CVE-2026-15027

Executive Summary

CVE-2026-15027 is an unauthenticated OS command injection vulnerability (CWE-78) in CGServiSign, a product developed by Changing. The flaw is reachable through a local service interface and requires user interaction: a remote attacker must induce a victim to visit a malicious web page that triggers the vulnerable local service, after which arbitrary OS commands can be injected and executed on the victim's local computer. With a CVSS 3.1 base score of 8.8, successful exploitation yields high confidentiality, integrity, and availability impact on the targeted host, though no public exploit, Nuclei template, or patch currently exists.

Technical Analysis

The vulnerability class is operating system command injection (CWE-78). According to the NVD description, CGServiSign exposes a "local service interface" that accepts input which is passed to an operating system shell without adequate neutralization of shell metacharacters. Because input reaching the service is not properly sanitized, an attacker who can cause the service to process crafted content can append or embed OS command syntax that the underlying shell then executes with the privileges of the service process.

Two characteristics of this CVE are operationally important:

1. Local service interface, remote trigger via browser. The vulnerable component is described as running on the victim's local computer as a service. The attack is delivered remotely by luring the victim to a malicious web page. This is a classic "local service exposed to the browser" pattern: a page controlled by the attacker issues requests to the loopback / local service endpoint, and the attacker-supplied payload is relayed into the command-execution sink. This matches the CVSS vector, which is network-vector (AV:N) yet requires user interaction (UI:R) — the network attack is mediated by the victim's browser reaching the local service.

2. No authentication required. The service does not demand credentials before processing the malicious input (PR:N). This lowers the bar for exploitation once the victim is induced to load the attacking page.

The supplied description does not name specific functions, parameters, endpoints, request formats, or file paths inside CGServiSign. No code-level specifics are therefore asserted here; the mechanism below is described strictly at the vulnerability-class level. The CVSS vector (AC:L) indicates the attack conditions are not considered complex once the interaction precondition is met, and the scope is unchanged (S:U), meaning the vulnerable component and the impacted system are the same security authority — the victim's local machine.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-15027. The following chain is inferred from the NVD description and the CWE-78 vulnerability class; it does not assert any specific internal function, endpoint, or parameter names.

1. Reconnaissance / target identification. The attacker identifies victims who run CGServiSign with its local service interface active. Because the vulnerable service is local to the victim, the attacker does not need to reach the victim's machine directly; the attacker instead needs only a delivery channel — a web page the victim can be persuaded to load. No prior knowledge of the victim's network position is required (AV:N, PR:N).

2. Triggering the flaw. The attacker hosts or compromises a web page that, when rendered by the victim's browser, causes the victim's local CGServiSign service to be contacted and to receive attacker-controlled input. The malicious page is the delivery vector; the local service interface is the entry point into the command-injection sink. The exact request mechanism is not disclosed in the provided description.

3. Payload / execution. Once the crafted input reaches the service, shell metacharacters embedded in that input are interpreted by the operating system shell that the service invokes. This results in execution of arbitrary OS commands on the victim's local computer, under the privileges of the CGServiSign service process. The disclosure does not describe the command-execution sink's internals, so command syntax specifics are not provided.

4. Post-exploitation impact. Per the CVSS vector, exploitation yields high confidentiality (C:H), integrity (I:H), and availability (A:H) impact against the victim host. In practice this means an attacker capable of injecting arbitrary commands has the potential to read local data, modify or destroy local files and settings, and disrupt the machine — all bounded by the privilege level of the vulnerable service (S:U). Full compromise of the host's security authority is not asserted because the scope is unchanged and the service's privilege level is not disclosed.

Impact Assessment

• Score and vector: CVSS 3.1 base 8.8 (High). AV:N / AC:L / PR:N / UI:R / S:U / C:H / I:H / A:H.
• Business impact: An unauthenticated remote attacker who successfully lures a user to a malicious page can execute arbitrary OS commands on that user's computer. This enables data theft, tampering, and denial of service on the affected endpoint. Because exploitation targets the endpoint rather than a server, the blast radius is bounded to machines running the vulnerable CGServiSign service, but each such machine is subject to high-severity impact.
• Attack preconditions: The two gating factors are (a) presence of the vulnerable local CGServiSign service on the victim host, and (b) successful user-interaction lure (UI:R). The absence of any authentication requirement (PR:N) removes the credential barrier entirely.
• Severity drivers: The high CIA impact and low attack complexity make 8.8 an accurate reflection of risk, tempered only by the required user interaction.
• EPSS: Not yet scored by FIRST.org. No exploit-in-the-wild evidence is available, and CISA has not listed this CVE in the Known Exploited Vulnerabilities catalog.

