Vulnerability in github.com/syncthing/syncthing (CVE-2021-21404)
Summary
Vulnerability in github.com/syncthing/syncthing (CVE-2021-21404). Risk of unauthorized operations or information disclosure. Exploitable via strelaysrv. Mitigation: upgrade to 1.15.0 or later.
Original description
The description as published by the source.
Syncthing is a continuous file synchronization program. In Syncthing before version 1.15.0, the relay server strelaysrv can be caused to crash and exit by sending a relay message with a negative length field. Similarly, Syncthing itself can crash for the same reason if given a malformed message from a malicious relay server when attempting to join the relay. Relay joins are essentially random (from a subset of low latency relays) and Syncthing will by default restart when crashing, at which point it's likely to pick another non-malicious relay. This flaw is fixed in version 1.15.0.
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-2021-21404 has been found in github.com/syncthing/syncthing.
Attackers can target a specific entry point like strelaysrv over the network to misuse the product.
Risk of unauthorized operations or information disclosure.
What to do: upgrade github.com/syncthing/syncthing to 1.15.0 or later.
If unsure, ask your IT team or search "github.com/syncthing/syncthing CVE-2021-21404" on the vendor's site.
CVE-2021-21404 (github.com/syncthing/syncthing) — CWE-20
Attack vector: remote (network-reachable) / unauthenticated / no user interaction
Attack surface: strelaysrv
Patched: 1.15.0 — 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.
A vulnerability (CWE-20) exists in github.com/syncthing/syncthing. Attackers reach the vulnerable code path via strelaysrv without authentication.
Check the vendor advisory for the affected versions of github.com/syncthing/syncthing. Attack surface: strelaysrv.
Severity: ?. Risk of unauthorized operations or information disclosure.
Update to 1.15.0.
Until the patch is applied: disable the affected feature, apply WAF rules, or restrict access via network ACLs.
Search webserver/proxy logs for unusual request patterns matching this CVE's known IOCs. Run grep -r 'github.com/syncthing/syncthing' . 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
-
1Identify exposure identify
grep -r 'github.com/syncthing/syncthing' . | 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
github.com/syncthing/syncthingを grep し、稼働しているサービス・バージョンを把握する。 -
2Apply patch patch
Upgrade github.com/syncthing/syncthing to 1.15.0ステージング環境で 1.15.0 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。
-
3Post-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
Fix status by distribution
Fixed packages published by each distribution for this CVE
- Ubuntu UBUNTU-CVE-2021-21404 syncthing
References
- advisory github.com
- advisory nvd.nist.gov
- advisory github.com
- advisory pkg.go.dev
- patch github.com