CVE-2026-45076Network attack vector

Room-history denial via crafted federated events

Published May 28, 2026 · Updated May 28, 2026

Improper input validation in Element Synapse before 1.152.1 allows remote attackers to suppress room history via federated events. A malicious homeserver can craft room events that cause Synapse pagination to omit full history, but Element has not published the affected event fields or validation path. Exploitation requires a federated room and control of a participating remote homeserver; clients can fail to display history while other server functions remain unreported as affected.

CVSS severity5.1
Medium
EPSS probability0.37%
Next 30 days · Sep 16, 2026
Known exploitationUnconfirmed
Based on sourced intelligence
Hinoki checkNot available
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Summary

What happened

Improper input validation in Element Synapse before 1.152.1 allows remote attackers to suppress room history via federated events. A malicious homeserver can craft room events that cause Synapse pagination to omit full history, but Element has not published the affected event fields or validation path. Exploitation requires a federated room and control of a participating remote homeserver; clients can fail to display history while other server functions remain unreported as affected.

The record

CVE
CVE-2026-45076
Published
May 28, 2026
Updated
May 28, 2026
Vendor
Element HQ
Product
Synapse
Classifications
CWE-20, T1499.004
Attack vector
network
Privileges
authenticated

Timeline

How it unfolded

  1. May 28, 2026CVE publishedPublication date reported by the CVE source.
  2. May 28, 2026Record 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=< 1.152.1

What conditions does exploitation require?

Attack vectornetwork
Required privilegesauthenticated

What is affected?

Element HQ · Synapseversion=< 1.152.1

Published CVSS scores

5.1GitHub, Inc.CVSS:4.0/AV:N/AC:L/AT:N/PR:H/UI:N/VC:N/VI:N/VA:L/SC:N/SI:N/SA:N
2.7NIST NVDCVSS:3.1/AV:N/AC:L/PR:H/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-20Improper Input Validation
T1499.004Application or System Exploitation

Public exploit references

No public exploit references are available in this record.

Labels summarize the accepted research assessment. They do not indicate a test against your environment.

Technologies

Your stack

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Your stack

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