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CVE-2021-21332

Cross-Site Scripting (XSS) in matrix-synapse (CVE-2021-21332)

Summary

Cross-site scripting in matrix-synapse (CVE-2021-21332). Confidential information can be exposed externally. Exploitable via account_threepid_delegates.email. Mitigation: upgrade to 1.27.0 or later.

Original description

The description as published by the source.

Impact

The password reset endpoint served via Synapse was vulnerable to cross-site scripting (XSS) attacks. The impact depends on the configuration of the domain that Synapse is deployed on, but may allow access to cookies and other browser data, CSRF vulnerabilities, and access to other resources served on the same domain or parent domains.

Patches

This is fixed in #9200.

Workarounds

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

  1. Password resets can be disabled by delegating email to a third-party service (via the account_threepid_delegates.email setting) or disabling email (by not configuring the email setting).

  2. If the homeserver is not configured to use passwords (via the password_config.enabled setting) then the affected endpoint can be blocked at a reverse proxy:

    • /_synapse/client/password_reset/email/submit_token

  3. The password_reset_confirmation.html template can be overridden with a custom template that manually escapes the variables using JInja2's escape filter. See the email.template_dir setting.

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-2021-21332 has been found in matrix-synapse.
Attackers can target a specific entry point like account_threepid_delegates.email over the network to misuse the product.
Confidential information can be exposed externally.
What to do: upgrade matrix-synapse to 1.27.0 or later.
If unsure, ask your IT team or search "matrix-synapse CVE-2021-21332" on the vendor's site.

CVE-2021-21332 (matrix-synapse) — CWE-79
Attack vector: remote (network-reachable) / unauthenticated
Attack surface: account_threepid_delegates.email / email / password_config.enabled / password_reset_confirmation.html
Patched: 1.27.0 — 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

Cross-site scripting (XSS) (CWE-79) exists in matrix-synapse. Attackers reach the vulnerable code path via account_threepid_delegates.email without authentication.

📍 Affected scope

Check the vendor advisory for the affected versions of matrix-synapse. Attack surface: account_threepid_delegates.email / email / password_config.enabled / password_reset_confirmation.html.

🔥 Severity

Severity: ?. Confidential information can be exposed externally.

🔧 How to fix

Update to 1.27.0.

🛡️ 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 1.27.0

    ステージング環境で 1.27.0 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":"1.27.0"}]}]

Fix status by distribution

Fixed packages published by each distribution for this CVE

References

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