CVE-2021-21404: Github.com/syncthing/syncthing Crash Due To Malformed Relay Vulnerability

LOW
Advisory Identifier: GHSA-x462-89pf-6r5h
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CVSS 7.5
CWE-20
Published
Advisory Summary

CVE-2021-21404 is a LOW (CVSS 7.5) crash due to malformed relay vulnerability (CWE-20: Improper Input Validation) affecting Github.com/syncthing/syncthing libraries in the Go ecosystem. The issue involves crash due to malformed relay protocol message. Remediation is available by upgrading affected packages to version 1.15.0.

CVE-2021-21404 at a Glance

CVE Identifier CVE-2021-21404
GHSA Identifier GHSA-x462-89pf-6r5h
Severity LOW
CVSS v3.1 Base Score 7.5
CWE Classification CWE-20: Improper Input Validation
Affected Product Github.com/syncthing/syncthing
Package Ecosystem Go
Published Date
Last Updated

Security Impact

CVSS: 7.5

The security impact metrics below are parsed directly from the CVSS v3.1 vector string (CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H).

Attack Vector Network (AV:N)
Attack Complexity Low (AC:L)
Privileges Required None (PR:N)
User Interaction None (UI:N)
Scope Unchanged (S:U)
Confidentiality None (C:N)
Integrity None (I:N)
Availability High (A:H)

How to Fix CVE-2021-21404

To remediate this vulnerability, upgrade all affected Github.com/syncthing/syncthing dependencies to a patched release. Both direct and transitive dependencies should be checked, as frameworks or security libraries may pull in vulnerable versions transitively.

github.com/syncthing/syncthing fixed: 1.15.0 
Upgrade github.com/syncthing/syncthing to 1.15.0
Verify Direct and Transitive Dependencies

Run the following command in your project directory to verify that all resolved dependencies use the patched release: 

inspect dependencies for github.com/syncthing/syncthing

How to Check if You Are Affected

To determine whether your application is affected by CVE-2021-21404, inspect your project dependencies for Github.com/syncthing/syncthing:

inspect dependencies for github.com/syncthing/syncthing

Analyze the terminal output to locate any version of Github.com/syncthing/syncthing in your dependency tree. If any resolved version matches the vulnerable ranges listed in the table below (for example, versions earlier than 1.15.0), your application is affected.

Note: Verify declared build manifests and deployed runtime container environments.

Technical Details

Vulnerability Mechanism & Technical Analysis

This vulnerability is classified under CWE-20: Improper Input Validation, where the product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly. In Github.com/syncthing/syncthing, software state validation or input boundaries are circumvented during processing. Review the official source advisory statement below for specific affected components, classes, and reproduction details.

Source Advisory Statement

### 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 workaround for `strelaysrv`. 2. `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](https://forum.syncthing.net/). Thanks to Wojciech Paciorek for discovering and reporting this issue.

CVE-2021-21404 Affected Github.com/syncthing/syncthing Versions

1 component(s)
Fixed Versions:
1.15.0
Product / Package Ecosystem Affected Versions Fixed Version
github.com/syncthing/syncthing
golang/github.com/syncthing/syncthing
Go
< 1.15.0
1.15.0

References

GitHub Advisory / Wiki ADVISORY
https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h
NVD — CVE-2021-21404 NVD
https://nvd.nist.gov/vuln/detail/CVE-2021-21404
Security Fix Commit FIX COMMIT
https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97
Github.com/syncthing/syncthing Release RELEASE
https://github.com/syncthing/syncthing/releases/tag/v1.15.0
pkg.go.dev/github.com/syncthin... WEB
https://pkg.go.dev/github.com/syncthing/syncthing

