NVD / CVE Program· CNA GitHub_M

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

Noroxi assessment

Priority: Monitor (action score 21/100)

The sources we track (CISA KEV, Exploit-DB, Metasploit, Nuclei, GitHub) show no public exploit code or exploitation in the wild.
Probability of exploitation in the next 30 days: 1.6% (FIRST EPSS); higher than 76.6% of all records.
Anyone who can reach it over the internet can trigger it. No account or password is required. No user action is required. No special conditions are required; it is repeatable.
Fixed version: matrix-org synapse 1.33.2.

Low priority for now. Upgrade to matrix-org synapse 1.33.2 or later.

Generated automatically from the record's data: CVSS (NVD and CNA), CISA KEV and SSVC, FIRST EPSS, exploit sources, fixed versions. How we calculate it

Primary sources: NVD · CVE.org · FIRST EPSS

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.

MediumCVSS 5.3 · v3.1—No exploit Fix available

Published: May 11, 2021
Updated: Jun 16, 2026
EPSS: 1.6% · 76th percentile
CWE: CWE-400

Report tools

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Action score

Monitor

Low priority for now.

CVSS: 21 / 40 · 5.3 / 10
CISA KEV: 0 / 30 · Not listed
EPSS: 0 / 30 · 1.6%

CNA vs NVD score

NVD: 5.3
CNA · GitHub_M: 3.7; 1.6 point gap

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

The score given by the assigning authority versus NVD’s independent score. A gap means the severity is contested.

Get told when this record changes

Not in KEV: no record of exploitation in the wild
No public exploit code on record
A fix has been released

If any of these changes, we'll tell you in the daily email digest.

Is your version affected? Check it

Noroxi analysis

What does it mean in your environment?

The assessment above is generated automatically from the record's data; it doesn't know your environment. Our team verifies on your systems whether this vulnerability is actually exploitable and which mitigation is enough.

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Affected systems

Is your version affected?

Pick the product and type the version you run. It's compared with this record's version ranges in your browser; the version isn't sent anywhere.

Vendor	Product	CPE
matrix	synapse	cpe:2.3:a:matrix:synapse
fedoraproject	fedora	cpe:2.3:o:fedoraproject:fedora

Affected versions

NVD version ranges (for catalog products). Add the product to your stack with a version and matching uses these.

fedoraproject fedora34
matrix synapsebefore 1.33.2

Versions reported by the vendor

Affected version ranges reported by the assigning authority (GitHub_M). Independent of NVD's CPE analysis and usually ahead of it.

matrix-org synapse
- < 1.33.2affected

Package-level exposure

OSV and GitHub Advisory data: ecosystem, package and range. SBOM matching uses this table.

Ecosystem	Package	Affected range	Fix
Alpine:v3.13	synapse	before 1.34.0-r0	1.34.0-r0
Alpine:v3.14	synapse	before 1.33.2-r0	1.33.2-r0
openSUSE:Tumbleweed	matrix-synapse	before 1.43.0-1.1	1.43.0-1.1
PyPI	matrix-synapse	before 1.33.2	1.33.2

Same product

matrix: all records

Other highest-scoring records for the same primary product.

CVE-2019-18835Matrix Synapse before 1.5.0 mishandles signature checking on some federation APIs.
39Monitor
CVE-2018-16515Matrix Synapse before 0.33.3.1 allows remote attackers to spoof events and possibly have unspecified other impacts by leveraging improper…
35Monitor
CVE-2024-52815Synapse allows a a malformed invite to break the invitee's `/sync`
34Monitor

Published before and after this one in the same product

← EarlierCVE-2021-21392Apr 12, 2021 · Open redirect via transitional IPv6 addresses on dual-stack networks
25Monitor
← EarlierCVE-2021-21393Apr 12, 2021 · Denial of service (via resource exhaustion) due to improper input validation on groups/communities endpoints
26Monitor
Later →CVE-2021-39163Aug 31, 2021 · Adding a private/unlisted room to a community exposes room metadata in an unauthorised manner.
12Monitor

Remediation

Which version to upgrade to

Fix versions compiled from the vendor, package registries and Microsoft. Verify the vendor's note before upgrading.

Product / package	Fixed version	Source
matrix-org synapse	1.33.2 · description	Vendor (CNA)
alpine:synapse	1.34.0-r0 · Alpine:v3.13	Package registry (OSV)
debian:matrix-synapse	1.33.2-1 · Debian:14	Package registry (OSV)
opensuse:matrix-synapse	1.43.0-1.1 · openSUSE:Tumbleweed	Package registry (OSV)
PyPI:matrix-synapse	1.33.2	Package registry (OSV)

Exploit status

No known public exploit

No public exploit has been observed yet. That doesn't mean you're safe, only that the bar is a little higher.

Research context

For pentesters and researchers: attack profile, score disagreement, timeline, patch commits, credits, variant and chain candidates, bug bounty scope. All derived from existing data; no exploit code.

Timeline

From publication to today: proof of concept, Metasploit module, CISA KEV and fix record. Dates are as reported by the sources.

No dated events beyond publication.

No history yet (it accumulates with the daily EPSS job).

Patch and commit links

Commit, PR and diff links among the references. A starting point for patch-diffing and variant hunting; fixes, not exploits.

matrix-org/synapsecommit 03318a7

Credits

All researchers

Finders, reporters and analysts named in the CNA record. Click a name for that researcher’s other records.

No credits in the CNA record.

Variant candidates

Same product, same weakness class, within 18 months. If the patch missed the root cause, the sibling bug is here.

No nightly-computed relations.

Chain candidates

An authentication bypass and a privilege-requiring bug in the same product, published close together: combined they may become an unauthenticated path.

—

Bug bounty scope

No known public program.

Source: bounty-targets-data (public HackerOne, Bugcrowd, Intigriti, YesWeHack listings).

Technical details

Attack conditions

Anyone who can reach it over the internet can trigger it.
No account or password is required.
No user action is required.
No special conditions are required; it is repeatable.

If successful

Confidentiality: none
Integrity: none
Availability: low · the service can be disrupted

Attack vector: Network
Attack complexity: Low
Privileges required: None
User interaction: None
Scope: Unchanged
Confidentiality impact: None
Integrity impact: None
Availability impact: Low

Weakness class (CWE)

CWE-400 · Uncontrolled Resource Consumption

CVSS vector

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

nvd-primary

Root cause

Code that parses nested structures sets no depth or size limit. A small input can turn into a disproportionate processing load.

The bug in code

The whole CWE-400 class

A representative example of this vulnerability class. Not the vendor's source code; it shows the faulty pattern and its fix.

Vulnerable

1function expand(node) {
2  return node.children.map(expand);
3}

Fixed

1const MAX_DEPTH = 32;
2function expand(node, depth = 0) {
3  if (depth > MAX_DEPTH) throw new Error("too deep");
4  return node.children.map((c) => expand(c, depth + 1));
5}

Attack context

MITRE CAPEC attack patterns and ATT&CK techniques for this weakness class (CWE). A starting point for detection rules and threat hunting.

Attack patterns (CAPEC)

ATT&CK techniques

Endpoint Denial of ServiceT1499

Change log

Fix✗ → ✓Sep 28, 2026, 10:44 PM UTC

For records you follow, these changes also arrive as notifications. →

References

github.com/matrix-org/synapse/commit/03318a766cac9f8b053db2214d9c332a977d226c

Show all references (3 more)

Vendor advisories and official records. Exploit/PoC links are deliberately left out.

All records