CVE-2026-76663

Executive Summary

CVE-2026-76663 is a stack-based buffer overflow vulnerability in HPE NonStop TELSERV, the telnet server component of the HPE NonStop (formerly Tandem) fault-tolerant operating system environment. The flaw is remotely reachable and requires no authentication or user interaction, allowing an unauthenticated network attacker to corrupt process memory and trigger a denial-of-service condition. With a CVSS v3.1 base score of 7.5, the vulnerability primarily threatens availability of the NonStop TELSERV service and any dependent terminal-access workflows, though memory corruption in a privileged network service carries inherent risk of further exploitation beyond simple DoS. No public proof-of-concept, exploit code, or Nuclei template is currently available, and no vendor patch has been confirmed from any tracked source.

Technical Analysis

CVE-2026-76663 is classified under CWE-121, Stack-based Buffer Overflow. The NVD description states that the vulnerability lies in HPE NonStop TELSERV and "can cause memory corruption that may result in DoS." TELSERV is the Telnet server process within the NonStop environment, responsible for handling inbound terminal session negotiation and connectivity to the NonStop Kernel. Because the vulnerability is exposed over the network and is triggerable without credentials, the attack surface is any TELSERV listener reachable from an untrusted network segment.

The underlying mechanism, per the CWE-121 classification, is that input supplied to a stack-allocated buffer is written without adequate bounds checking, allowing data to exceed the buffer's capacity and overwrite adjacent stack memory — typically including saved control data used to direct program flow after the function returns. The NVD description does not disclose the specific protocol field, command verb, or parsing routine that houses the overflow, and no CNA title was provided. Consequently, the exact trigger point within TELSERV's Telnet option negotiation or command handling cannot be stated with confidence. What can be stated factually is the class of defect and its consequence: an unchecked copy into a stack buffer within a network-facing service, resulting in memory corruption that the vendor characterizes as potentially leading to denial of service. Memory corruption of this nature is frequently weaponizable beyond DoS (e.g., control-flow hijack), but no evidence in the available data confirms arbitrary code execution, and the CVSS vector assigns no Confidentiality or Integrity impact (C:N/I:N) while rating Availability as High (A:H), which is consistent with a crash/DoS-centric assessment. The PR:N and UI:N metrics confirm the service processes attacker-controlled input before any authentication stage.

Exploitation Scenarios

No public proof-of-concept or exploit code exists for CVE-2026-76663. The following attack chain is inferred strictly from the vulnerability class (remote stack-based buffer overflow in a network service) and the CVSS vector; no specific internal function names, protocol fields, or parameters are asserted because the NVD description does not provide them.

1. Reconnaissance / target identification. An attacker scans for exposed HPE NonStop TELSERV endpoints. Because TELSERV is a Telnet-based service, discovery typically involves identifying hosts with Telnet listeners associated with NonStop systems. Shodan queries such as product:"NonStop TELSERV" or enumeration of Telnet ports (commonly 23) can identify candidate targets. The attacker confirms reachability of the TELSERV listener from an untrusted network position, matching the AV:N metric.

2. Triggering the flaw class. The attacker establishes a connection to the TELSERV service and transmits crafted input that is processed by the vulnerable buffer-copy path. Per CWE-121, the triggering condition is input whose length exceeds the destination stack buffer's bounds, causing a write past the end of the buffer. Because the description does not identify the exact command or field, the attacker's task at this stage is to fuzz or enumerate the input surface (e.g., Telnet negotiation sequences or service commands) until the overflow condition is reached. No authentication is required (PR:N) and no user action is needed (UI:N), so the payload can be delivered directly over the socket.

3. Payload / execution method at the class level. The overflow corrupts adjacent stack memory. At minimum, the corrupted memory causes the TELSERV process to crash or behave unpredictably, producing the denial-of-service outcome the vendor describes. In a general stack-overflow exploitation pattern, an attacker would attempt to overwrite saved control data to redirect execution; however, the available data provides no evidence that this has been achieved for this CVE, and the CVSS vector reflects availability impact only.

4. Post-exploitation impact. Successful triggering causes memory corruption within TELSERV, potentially terminating or destabilizing the service process. Because TELSERV provides terminal access into the NonStop environment, a sustained DoS against it can disrupt administrative and operational access to affected systems. No data exfiltration or integrity impact is indicated by the CVSS subscores.

