maintenance window, with faster action for public, federated, or heavily used homeservers.

Technical view

Synapse push rules support event_match conditions with wildcard patterns. In versions before 1.33.2, certain patterns trigger very poor matching performance when processing moderate length events, creating a denial-of-service condition. The issue is tracked as CWE-400 and fixed in Synapse 1.33.2.

Likely exposure

Organizations running Matrix Synapse homeservers before 1.33.2 are exposed, especially user-facing or federated deployments where users can define custom push rules. The source bundle identifies matrix-org Synapse only and does not name other affected products.

Exploitation context

The sources describe a denial-of-service issue but do not state active exploitation. The KEV flag is false. CVSS rates this as low severity with high attack complexity, no confidentiality or integrity impact, and limited availability impact.

Researcher notes

The evidence supports a bounded DoS caused by inefficient push-rule pattern matching. The bundle includes the fix release, commit, and advisory, but does not provide exploitation-in-the-wild evidence or broader product impact.

Mitigation direction

  • Upgrade Matrix Synapse to version 1.33.2 or later.
  • If upgrade is delayed, block custom push-rule creation at a reverse proxy.
  • Use vendor or distribution packages that include the Synapse 1.33.2 fix.
  • Track Matrix Synapse advisory updates for any revised operational guidance.

Validation and detection

  • Inventory all Matrix Synapse homeservers and record installed versions.
  • Confirm no Synapse instance remains below version 1.33.2.
  • Review reverse-proxy controls if relying on the custom push-rule workaround.
  • Check monitoring for unexplained Synapse CPU spikes or availability degradation.
Prepared
Confidence
high
Sources
6

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

These mappings and lookup hints may be relevant to the vulnerability behavior, CWE, affected product, or exposure path. Glexia-inferred context is not an official MITRE, ATT&CK, CWE, or CVE Program mapping.

ATT&CK lookup starting points

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cwe · low confidence lookup

CWE-400: Exact CWE lookup

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cve · low confidence lookup

CVE-2021-29471 mapping review

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Vulnerability profileCVE Program record
Severity
Low
CVSS
3.7 (3.1)
Known Exploited
No
Published

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

1CVSS vectors
0Timeline events
0ADP providers
5Source links

CVSS vector scores

1 official score

We collect every scored CVSS vector available in the official CNA and ADP containers. When more than one version is present, the table keeps the source vectors side by side instead of collapsing them into the highest score.

ScoreVersionSeverityVectorExploitImpactSource
3.7CVSS 3.1LowCVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L2.21.4Primary CVE score

Vulnerability scoring details

Base CVSS 3.1 score

3.7Low
CVSS 3.1 vector shape for CVE-2021-29471Attack VectorAttack ComplexityPrivileges RequiredUser InteractionScopeConfidentiality ImpactIntegrity ImpactAvailability Impact

Vector: CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:N/A:L

Attack Vector
NetworkAdjacentLocalPhysical
Attack Complexity
LowHigh
Privileges Required
NoneLowHigh
User Interaction
NoneRequired
Scope
ChangedUnchanged
Confidentiality Impact
HighLowNone
Integrity Impact
HighLowNone
Availability Impact
HighLowNone

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
matrix-orgsynapse< 1.33.2Listed
Weakness

CWE details

CWE-400

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.

CWE-400 · source CWE mapping

Uncontrolled Resource Consumption

Uncontrolled Resource Consumption represents a recurring weakness pattern that can create exploitable paths when design, validation, or implementation controls are missing.

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