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CVE Record

CVE-2020-26890: Matrix Synapse before 1.20.0 erroneously permits non-standard NaN, Infinity, and -Infinity JSON values in f...

Matrix Synapse before 1.20.0 erroneously permits non-standard NaN, Infinity, and -Infinity JSON values in fields of m.room.member events, allowing remote attackers to execute a denial of service attack against the federation and common Matrix clients. If such a malformed event is accepted into the room's state, the impact is long-lasting and is not fixed by an upgrade to a newer version, requiring the event to be manually redacted instead. Since events are replicated to servers of other room members, the impact is not constrained to the server of the event sender.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysishigh

Security readout for executives and security teams

Plain-English summary

CVE-2020-26890 affects Matrix Synapse before 1.20.0. A remote attacker could introduce malformed membership data that causes denial of service for federation and common Matrix clients. The business issue is persistence: if the bad event enters room state, upgrading alone does not clean it up.

Executive priority

Prioritize remediation for internet-facing or federated Synapse deployments. The main risk is service disruption with cleanup that may require manual intervention, especially in shared rooms where replicated events can affect multiple servers.

Technical view

Synapse accepted non-standard JSON values NaN, Infinity, and -Infinity in fields of m.room.member events. Because Matrix events replicate across servers for room members, the impact can spread beyond the sender's server. Existing malformed room-state events require manual redaction after upgrade.

Likely exposure

Organizations operating Matrix Synapse versions before 1.20.0 are the primary exposure. Federated rooms increase operational risk because events replicate to other member servers. Fedora-packaged Synapse users should check the referenced Fedora advisories.

Exploitation context

The provided sources describe remote denial of service, but do not state active exploitation. The CVE is not listed as KEV in the source bundle. Evidence supports persistent impact after malformed event acceptance, not public weaponization status.

Researcher notes

The source bundle lacks CVSS, CWE, and detailed affected CPE data. Analysis relies on the CVE description and Synapse/GitHub and Fedora references. Avoid assuming broader Matrix software exposure beyond Synapse and common clients affected by malformed replicated events.

Mitigation direction

  • Upgrade Matrix Synapse to version 1.20.0 or later.
  • Apply relevant Fedora package updates if using Fedora builds.
  • Manually redact any malformed accepted events from affected room state.
  • Review Matrix/Synapse vendor guidance before operational cleanup actions.

Validation and detection

  • Inventory all Synapse instances and confirm versions are 1.20.0 or later.
  • Check whether any exposed servers participate in federated Matrix rooms.
  • Review room-state data for malformed m.room.member events described by the advisory.
  • Confirm remediation includes redaction where malformed events were already accepted.
Prepared
Confidence
high
Sources
5

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
0Timeline events
0ADP providers
4Source links

CVSS and timeline data

No CVSS vectors or timeline events were available in the normalized CVE source material.

Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
n/an/an/aListed
Weakness

CWE details

No CWE listed

CWE links open Glexia weakness intelligence pages with official CWE context, developer remediation guidance, and related CVE mappings.