Detection Guidance

Detection must account for two layers: endpoint activity from the local CGServiSign service (child shell processes, suspicious command lines) and network/browser activity that references the local service from a non-local or page-driven context. The rules below are written to be tuned; placeholder fields (e.g., signature names, local ports) should be adjusted to the environment because the disclosure does not specify them.

Sigma Rule

title: Potential OS Command Injection via CGServiSign Local Service
id: 6f3a2c1e-1502-7a27-9e00-cgservisign01
status: experimental
description: Detects suspicious child processes or shell invocation potentially spawned by the CGServiSign local service, consistent with CWE-78 command injection (CVE-2026-15027).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-15027
author: Threat Intelligence
date: 2026/01/01
logsource:
  category: process_creation
  product: windows
detection:
  selection_parent:
    ParentImage|contains:
      - 'CGServiSign'
      - 'CGServiSign'
  selection_child:
    Image|endswith:
      - '\cmd.exe'
      - '\powershell.exe'
      - '\pwsh.exe'
      - '\wscript.exe'
      - '\cscript.exe'
      - '\mshta.exe'
  selection_shellmeta:
    CommandLine|contains:
      - '& '
      - '&& '
      - '| '
      - '|| '
      - '; '
      - '`'
      - '$('
      - '> /'
      - '/etc/passwd'
      - 'whoami'
      - 'certutil -urlcache'
  condition: selection_parent and (selection_child or selection_shellmeta)
falsepositives:
  - Legitimate administrative scripts invoking the local service
  - Software update routines that launch signed shell binaries
level: high
tags:
  - attack.t1203
  - attack.execution
  - cve.2026.15027

YARA Rule

rule CVE_2026_15027_CGServiSign_CommandInjection
{
    meta:
        description = "Detects artifacts referencing CGServiSign and OS command injection indicators associated with CVE-2026-15027"
        author = "Threat Intelligence"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-15027"
        date = "2026-01-01"
        cve = "CVE-2026-15027"
        severity = "high"

    strings:
        $prod1 = "CGServiSign" ascii wide nocase
        $prod2 = "CGServiSign" ascii wide nocase
        $meta1 = "cmd.exe" ascii wide
        $meta2 = "powershell" ascii wide nocase
        $meta3 = "cmd.exe /c" ascii wide nocase
        $meta4 = "& whoami" ascii wide nocase
        $meta5 = "| whoami" ascii wide nocase
        $meta6 = "; whoami" ascii wide nocase
        $meta7 = "$(" ascii wide
        $loop1 = "127.0.0.1" ascii wide

    condition:
        (uint16(0) == 0x5A4D or true) and
        ($prod1 or $prod2) and
        (2 of ($meta*) or ($loop1 and 1 of ($meta*)))
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-15027 CGServiSign OS Command Injection Attempt"; flow:established,to_server; http.method; content:"GET"; http.header; content:"Referer"; content:"http"; pcre:"/(%26%26|%7C%7C|%3B|%60|\$\(|%24%28)/Ui"; http.uri; pcre:"/(CGServiSign|CGServiSign|cgservisign)/Ui"; classtype:web-application-attack; reference:cve,2026-15027; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-15027; sid:202615027; rev:1;)

Note: this rule is intentionally generic because no specific endpoint or parameter name is disclosed for CVE-2026-15027. Tune the http.uri PCRE to the actual local-service path observed in the environment.

Elastic Detection

sequence by process.parent.pid with maxspan=5s
  [process where process.name : ("cmd.exe","powershell.exe","pwsh.exe") and
     process.parent.name : ("CGServiSign*","*cgservisign*")]
  [process where process.name : ("whoami.exe","net.exe","certutil.exe","curl.exe","systeminfo.exe")]

EQL sequence capturing a shell spawned by the CGServiSign service immediately followed by common post-exploitation reconnaissance binaries.

