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Fencer Vulns
high 7.5

CVE-2021-21404 json

Crash due to malformed relay protocol message

published 2021-04-06 5 y ago modified 2026-09-08 2 mo ago also BIT-syncthing-2021-21404 , EUVD-2021-1226 , GHSA-x462-89pf-6r5h , GO-2022-0888

cvss
7.5
version 3.1, CNA
cisa kev
Not listed
not in CISA's catalog
epss
2.0%
EPSS 2.0% chance of exploitation in 30 days, percentile 80%
public exploits
0
none known

Summary

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.

Patch / fix

  • go github.com/syncthing/syncthing 1.15.0
  • bitnami syncthing 1.15.0
  • syncthing syncthing 1.15.0
  • syncthing Debian 14, Debian 13, Debian 12 1.12.1~ds1-3

debian advisories DEBIAN-CVE-2021-21404

  • syncthing openSUSE Tumbleweed 1.18.2-2.1
  • syncthing Package Hub 15 SP2 1.15.1-bp152.2.3.1
  • syncthing openSUSE Leap 15.2 1.15.1-lp152.2.3.1

opensuse advisories openSUSE-SU-2024:11417-1openSUSE-SU-2021:0713-1openSUSE-SU-2021:0688-1

  • syncthing Alpine 3.13 1.12.1-r1
  • syncthing Alpine 3.24, Alpine 3.23, Alpine 3.22 +8 more 1.15.1-r0

alpine advisories ALPINE-CVE-2021-21404

Alpine secdb © Alpine Linux, from https://secdb.alpinelinux.org via OSV; changed: normalised to fixed package versions per release. CC BY-SA 4.0

  • syncthing Arch Linux 1.15.1-1
  • syncthing-relaysrv Arch Linux 1.15.1-1

arch linux advisories AVG-1778

Vendor advisories

What the vendors themselves say about their products: affected, fixed or not affected, and what to do.

red hat product security vex CVE-2021-21404 updated cvss 7.5

syncthing: sending a relay message with a negative length field may lead to crash

  • Red Hat All currently supported Red Hat products
    not affected: vulnerable code not present

Red Hat CSAF VEX © Red Hat, Inc., from https://security.access.redhat.com/data/csaf/v2/vex/; changed: reduced to product statuses and remediations. CC BY 4.0 · csaf

Exploit prediction

1.97% chance of exploitation in the next 30 days, higher than 80% of all scored CVEs (FIRST EPSS, 2026-11-08).

CVSS scores

assigned by version score vector
CNA 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
GitHub 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
NVD 2.0 5.0 AV:N/AC:L/Au:N/C:N/I:N/A:P
OSV 3.1 7.5 CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H

Affected packages

ecosystem package affected fixed in
go github.com/syncthing/syncthing
<1.15.0
1.15.0
bitnami syncthing
<1.15.0
1.15.0

Affected products

  • syncthing syncthing

Classification

Weakness
  • CWE-20 Improper Input Validation

References

Sources

Fencer Vulns keeps one entry per vulnerability: the CVE id wins, then GHSA, and records that alias each other are merged, except that two CVEs always keep separate entries. The same data is available as JSON.