Impact Assessment

The CVSS v3.1 base score is 7.5 (High), with vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Breaking this down: the attack is network-reachable (AV:N) at low complexity (AC:L), requires no privileges (PR:N) and no user interaction (UI:N). The scope is unchanged (S:U), meaning impact is confined to TELSERV and its security authority. Confidentiality and Integrity impacts are None, while Availability impact is High — the defining business risk is service disruption rather than data compromise.

Affected systems are HPE NonStop environments running TELSERV. Because TELSERV is a core access service, a DoS condition can impede operator and administrative access to NonStop hosts, potentially disrupting dependent business processes hosted on the platform. The blast radius is greatest where TELSERV is reachable from untrusted networks, where NonStop systems support critical transactional workloads, and where redundancy or failover does not mask a crashed listener. The EPSS score is not yet available from FIRST.org; no exploitation probability estimate can be provided. CVE-2026-76663 is not listed in CISA KEV, and no in-the-wild exploitation has been reported in the available data. The residual concern is that memory corruption in a privileged network daemon can, in principle, be developed into code execution; however, no evidence supports that for this CVE, and defenders should prioritize availability protection while monitoring for anomalous TELSERV input.

Detection Guidance

Sigma Rule

This Sigma rule targets anomalous TELSERV process behavior consistent with a crash or memory-corruption event on HPE NonStop and Linux-hosted monitoring endpoints. Because TELSERV runs on the NonStop platform and specific process telemetry names are not disclosed in the available data, the rule focuses on generic process termination and fault indicators that SOC analysts can adapt to their endpoint schema.

title: Potential HPE NonStop TELSERV Stack Buffer Overflow Activity
id: 7c4f2b91-3a8e-4d62-9f10-2b6a5c8e1d44
status: experimental
description: Detects process faults or terminations associated with TELSERV that may indicate exploitation of CVE-2026-76663 stack-based buffer overflow.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-76663
author: valtersit.com Threat Intelligence
date: 2026/01/01
tags:
  - attack.t1190
  - cve.2026-76663
logsource:
  category: process_creation
  product: linux
detection:
  selection_name:
    Image|endswith:
      - '/telserv'
      - '/TELSERV'
  selection_fault:
    - CommandLine|contains:
        - 'TELSERV'
    - ExitCode: '139'
    - ExitCode: '11'
  condition: selection_name or selection_fault
falsepositives:
  - Legitimate service restarts or planned maintenance
  - Administrative termination of TELSERV
level: medium

YARA Rule

The following YARA rule is a heuristic memory/process-scanning rule for detecting signs of stack corruption or injected payloads in processes associated with TELSERV. Because no exploit sample exists, this rule uses generic overflow-indicator and Telnet-negotiation patterns rather than CVE-specific byte signatures.

rule CVE_2026_76663_TELSERV_StackOverflow_Indicator
{
    meta:
        author = "valtersit.com Threat Intelligence"
        description = "Heuristic detection of potential stack-based buffer overflow exploitation targeting HPE NonStop TELSERV (CVE-2026-76663)"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-76663"
        cve = "CVE-2026-76663"
        date = "2026-01-01"
        severity = "high"

    strings:
        $telopt_iac = { FF F0 }
        $telopt_sb  = { FF FA }
        $telopt_se  = { FF F0 }
        $long_a    = "AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA"
        $telnet_srv = "TELSERV" ascii wide nocase

    condition:
        (
            uint16(0) == 0x5A4D or
            uint16(0) == 0x7F45
        )
        and $telnet_srv
        and ( $telopt_iac or $telopt_sb )
        and $long_a
}

Suricata IDS Rule

The Suricata rule inspects TCP sessions to the Telnet service port for oversized input combined with Telnet IAC (0xFF) option-negotiation markers, which is a plausible network signature for attempting to reach a TELSERV buffer overflow. Port 23 is used as the conventional Telnet port; analysts should adjust the destination port to the environment's TELSERV listener.

alert tcp $EXTERNAL_NET any -> $HOME_NET 23 ( \
  msg:"CVE-2026-76663 Possible HPE NonStop TELSERV Stack Buffer Overflow Attempt"; \
  flow:established,to_server; \
  content:"|FF|"; offset:0; depth:64; \
  byte_test:1,>,200,0,relative; \
  threshold: type both, track by_src, count 5, seconds 60; \
  classtype:attempted-dos; \
  reference:cve,CVE-2026-76663; \
  reference:url,nvd.nist.gov/vuln/detail/CVE-2026-76663; \
  sid:2026766; rev:1; \
  metadata:created_at 2026_01_01, updated_at 2026_01_01, confidence Low; )

