CVE-2026-108892

Executive Summary

CVE-2026-108892 is a remotely exploitable, unauthenticated stack-based buffer overflow in the Linksys E1700 wireless router running firmware version 1.0.0.4.003. The flaw is reachable through the /goform/ssid1MACFilter endpoint, where manipulation of the newap_text_%d argument in the ssid1MACFilter function corrupts the stack. With a CVSS v3.1 base score of 10.0 (AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H), successful exploitation allows an unauthenticated network attacker to fully compromise confidentiality, integrity, and availability, potentially achieving remote code execution on the device.

Technical Analysis

The vulnerability resides in the ssid1MACFilter function, which handles requests directed at the /goform/ssid1MACFilter file path/endpoint. This endpoint belongs to the router's MAC Filter configuration component — a feature that lets administrators whitelist or blacklist client devices by hardware address. The parameter newap_text_%d is parsed from the HTTP request and copied into a fixed-size stack buffer without bounds validation. Because the function performs an unchecked copy of attacker-supplied data, an oversized value overwrites adjacent stack memory, producing a classic stack-based buffer overflow (CWE-119).

Key characteristics:

• Attack surface: The MAC Filter form handler exposed through the router's embedded web management interface (/goform/ is the standard CGI dispatch path on Linksys/Broadcom-derived firmware). goform handlers are typically reachable on the LAN side and, on affected Linksys models, are frequently reachable from the WAN if remote management is enabled or if the handler lacks session enforcement.
• Triggers: The overflow is triggered by supplying an over-long value to the newap_text_%d argument. The %d suffix indicates an indexed parameter family (e.g., newap_text_0, newap_text_1, ...), consistent with per-entry MAC filter slots. Each indexed instance is a candidate injection point, multiplying the attack surface.
• Memory-safety class: Stack-based buffer overflow. No mitigation (stack canary, bounds check, safe copy routine) is evident given a CVSS 10.0 rating with Scope:Changed, indicating that the corruption can affect resources beyond the vulnerable component — a hallmark of memory-corruption impacting control flow (e.g., saved return address or function pointer overwrite).
• Preconditions: No authentication (PR:N) and no user interaction (UI:N) are required per the vector. Attack complexity is Low (AC:L), meaning the overflow is deterministically triggerable without specialized conditions.

The exact stack layout, buffer size, and overwrite delta are not disclosed in the NVD record and are therefore not asserted here. The ssid1MACFilter function name, the /goform/ssid1MACFilter path, and the newap_text_%d argument are the only concrete implementation artifacts confirmed by the advisory.

The vendor was contacted early but did not respond, leaving the flaw unpatched. Public disclosure means the technical details of the overflow are in the public domain, and the advisory explicitly states the exploit "may be utilized."

Exploitation Scenarios

No public PoC file is attributed in the tracked sources, so the following chain is inferred from the disclosed vulnerability class (unauthenticated stack buffer overflow in a goform handler) and the confirmed function/parameter names. It describes the mechanism at the vulnerability-class level.

1. Reconnaissance / target identification. The attacker scans a network range for Linksys E1700 management interfaces — commonly via the http.title:"Router" or vendor/product fingerprints — and inspects banner/Server headers or the presence of the /goform/ CGI path to confirm the Linksys embedded web stack. Because the vector is AV:N with no privileges required, the endpoint is reachable directly over the network; if WAN-side management is enabled, no internal foothold is needed.

2. Triggering the flaw. The attacker issues an HTTP request to /goform/ssid1MACFilter. In the body (or query string), they submit the newap_text_%d parameter with a value substantially longer than the expected MAC-filter entry, exceeding the destination buffer in ssid1MACFilter. The indexed suffix (newap_text_0, newap_text_1, etc.) lets the attacker target the vulnerable copy directly, and each index represents an independent corruption primitive.

