CVE-2026-55591 SignalK · signalk-server

Signal K Server: Server-Side Request Forgery via Remote Connection Endpoints

Vulnerability at a glance

Severity, exploitation likelihood & attack requirements

CVSS v3.1 severity
5.8Medium
EPSS · exploitation likelihood
0.274% Probability within 30 daysPercentile 19.8 FIRST · 2026-09-16
Known exploitation
Not in KEV CISA KEV catalog Absence does not establish safety
Attack vector
NetworkRemotely reachable
Attack complexity
LowFewer obstacles to exploitation
Privileges required
NoneNo authentication required
User interaction
NoneNo user action needed
Exploit availability & maturity
Not scored Availability not established No explicit exploit maturity assessment View exploit evidence
Already protected by existing Atomicorp rules No new CVE-specific rule required.

Atomicorp ModSecurity protection

Protected by Atomicorp rules

Atomicorp ModSecurity rules block attacks exploiting this vulnerability when blocking is enabled. Keep your rules current.

By targeting underlying attack techniques and CWE weakness classes, existing rules can protect against newly disclosed vulnerabilities. Rule changes are regression-tested against attack and false-positive corpora.

7 rules identified Verified 2026-09-16 View rules & research details →

WAF protection applies to the tested attack paths; it does not change the CVSS or EPSS score. Keep your software patched and your Atomicorp rules current.

CISA SSVC assessment

CISA ADP · Assessed 2026-09-15 · SSVC 2.0.3

Exploitation
Proof-of-concept
Automatable
Yes
Technical Impact
Partial

Stakeholder-Specific Vulnerability Categorization records CISA’s assessment of exploitation, automation, and technical impact. “None reported” reflects the assessment date; it does not establish that no exploit exists. View SSVC source details.

On this page

CVE Details

  • Protection status: Protected by Atomicorp rules · About protection statuses
  • CVE: CVE-2026-55591
  • Name: Signal K Server: Server-Side Request Forgery via Remote Connection Endpoints
  • Severity: Medium
  • CVSS: 5.8 (v3.1) (Medium)
  • CVSS version: 3.1
  • CVSS source: GitHub_M CNA
  • CVSS vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
    • Attack vector: Network
    • Attack complexity: Low
    • Privileges required: None
    • User interaction: None
    • Scope: Changed
    • Confidentiality impact: Low
    • Integrity impact: None
    • Availability impact: None
  • CWE: CWE-918
  • Vendor: SignalK
  • Vulnerable product: signalk-server
  • CVE published: 2026-09-15
  • CVE last modified: 2026-09-15
  • EPSS: 0.0027; 19.8th percentile as of 2026-09-16 (FIRST )
  • Latest Update: 2026-09-16
  • Selected Rule IDs observed: 337109 , 337110 , 340162 , 340163 , 344360 , 398001 , 398008 , 398021 , 398022
  • Matching rules: 9; rules with blocking actions: 7
  • Would-block Rule IDs: 337109 , 337110 , 340162 , 340163 , 344360 , 398021 , 398022
  • Request surfaces inspected: JSON request data, Request URI, Request argument names, Request arguments, Request cookies, SOAP request data, XML request data
Technical vulnerability details · CVSS assessments, exploit evidence & CPE data

CVSS assessments

CVSS v3.1 · security-advisories@github.com

  • Base score: 5.8
  • Base severity: MEDIUM
  • Exploitability subscore: 3.9
  • Impact subscore: 1.4
  • Assessment type: Secondary
  • Source identifier: security-advisories@github.com
  • Full vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:L/I:N/A:N
    • Attack vector: Network
    • Attack complexity: Low
    • Privileges required: None
    • User interaction: None
    • Scope: Changed
    • Confidentiality impact: Low
    • Integrity impact: None
    • Availability impact: None
  • Exploit maturity: Not scored

Exploitability and impact subscores describe the vulnerability’s technical characteristics. Exploit maturity describes the reported state of exploitation; these are separate measures.

Assessment source: NVD record · CVE record

CISA SSVC assessments

These are CISA’s published decision points at the assessment time. They are not a complete organization-specific prioritization decision.

