CVE-2025-30355: Synapse is an open source Matrix homeserver implementation

Q: How do I fix CVE-2025-30355?

synapse: upgrade to 1.127.1. Exact commands for each OS are on this page.

Q: Is CVE-2025-30355 being exploited?

CVE-2025-30355 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

Q: What does CVE-2025-30355 affect?

CVE-2025-30355 affects synapse. Severity is high (CVSS 7.1). Synapse is an open source Matrix homeserver implementation. A malicious server can craft events which, when received, prevent Synapse version up to 1.127.0 from federating with other servers. The vulnerability has been exploited in the wild

This month

HIGH 7.1UPGRADE

CVE-2025-30355 is a high-severity flaw (CVSS 7.1) affecting synapse. Below: why it ranks where it does, which versions are affected, what to do, and the exact commands.

Synapse is an open source Matrix homeserver implementation. A malicious server can craft events which, when received, prevent Synapse version up to 1.127.0 from federating with other servers. The vulnerability has been exploited in the wild and has been fixed in Synapse v1.127.1. No known workarounds are available.

Why this priority

High severity

A fixed release or distro patch exists.

Risk facts

CVSS score	7.1 (High)
Attack vector	Network (remote)
Attack complexity	Low
Privileges needed	Low
User interaction	None
EPSS exploit chance	1.2% (top 31%)
CISA exploited list	No
Weakness	CWE-20
Published	2025-03-27
Last modified	2026-06-17

Publisher (CNA): GitHub, Inc. · Vendor: matrix

Last updated 2026-06-17 · Sources: CVE Program, NVD, CISA KEV, EPSS and vendor advisories · How we score

Am I affected?

Timeline

2025-03-27 01:15 UTCPublished CVE record published
2026-06-17 09:08 UTCUpdated Record last modified

matrix synapse UPGRADE

Affected: < 1.127.1

Fixed in: 1.127.1

Safe to downgrade to: no unaffected older release

1. Check installed version

apt-cache policy synapse || rpm -q synapse || synapse --version

2. UPGRADE to 1.127.1 (Debian / Ubuntu)

sudo apt update && sudo apt install --only-upgrade synapse

2. UPGRADE (RHEL / Amazon Linux / Fedora)

sudo dnf upgrade synapse

Note

Distros often backport fixes without changing the version number. Confirm with the distro advisory or changelog.

Verify at the official source NVD · CVE Program · Report a problem with this page
Scores and fixes here are computed automatically from those and other public sources. Confirm with the vendor advisory before changing production systems.

Quick answers

How do I fix CVE-2025-30355?

synapse: upgrade to 1.127.1. Exact commands for each OS are on this page.

Is CVE-2025-30355 being exploited?

CVE-2025-30355 is not on the CISA Known Exploited Vulnerabilities list at the time of the last update. Check again, as this can change.

What does CVE-2025-30355 affect?

CVE-2025-30355 affects synapse. Severity is high (CVSS 7.1). Synapse is an open source Matrix homeserver implementation. A malicious server can craft events which, when received, prevent Synapse version up to 1.127.0 from federating with other servers. The vulnerability has been exploited in the wild

How urgent is CVE-2025-30355?

Priority 42/100. High severity

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References

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