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CVE-2021-21273

Open Redirect in matrix-synapse (CVE-2021-21273)

Summary

Vulnerability in matrix-synapse (CVE-2021-21273). Risk of unauthorized operations or information disclosure. Exploitable via federation_ip_range_blacklist. Mitigation: upgrade to 1.25.0 or later.

Original description

The description as published by the source.

Impact

Requests to user provided domains were not restricted to external IP addresses when calculating the key validity for third-party invite events and sending push notifications. This could cause Synapse to make requests to internal infrastructure. The type of request was not controlled by the user, although limited modification of request bodies was possible.

For the most thorough protection server administrators should remove the deprecated federation_ip_range_blacklist from their settings after upgrading to Synapse v1.25.0 which will result in Synapse using the improved default IP address restrictions. See the new ip_range_blacklist and ip_range_whitelist settings if more specific control is necessary.

Patches

Issue is resolved by #8821. Further improvements to protect homeservers by default were made in #8870 and #8954.

Workarounds

Requests to internal IP addresses could be blocked at the system or network level.

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-2021-21273 has been found in matrix-synapse.
Attackers can target a specific entry point like federation_ip_range_blacklist over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade matrix-synapse to 1.25.0 or later.
If unsure, ask your IT team or search "matrix-synapse CVE-2021-21273" on the vendor's site.

CVE-2021-21273 (matrix-synapse) — CWE-601
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: federation_ip_range_blacklist / ip_range_blacklist / ip_range_whitelist
Patched: 1.25.0 — apply immediately
Workaround: Requests to internal IP addresses could be blocked at the system or network level.
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-601) exists in matrix-synapse. Attackers reach the vulnerable code path via federation_ip_range_blacklist without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of matrix-synapse. Attack surface: federation_ip_range_blacklist / ip_range_blacklist / ip_range_whitelist.

🔥 Severity

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

🔧 How to fix

Update to 1.25.0.

🛡️ Workaround

Workaround: Requests to internal IP addresses could be blocked at the system or network level.

🔍 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 temporary workaround mitigate 
    Requests to internal IP addresses could be blocked at the system or network level.

    パッチが適用されるまでの応急処置として、Requests to internal IP addresses could be blocked at the system or network level. を実施。回避策の副作用 (機能低下) を確認した上で。

  3. 3
    Apply patch patch 
    Upgrade matrix-synapse to 1.25.0

    ステージング環境で 1.25.0 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。

  4. 4
    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":"1.25.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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