  • Provider: CISA ADP
  • Provider identifier: 134c704f-9b21-4f2e-91b3-4a467353bcc0
  • SSVC version: 2.0.3
  • Role: CISA Coordinator
  • Assessment timestamp: 2026-09-15T17:23:28.984905Z
  • Exploitation: poc
  • Automatable: yes
  • Technical Impact: partial
  • Source: CISA ADP in the CVE record

Vulnerable CPE configurations

No vulnerable CPE configurations are supplied in the current NVD record. See the affected-product information and vendor guidance on this page.

Vulnerability Description

Signal K Server is a server application that runs on a central hub in a boat. Prior to 2.28.0, makeRemoteRequest() in src/serverroutes.ts accepted attacker-controlled host, port, useTLS, and selfsignedcert parameters from the testSignalKConnection, requestAccess, and checkAccessRequest endpoints without validating the destination. When security was not configured, addAdminMiddleware() was a no-op in dummysecurity.ts, leaving all three endpoints accessible without authentication. The server could be forced to contact loopback, private, link-local, cloud metadata, or arbitrary external destinations, and selfsignedcert could disable certificate verification for outbound HTTPS requests. The checkAccessRequest endpoint also interpolated requestId into its destination path, allowing traversal to other paths on the selected host. Distinct success, connection-refused, and timeout responses enabled internal port scanning; returned response bodies enabled cloud metadata and internal-service data exfiltration; requestAccess enabled server-side POST requests with attacker-controlled JSON; and access to cluster-internal services could support lateral movement. This issue is fixed in version 2.28.0.

  1. Maintain the observed Atomicorp protections. Keep your Atomicorp commercial rules current and confirm that the selected observed rules (337109 , 337110 , 340162 , 340163 , 344360 , 398021 , 398022 ) have not been locally disabled. Atomicorp commercial rules are enabled by default. If you use Atomic WAF, review the applicable policy and rule settings.

  2. Apply the vendor’s remediation. WAF protection is defense in depth and does not replace upgrading or following the affected vendor’s guidance.

    • Affected versions: SignalK signalk-server version < 2.28.0.
    • No specific CNA/vendor solution or workaround is included in the current CVE Record . Review the vendor’s security advisories and supported-release guidance before making changes.

  3. Validate after changes. Confirm rule updates are current, review WAF events for the affected application, and test application behavior after vendor upgrades, mitigations, or local policy changes.

Research Notes

Atomicorp WAF protections are designed around attack techniques and exploit behaviors, allowing existing protections to apply across multiple vulnerabilities without requiring a unique rule for every CVE.

Public Vulnerability References

Test or Research Source

  • Source: Atomicorp AI Research Framework
  • Tags: cve, derived, ssrf, waf-probe

Research Note Context

This research note documents exploit-technique analysis associated with this vulnerability and references Atomicorp WAF protections observed during engineering review, testing, or validation activities.

Atomicorp security controls are generally designed to identify and mitigate attack techniques and exploit behaviors rather than individual CVE identifiers. As a result, protections developed for one vulnerability may also apply to related attack variants and future vulnerabilities using similar techniques.

Learn More

Atomicorp develops commercially supported security technologies for web applications, servers, endpoints, workloads, and legacy enterprise platforms.

Organizations interested in protecting systems against exploit techniques associated with vulnerabilities such as this one can learn more about:

  • Atomic ModSecurity Rules: commercially maintained WAF rules for ModSecurity deployments, including protections for web attacks, bots, scanners, malware, exploitation attempts, and emerging vulnerability classes.
  • Atomic WAF: an enterprise web application firewall with Atomicorp rules, centralized management, reporting, authentication controls, and operational tooling.
  • Atomic ModSecurity Integrator: a special licensing option for MSPs, MSSPs, OEMs, VARs, reverse proxy providers, and other integrators who want to incorporate Atomicorp ModSecurity rule technology into their own products or services.
  • Atomic OSSEC: an enterprise OSSEC-based intrusion detection, compliance, and security monitoring platform.
  • Atomic Protector: workload protection for Linux, web, cloud, data center, and hybrid environments.

For additional information, product evaluations, or technical discussions, visit https://www.atomicorp.com/ or https://modsecurity.io/ .