CVE-2025-30355 | Synapse vulnerable to federation denial of service via malformed events

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.

Published: 2025-03-27 Last update: 2025-08-26 Assigner: [email protected] Source: [email protected]
NVD Status：Analyzed，CVE State: published
View at NVD, CVE.org, EUVD

CVE-2025-30355 is rated High Risk (66.4/100): CVSS High severity, with high exploitation likelihood (EPSS 13.20%, 94th percentile). EPSS ranks this CVE among the most likely to be exploited in the near term. EPSS rose +5.66% over the last day, indicating growing attacker interest. High exploitation likelihood—assess exposure and prioritize remediation.

Risk is dynamic; we continuously reassess and refresh what is shown on this page as upstream context changes.

Exploit prediction scoring system (EPSS) score for CVE-2025-30355

EPSS lead: Daily EPSS estimates relative likelihood of exploitation; percentile ranks this CVE among scored vulnerabilities (higher = more severe relative rank).

# Date Old EPSS score New EPSS score Delta (New - Old)
1 2026-05-05 7.54% 13.20% +5.66%
2 2026-01-21 7.17% 7.54% +0.37%
3 2025-12-21 — 7.17% —

Full EPSS history (19 records total)

Common vulnerability scoring system (CVSS) metrics for CVE-2025-30355

CVSS metrics for this CVE.

Base score Version Severity Vector Exploitability Impact Score source
7.1 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:U/C:N/I:L/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:L)
A normal user session is enough; they don’t have to be admin.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:L)
Attackers could change some data, but it’s limited—not everything goes.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 2.8 4.2 [email protected]
7.5 3.1 HIGH
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H Click to expand
Attack vector (AV:N)
Could be attacked over the internet or any normal routed network—not just someone sitting at the machine.
Attack complexity (AC:L)
Once they can reach the bug, pulling it off is straightforward—no weird race conditions or rare setup.
Privileges required (PR:N)
No account or special rights needed—anonymous or random user is enough.
User interaction (UI:N)
Nobody has to click “OK” or open a trap file; it can work without a victim helping.
Scope (S:U)
Damage stays in the same “trust bubble” as the broken component—no big spill into unrelated systems.
Confidentiality (C:N)
Doesn’t really leak secrets in a meaningful way.
Integrity (I:N)
Data isn’t meaningfully altered or forged.
Availability (A:H)
Could take the service down hard or make it unusable for people who depend on it.
 3.9 3.6 [email protected]

Weakness enumeration for CVE-2025-30355

GitHub Security Advisory for CVE-2025-30355

GHSA-v56r-hwv5-mxg6 · Severity: high · Ecosystem: pip — Synapse vulnerable to federation denial of service via malformed events

OS Trackers for CVE-2025-30355

vendor priority summary link
alpine — CVE-2025-30355: 1 source package rows (synapse); 48 state rows across 3 repos (3.21-community, 3.22-community, edge-community); fixed 3, open 45. https://security.alpinelinux.org/vuln/CVE-2025-30355
debian not yet assigned CVE-2025-30355 not yet assigned priority: Debian including 1 source packages (matrix-synapse), 2 status rows across 2 suites (forky, sid): resolved 2. https://security-tracker.debian.org/tracker/CVE-2025-30355
ubuntu medium CVE-2025-30355 medium priority: Ubuntu including 1 source packages (matrix-synapse), 6 status rows across 6 suites (bionic, focal, jammy, noble, oracular, upstream): needs-triage 4, ignored 1, released 1. https://ubuntu.com/security/CVE-2025-30355

Affected software / configurations for CVE-2025-30355

Vendor Product Version Raw CPE
matrix synapse < 1.127.1 cpe:2.3:a:matrix:synapse:*:*:*:*:*:*:*:*

References for CVE-2025-30355

cvelogic Threat Intelligence