Elastic Detection

This Elastic EQL rule correlates inbound network connections to the Telnet service with process crash/termination telemetry, which is the observable pattern for a successful CVE-2026-76663 DoS trigger. Field names should be mapped to the environment's ECS-compliant indices.

sequence by host.id with maxspan=2m
  [ network where destination.port == 23 and event.action == "connection_accepted" ]
  [ process where process.name : ("telserv", "TELSERV") and event.action in ("process_stopped", "process_crashed", "fault") ]

Splunk SPL Query

This SPL search correlates Telnet service connections with subsequent TELSERV process termination or crash events, an indicator of a successful availability-impacting overflow.

index=network OR index=endpoint
| eval telnet_conn = if(dest_port=23 AND action="allowed", 1, 0)
| eval telserv_fault = if(match(process_name, "(?i)telserv") AND match(action, "(?i)stopped|crashed|fault|terminated"), 1, 0)
| where telnet_conn=1 OR telserv_fault=1
| stats count values(src_ip) as sources values(dest_ip) as targets values(process_name) as procs by host
| where count > 0
| table host, count, sources, targets, procs

Wazuh OSSEC Rule

This Wazuh rule fires when a TELSERV-related process fault or termination is recorded, supporting detection of the DoS consequence of this overflow. The parent rule ID (100600) is a custom range placeholder for deployment.

<group name="hpe_nonstop,telserv,cve_2026_76663,">
  <rule id="100601" level="10">
    <decoded_as>json</decoded_as>
    <field name="process.name">telserv|TELSERV</field>
    <field name="event.action">crashed|stopped|fault|terminated</field>
    <description>CVE-2026-76663: Potential HPE NonStop TELSERV crash or fault (possible stack buffer overflow DoS)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <options>no_full_log</options>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence is 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, affected version range, or workaround has been identified in the available data. The following guidance is therefore defensive and environment-adaptive.

1. Immediate mitigations. Restrict network access to TELSERV so that it is reachable only from trusted management segments — apply firewall or ACL rules blocking untrusted sources from reaching the TELSERV listener port. Where TELSERV is not operationally required, disable or stop the service. Isolate NonStop systems from flat or untrusted networks to reduce the AV:N exposure. Because the vulnerability requires no authentication, network segmentation is the most effective compensating control until a patch is available. Monitor TELSERV availability closely so that a crash-based DoS is detected quickly and service can be restored.

2. Patch / upgrade steps. No patch is currently confirmed. Track HPE NonStop security bulletins and the HPE Support Center for an advisory and fixed release specific to TELSERV. Once a vendor fix is published, validate it in a non-production environment, schedule deployment, and verify that the TELSERV listener version reflects the patched build. Until then, treat the system as unpatched and rely on network controls.

3. Long-term hardening. Reduce reliance on unencrypted Telnet-style access in favor of authenticated, encrypted management channels where supported. Apply least-privilege network zoning for NonStop management interfaces. Implement continuous monitoring and alerting on TELSERV input anomalies and process faults. Maintain an inventory of network-exposed NonStop services and periodically reassess their external reachability. Include NonStop components in vulnerability management and threat-intelligence workflows so that future TELSERV advisories are actioned promptly.

References

• NVD — CVE-2026-76663: https://nvd.nist.gov/vuln/detail/CVE-2026-76663
• CWE-121: Stack-based Buffer Overflow — https://cwe.mitre.org/data/definitions/121.html
• MITRE ATT&CK T1190 — Exploit Public-Facing Application: https://attack.mitre.org/techniques/T1190/
• HPE Support Center / NonStop security bulletins (vendor advisory pending): https://support.hpe.com/
• CISA Known Exploited Vulnerabilities Catalog (CVE-2026-76663 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No public proof-of-concept, exploit code, or Nuclei template is available for CVE-2026-76663. The vulnerability is not listed in CISA KEV, and no threat actor campaigns or exploit-in-the-wild evidence have been reported in the available data. EPSS has not yet assigned a score.