3. Payload / execution method. At the vulnerability-class level, the overflow overwrites stack-adjacent memory. Depending on the firmware's memory protections, this typically proceeds to (a) overwrite of a saved return address or function pointer to redirect control flow to attacker-controlled data, or (b) corruption of adjacent stack variables that influence later execution. Because Scope is Changed and C/I/A impacts are High, the advisory implies control-flow or data-structure corruption with device-wide effect. A weaponized payload would embed shellcode or a ROP chain in the oversized value; no specific offsets or gadget addresses are disclosed here.

4. Post-exploitation impact. Successful exploitation yields code execution in the context of the router's web/CGI process, which on this class of device typically runs as root. This enables full device takeover: exfiltration or modification of router configuration, credential capture, traffic interception/redirection, establishment of persistence, and use of the device as a pivot into the internal network (Scope:Changed). Because the flaw is unauthenticated and remote, it is an ideal initial-access primitive against SOHO networks.

Impact Assessment

Exploitation provides complete loss of confidentiality, integrity, and availability of the affected router (all three CVSS subscores are High). The Scope:Changed rating reflects that the attacker's control extends beyond the vulnerable ssid1MACFilter component — consistent with full device compromise and downstream impact on systems behind the router. Blast radius includes:

• Network position: The router is a network chokepoint. Full compromise enables DNS manipulation, man-in-the-middle, traffic redirection, and lateral movement to all attached hosts.
• SOHO / branch deployments: Linksys E1700 is a consumer/SOHO device, frequently deployed in home offices, small businesses, and branch locations where it may anchor sensitive traffic but receives little security monitoring.
• Chaining risk: An unauthenticated remote root primitive on a perimeter device is a high-value, low-friction initial-access vector for ransomware and botnet operators.
• Reputational / compliance: Physical or logical exposure of internal traffic and credentials may trigger breach-notification and regulatory obligations.

CVSS v3.1: 10.0 — Critical (AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H). EPSS is not yet scored by FIRST.org; no EPSS value is asserted. CISA KEV: Not listed.

Detection Guidance

Sigma Rule

title: Linksys E1700 ssid1MACFilter Stack Buffer Overflow Attempt
id: 3f9c2a10-7d4e-4b6f-9a21-1c0d5e8f2b47
status: experimental
description: Detects oversized newap_text parameter submissions to /goform/ssid1MACFilter consistent with CVE-2026-108892 stack buffer overflow.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-108892
author: valtersit.com
date: 2026-01-01
logsource:
  category: webserver
detection:
  selection_uri:
    cs-uri-stem|contains: '/goform/ssid1MACFilter'
  selection_param:
    cs-uri-query|contains: 'newap_text'
  selection_length:
    cs-uri-query|re: 'newap_text_[0-9]+=.{256,}'
  condition: selection_uri and (selection_param and selection_length)
falsepositives:
  - Legitimate long MAC filter labels (unlikely to exceed 256 chars)
level: high
tags:
  - attack.initial_access
  - attack.t1190
  - cve.2026.108892

YARA Rule

rule CVE_2026_108892_Linksys_E1700_ssid1MACFilter
{
    meta:
        description = "Detects HTTP requests targeting the Linksys E1700 ssid1MACFilter endpoint with oversized newap_text parameter (CVE-2026-108892)"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-108892"
        date = "2026-01-01"
    strings:
        $uri = "/goform/ssid1MACFilter" ascii nocase
        $param = "newap_text_" ascii nocase
        $param2 = "newap_text" ascii nocase
    condition:
        $uri and ($param or $param2)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-108892 Linksys E1700 ssid1MACFilter Stack Buffer Overflow Attempt"; flow:established,to_server; http.uri; content:"/goform/ssid1MACFilter"; http.uri; content:"newap_text"; pcre:"/(newap_text(_[0-9]+)?)=.{256,}/Ui"; clamp:1; sid:2026108892; rev:1; metadata:created_at 2026_01_01, cve CVE-2026-108892, affected_product Linksys_E1700; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-108892; classtype:attempted-admin; priority:1;)

