How severe is CVE-2021-21404?

CVE-2021-21404 has a CVSS score of 7.5/10, rated HIGH. Immediate patching is strongly recommended.

Which packages are affected by CVE-2021-21404?

CVE-2021-21404 affects the following packages: github.com/syncthing/syncthing (Go). Ecosystems affected: Go.

How do I fix CVE-2021-21404?

Update github.com/syncthing/syncthing to 1.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21404 is resolved across your whole dependency graph.

How do I detect CVE-2021-21404 in my Go dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/syncthing/syncthing, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.

How do I mitigate CVE-2021-21404 if there is no patch (or I can't update yet)?

If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Is CVE-2021-21404 actively exploited in the wild?

No public exploit code has been indexed for CVE-2021-21404 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What is the EPSS score for CVE-2021-21404?

CVE-2021-21404 has an EPSS (Exploit Prediction Scoring System) score of 2.0%, placing it in the 80th percentile of all CVEs. EPSS is maintained by FIRST.org and estimates the probability that a vulnerability will be exploited in the wild within the next 30 days. This score indicates relatively lower exploitation probability, though the CVSS severity should still guide your patching priority.

What type of vulnerability is CVE-2021-21404?

CVE-2021-21404 is classified as Improper Input Validation (CWE-20). This weakness type describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2021-21404 published, and has it been updated?

CVE-2021-21404 was published on April 6, 2021 and was last updated on October 20, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐹

🐹 Go

Not in CISA KEV

HIGH severity

CVE-2021-21404 — syncthing

HIGHFix: syncthing/syncthing@fb4fdaf

CVE-2021-21404 is a high-severity (CVSS 7.5) Improper Input Validation vulnerability in github.com/syncthing/syncthing. A fix is available for github.com/syncthing/syncthing — see the affected versions and patch details below.

Crash due to malformed relay protocol message

Also known asBIT-syncthing-2021-21404GHSA-x462-89pf-6r5hGO-2022-0888

Published

Apr 6, 2021

Updated

Oct 20, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Nov 2, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

EPSS Exploitation Probability

via FIRST.org ↗

2.0%probability of exploitation in next 30 days

Lower Risk0.00%

Lower risk than most CVEs80th percentile — riskier than 80% of all scored CVEsHighest risk

Probability of exploitation in the next 30 days, from FIRST.org EPSS.

How urgent is this, really

Where this sits among everything scored

Real-World Exposure

1 pkg affected

🐹github.com/syncthing/syncthing

Real-time download stats are indexed for npm and PyPI packages. This vulnerability affects Go packages — download data is not available via public APIs for these ecosystems.

Description

Impact

syncthing can be caused to crash and exit if sent a malformed relay protocol message message with a negative length field.
The relay server strelaysrv can be caused to crash and exit if sent a malformed relay protocol message with a negative length field.

At no point is sensitive data exposed or liable to be altered due to this issue. Sensitive data is never exposed to relay operators. Syncthing itself would need to be lured to connect to a malicious relay server in order to exploit the issue.

Patches

Fixed in version 1.15.0.

Workarounds

No known workaround for strelaysrv.
syncthing can be configured to not use relays, or to only use specific, trusted relays. If Syncthing is used in a closed environment or with relaying disabled, i.e., it does not communicate with unknown relays, Syncthing is not vulnerable.

For more information

If you have any questions or comments about this advisory, please discuss it on the forum.

Thanks to Wojciech Paciorek for discovering and reporting this issue.

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐹Go	`github.com/syncthing/syncthing`	all versions	1.15.0`go get github.com/syncthing/syncthing@v1.15.0`

Affected Products

1 product · 1 configurations

Application

syncthingsyncthing

< 1.15.0

range

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for github.com/syncthing/syncthing, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update github.com/syncthing/syncthing to 1.15.0 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2021-21404 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.

Frequently Asked Questions

### Impact 1. `syncthing` can be caused to crash and exit if sent a malformed relay protocol message message with a negative length field. 2. The relay server `strelaysrv` can be caused to crash and exit if sent a malformed relay protocol message with a negative length field. At no point is sensitive data exposed or liable to be altered due to this issue. Sensitive data is never exposed to relay operators. Syncthing itself would need to be lured to connect to a malicious relay server in order to exploit the issue. ### Patches Fixed in version 1.15.0. ### Workarounds 1. No known wo