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
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2021-21404 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,567 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
github.com/syncthing/syncthingReal-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
-
syncthingcan be caused to crash and exit if sent a malformed relay protocol message message with a negative length field. -
The relay server
strelaysrvcan 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. -
syncthingcan 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
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐹Go | github.com/syncthing/syncthing | all versions | 1.15.0go get github.com/syncthing/syncthing@v1.15.0 |
Detection & mitigation playbook
Open-source dependencyDetect
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.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2021-21404 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2021-21404. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2021-21404 in your dependencies?
O3 Security finds CVE-2021-21404 across Go dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.