Frequently Asked Questions

What is CVE-2021-21404?
CVE-2021-21404 is a security vulnerability in Github.com/syncthing/syncthing involving Crash due to malformed relay protocol message under CWE-20: Improper Input Validation.
Which Github.com/syncthing/syncthing versions are affected?
Affected versions include releases prior to 1.15.0 across affected packages: github.com/syncthing/syncthing.
What version fixes CVE-2021-21404?
Upgrading to 1.15.0 (or the latest patched release for your respective branch) resolves the vulnerability.
Is github.com/syncthing/syncthing affected?
Yes, github.com/syncthing/syncthing is affected in releases prior to 1.15.0. Upgrade your dependency to 1.15.0 or later.
What is the CVSS score of CVE-2021-21404?
CVE-2021-21404 has a CVSS v3.1 base score of 7.5 (LOW) with vector string CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
What CWE is associated with CVE-2021-21404?
CVE-2021-21404 is classified under CWE-20: Improper Input Validation (The product receives input or data, but it does not validate or incorrectly validates that the input has the properties that are required to process the data safely and correctly.).
How can I check Go dependencies for CVE-2021-21404?
Run the dependency tree command: 'inspect dependencies for github.com/syncthing/syncthing'. This command lists direct and transitive dependencies in your build configuration so you can verify if a vulnerable version is resolved.
Machine-readable OSV JSON OSV v1.5 Schema
Normalized schema output
View Raw JSON 
{
  "schema_version" : "1.4.0",
  "id" : "GHSA-x462-89pf-6r5h",
  "modified" : "2021-05-20T20:52:26Z",
  "published" : "2021-05-21T16:23:17Z",
  "aliases" : [ "CVE-2021-21404" ],
  "summary" : "Crash due to malformed relay protocol message",
  "details" : "### Impact\n\n1. `syncthing` can be caused to crash and exit if sent a malformed relay\n   protocol message message with a negative length field.\n\n2. The relay server `strelaysrv` can be caused to crash and exit if sent\n   a malformed relay protocol message with a negative length field.\n\nAt no point is sensitive data exposed or liable to be altered due to this\nissue. Sensitive data is never exposed to relay operators. Syncthing itself\nwould need to be lured to connect to a malicious relay server in order to\nexploit the issue.\n\n### Patches\n\nFixed in version 1.15.0.\n\n### Workarounds\n\n1. No known workaround for `strelaysrv`.\n\n2. `syncthing` can be configured to not use relays, or to only use specific,\n   trusted relays. If Syncthing is used in a closed environment or with\n   relaying disabled, i.e., it does not communicate with unknown relays,\n   Syncthing is not vulnerable.\n\n### For more information\n\nIf you have any questions or comments about this advisory, please discuss it [on the forum](https://forum.syncthing.net/).\n\nThanks to Wojciech Paciorek for discovering and reporting this issue.",
  "severity" : [ {
    "type" : "CVSS_V3",
    "score" : "CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H"
  } ],
  "affected" : [ {
    "package" : {
      "ecosystem" : "Go",
      "name" : "github.com/syncthing/syncthing"
    },
    "ranges" : [ {
      "type" : "ECOSYSTEM",
      "events" : [ {
        "introduced" : "0"
      }, {
        "fixed" : "1.15.0"
      } ]
    } ]
  } ],
  "references" : [ {
    "type" : "WEB",
    "url" : "https://github.com/syncthing/syncthing/security/advisories/GHSA-x462-89pf-6r5h"
  }, {
    "type" : "ADVISORY",
    "url" : "https://nvd.nist.gov/vuln/detail/CVE-2021-21404"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/syncthing/syncthing/commit/fb4fdaf4c0a79c22cad000c42ac1394e3ccb6a97"
  }, {
    "type" : "WEB",
    "url" : "https://github.com/syncthing/syncthing/releases/tag/v1.15.0"
  }, {
    "type" : "WEB",
    "url" : "https://pkg.go.dev/github.com/syncthing/syncthing"
  } ],
  "database_specific" : {
    "cwe_ids" : [ "CWE-20" ],
    "severity" : "LOW",
    "github_reviewed" : true,
    "github_reviewed_at" : "2021-05-20T20:52:26Z",
    "nvd_published_at" : "2021-04-06T20:15:00Z"
  }
}