Elastic Detection

network where
  url.path == "/goform/ssid1MACFilter" and
  url.query contains "newap_text"

Companion EQL for length-based triggering where query is indexed as a full string:

network where
  url.path == "/goform/ssid1MACFilter" and
  length(url.query) > 256 and
  url.query contains "newap_text_"

Splunk SPL Query

index=web OR index=proxy OR index=suricata
| eval raw=coalesce(uri, url, cs_uri_stem, http_uri, request)
| where match(raw, "(?i)/goform/ssid1MACFilter")
| rex field=raw "(?i)newap_text(_\d+)?=(?<macfilter_val>[^&\s]+)"
| where len(macfilter_val) > 200
| stats count min(_time) as first_seen max(_time) as last_seen values(src_ip) as source_ips by dest, macfilter_val
| sort - count

Wazuh OSSEC Rule

<group name="web,linksys,cve-2026-108892,">
  <rule id="108892" level="12">
    <if_sid>31100,31101,31108</if_sid>
    <url>/goform/ssid1MACFilter</url>
    <match>newap_text</match>
    <description>CVE-2026-108892: Linksys E1700 ssid1MACFilter stack buffer overflow attempt via oversized newap_text parameter.</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>cve_2026_108892,attack_initial_access,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The vendor was contacted early and did not respond. No firmware update, hotfix, or workaround has been published.

1. Immediate mitigations

• Disable WAN-side / remote management on the E1700 so the /goform/ interface is not reachable from untrusted networks. Restrict management to the LAN and behind a firewall.
• Segment the router onto an isolated management VLAN; block inbound access to TCP/80 and TCP/443 from untrusted zones at the perimeter firewall.
• Apply upstream IPS/IDS signatures (Suricata/Snort rules above) at the network edge to block or alert on requests to /goform/ssid1MACFilter containing oversized newap_text values.
• Where feasible, replace the E1700 with a supported device, or place a hardened firewall/router in front of it and disable its routing/management functions (use as access point only, or retire it).

2. Patch / upgrade steps

• There is currently no vendor patch. Monitor the Linksys support site and security advisories for firmware later than 1.0.0.4.003. Do not assume a fix exists until a vendor advisory explicitly covers CVE-2026-108892.
• If a fixed firmware is released, verify the version before deployment, apply it during a maintenance window, and back up configuration beforehand. Confirm the /goform/ssid1MACFilter handler enforces bounds checking post-update.

3. Long-term hardening

• Maintain a hardware asset inventory with firmware-version tracking and lifecycle/EOL status for all SOHO and edge routers.
• Adopt a network architecture that terminates untrusted traffic at a monitored, patchable perimeter device rather than relying on SOHO router firmware.
• Enforce the principle that management interfaces are never WAN-exposed; standardize on VPN-only administrative access.
• Deploy continuous web-server/proxy logging with the detection rules above, and alert on oversized inputs to CGI handlers as a general memory-corruption signal.

References

• NVD — CVE-2026-108892: https://nvd.nist.gov/vuln/detail/CVE-2026-108892
• MITRE CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer): https://cwe.mitre.org/data/definitions/119.html
• MITRE ATT&CK T1190 (Exploit Public-Facing Application): https://attack.mitre.org/techniques/T1190/
• FIRST EPSS (score not yet published for this CVE): https://www.first.org/epss/
• CISA Known Exploited Vulnerabilities Catalog (CVE not listed as of report date): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

No public proof-of-concept exploit code or tracked in-the-wild campaign is attributed to this CVE in the available data. The advisory notes the exploit "has been publicly disclosed and may be utilized," indicating a realistic risk of weaponization despite the absence of a distributed PoC. Operators should treat exposure of the E1700 management interface to untrusted networks as a critical, actively-exploitable condition.