Splunk SPL Query

index=windows (source="XmlWinEventLog:Microsoft-Windows-Sysmon/Operational" OR source="WinEventLog:Security")
(EventCode=1 OR EventCode=4688)
| search ParentImage="*CGServiSign*" OR ParentImage="*CGServiSign*" OR CommandLine="*CGServiSign*"
| search (Image="*\\cmd.exe" OR Image="*\\powershell.exe" OR Image="*\\pwsh.exe"
   OR Image="*\\wscript.exe" OR Image="*\\mshta.exe" OR Image="*\\certutil.exe")
   OR (CommandLine="*&& *" OR CommandLine="*|| *" OR CommandLine="*; *" OR CommandLine="*$( *" OR CommandLine="*| whoami*")
| stats count min(_time) as firstTime max(_time) as lastTime values(CommandLine) by Computer, User, ParentImage, Image
| convert ctime(firstTime) ctime(lastTime)

Wazuh OSSEC Rule

<group name="windows,sysmon,cve-2026-15027,">
  <rule id="2026150" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.parentImage">CGServiSign</field>
    <field name="win.eventdata.image">\\cmd.exe|\\powershell.exe|\\pwsh.exe|\\wscript.exe|\\mshta.exe</field>
    <description>CVE-2026-15027: Possible OS command injection - shell spawned by CGServiSign local service</description>
    <mitre>
      <id>T1203</id>
    </mitre>
    <group>cve_2026_15027,command_injection,</group>
  </rule>

  <rule id="2026151" level="12">
    <if_sid>61603</if_sid>
    <field name="win.eventdata.commandLine">&& |\|\| |; |\$\(|\| whoami</field>
    <description>CVE-2026-15027: Shell metacharacters observed in command line associated with CGServiSign exploitation</description>
    <mitre>
      <id>T1059</id>
    </mitre>
    <group>cve_2026_15027,command_injection,</group>
  </rule>
</group>

Note: field names (win.eventdata.parentImage, etc.) and if_sid values should be validated against the deployed Sysmon/Wazuh version; adjust regex fields to the environment's log schema.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor advisory, fixed version, workaround, or affected-version list is available for CVE-2026-15027. Because the vulnerable component is a local service interface, mitigation focuses on reducing exposure and blocking the browser-to-local-service path until a fix exists.

1. Immediate mitigations

   • Restrict or disable the CGServiSign local service interface where operationally feasible. If the service is not required, stop and disable it to remove the attack surface entirely.
   • Bind the local service strictly to loopback and, where the service supports it, require authentication or a per-session token so that page-driven requests cannot silently invoke it.
   • Apply browser hardening: enforce strict same-origin / private-network-access controls, block mixed-content and private-network requests from public pages, and consider extensions or policies that prevent web pages from reaching loopback services.
   • Add network egress filtering and DNS/proxy controls to reduce the chance users reach attacker-controlled lure pages.
   • Deploy the detection rules above to monitor for shell processes and metacharacter-bearing command lines originating from the CGServiSign service.

2. Patch / upgrade steps

   • Monitor Changing's official CGServiSign distribution channel and the CVE-2026-15027 NVD entry for a vendor advisory or fixed release.
   • When a patched version is published, validate the advisory, test in a staging environment, and deploy promptly.
   • Treat the current version as unpatched; there is no confirmed fixed version to upgrade to at this time.

3. Long-term hardening

   • Require services that expose a local interface to authenticate all callers and to reject unauthenticated or cross-origin invocation.
   • Enforce secure coding practices for command construction: avoid shell invocation entirely and pass arguments to OS APIs rather than concatenating strings into shell command lines.
   • Maintain an inventory of local/loopback services and include them in vulnerability management so that similar local-service exposures are caught early.
   • Adopt user awareness controls around phishing and malicious-page lures, since user interaction (UI:R) is a required precondition.
   • Track the CVE for CISA KEV listing and for any EPSS score once FIRST.org publishes one.

References

• NVD — CVE-2026-15027: https://nvd.nist.gov/vuln/detail/CVE-2026-15027
• CWE-78 — Improper Neutralization of Special Elements used in an OS Command ('OS Command Injection'): https://cwe.mitre.org/data/definitions/78.html
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-15027 not listed as of this report): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• MITRE ATT&CK T1203 — Exploitation for Client Execution: https://attack.mitre.org/techniques/T1203/
• Changing (vendor) product page — no CVE-2026-15027 advisory identified; monitor vendor channel for updates. No public exploit, PoC, or exploit-in-the-wild campaign has been identified for this CVE.