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VulnerabilityPublished Updated

CVE-2025-61672: element-hq synapse · Improper Validation of Specified Type of Input

Affects element-hq synapseSource CVE Database V5

0

Not known to be exploited · CVSS 5.3 Medium : upgrade to 1.138.3 when convenient.

Data as of 26 Oct 2026, 10:12 UTC

CVSS 4.0 base score
5.3Medium
How severe the flaw is on the 0–10 CVSS scale.
EPSS
0.4% chance of exploitation in 30 days – higher than 40% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.
Patch
Available – fixed in 1.138.3
CVSS:4.0/AV:N/AC:L/AT:N/PR:L/UI:N/VC:N/VI:L/VA:L/SC:N/SI:N/SA:N

Exploitation evidence

  1. Disclosed: evidence found.

    CVE published 8 Oct 2025

  2. Detectable: no evidence on record.

    No public nuclei template indexed

  3. Weaponised: no evidence on record.

    No public exploit code indexed in Metasploit or Exploit-DB

  4. Exploited in the wild: no evidence on record.

    Not in CISA KEV and no source reports exploitation

  5. Observed by OffSeq Mirage: no evidence on record.

    No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 26 Oct 2026.

What to do

Upgrade element-hq synapse to 1.138.3.

Affected software

ProductAffectedFixed in
element-hq synapseAffected
  • <1.138.3
  • =1.139.0
Fixed in
  • 1.138.3

AI-suggested

European organizations should immediately plan to upgrade Synapse deployments to versions 1.138.4 or 1.139.2, explicitly avoiding versions 1.138.3 and 1.139.1 due to known regressions. Prior to upgrade, conduct thorough testing in staging environments to ensure federation functionality remains stable post-patch. Implement strict access controls and monitoring on user registrations to reduce the risk of malicious insiders exploiting the vulnerability. Enable detailed federation logging to detect anomalies or degradation in outbound federation early. Consider deploying federation health monitoring tools that can alert administrators to unexpected federation failures. Regularly audit device key management processes and validate inputs where possible to detect malformed or suspicious device keys. Engage with the Synapse community and maintain awareness of further updates or patches addressing related issues. Finally, incorporate this vulnerability into incident response plans to quickly remediate any federation disruptions.

Description

Synapse is an open source Matrix homeserver implementation. Lack of validation for device keys in Synapse before 1.138.3 and in Synapse 1.139.0 allow an attacker registered on the victim homeserver to degrade federation functionality, unpredictably breaking outbound federation to other homeservers. The issue is patched in Synapse 1.138.3, 1.138.4, 1.139.1, and 1.139.2. Note that even though 1.138.3 and 1.139.1 fix the vulnerability, they inadvertently introduced an unrelated regression. For this reason, the maintainers of Synapse recommend skipping these releases and upgrading straight to 1.138.4 and 1.139.2.

References

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