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CVE-2021-21394

Vulnerability in matrix-synapse (CVE-2021-21394)

Summary

Vulnerability in matrix-synapse (CVE-2021-21394). Risk of unauthorized operations or information disclosure. Exploitable via email. Mitigation: upgrade to 1.28.0 or later.

Original description

The description as published by the source.

Impact

Missing input validation of some parameters on the endpoints used to confirm third-party identifiers could cause excessive use of disk space and memory leading to resource exhaustion.

Patches

The issue is fixed by #9321.

Workarounds

Depending on the needs and configuration of the homeserver a few options are available:

  1. Using email as third-party identifiers be disabled by not configuring the email setting.

  2. Using phone numbers as third-party identifiers can be disabled by ensuring that account_threepid_delegates.msisdn is not configured.

  3. Additionally, the affected endpoint patterns can be blocked at a reverse proxy:

    • ^/_matrix/client/(r0|unstable)/register/email
    • ^/_matrix/client/(r0|unstable)/register/msisdn
    • ^/_matrix/client/(r0|unstable)/account/password
    • ^/_matrix/client/(r0|unstable)/account/3pid

Source: OSV.dev (Open Source Vulnerabilities) NVD GitHub Advisory OSV

Auto-generated summary

Assembled by SecPulse from the source data (CVSS, CWE, affected versions) using templates — no AI involved. Check the original description and references below for exact details.

A vulnerability tracked as CVE-2021-21394 has been found in matrix-synapse.
Attackers can target a specific entry point like email over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade matrix-synapse to 1.28.0 or later.
If unsure, ask your IT team or search "matrix-synapse CVE-2021-21394" on the vendor's site.

CVE-2021-21394 (matrix-synapse) — CWE-20
Attack vector: remote (network-reachable) / no user interaction
Attack surface: email / account_threepid_delegates.msisdn
Patched: 1.28.0 — apply immediately
Plan: 1) Audit SBOM/dependencies, 2) Stage→prod upgrade, 3) Add WAF/proxy monitoring on affected endpoints, 4) Hunt IOCs in logs.
Refs: see the GHSA / vendor advisory / patched release linked on this page.

❓ What is the problem

A vulnerability (CWE-20) exists in matrix-synapse. Attackers reach the vulnerable code path via email.

📍 Affected scope

Check the vendor advisory for the affected versions of matrix-synapse. Attack surface: email / account_threepid_delegates.msisdn.

🔥 Severity

Severity: ?. Risk of unauthorized operations or information disclosure.

🔧 How to fix

Update to 1.28.0.

🛡️ Workaround

Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.

🔍 Detection

Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'matrix-synapse' . against your dependency files (package-lock.json, requirements.txt, go.sum) to find affected services.

Response actions

Concrete steps and command examples for SOC/SRE teams to execute in order

  1. 1
    Identify exposure identify 
    grep -r 'matrix-synapse' . | grep -v node_modules

    リポジトリと本番環境の依存ファイル (package-lock.json / requirements.txt / go.sum / Gemfile.lock 等) で matrix-synapse を grep し、稼働しているサービス・バージョンを把握する。

  2. 2
    Apply patch patch 
    Upgrade matrix-synapse to 1.28.0

    ステージング環境で 1.28.0 に上げて回帰テスト → 本番反映。回帰テストはアプリの主要ハッピーパスを含めること。

  3. 3
    Post-deployment verification verify 
    Confirm patched version is live in production

    パッチ適用後、ステージングで PoC または同等の悪用パターンを再現して脆弱性が閉じたことを確認。本番ではアクセスログ・WAF でアラート再発が無いか継続監視。

Affected packages

PyPI matrix-synapse
[{"type":"ECOSYSTEM","events":[{"introduced":"0"},{"fixed":"1.28.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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