CVE-2026-108893

Executive Summary

CVE-2026-108893 is a stack-based buffer overflow in the Linksys E1700 wireless router (firmware up to 1.0.0.4.003) that is reachable remotely through manipulation of the vlan_set argument passed to the setVlan function in /goform/setVlan. The flaw requires low privileges (an authenticated or otherwise authorized session context) but carries a CVSS v3.1 base score of 9.9 because exploitation can cross a security boundary and fully compromise confidentiality, integrity, and availability. The vulnerability is unpatched, the exploit is publicly available, and the vendor has not responded to disclosure — making this a high-urgency exposure for any organization still running the affected device.

Technical Analysis

The vulnerability is classified as CWE-119 (Improper Restriction of Operations within the Bounds of a Memory Buffer) and is specifically a stack-based buffer overflow. According to the NVD description, the issue resides in the setVlan function exposed through the /goform/setVlan web endpoint. The vulnerable input path is the vlan_set argument, which is copied into a fixed-size stack buffer without adequate length validation.

The mechanics of a classic stack-based overflow apply here: when the input supplied to vlan_set exceeds the size of the destination stack buffer, adjacent stack memory is overwritten. This can corrupt saved registers and the saved return address. If the attacker controls the overwritten content, execution flow can be redirected, leading to arbitrary code execution in the context of the goform web service process running on the device.

The attack surface is the router's embedded HTTP management interface, which exposes goform handler endpoints. Because the handler is reachable over the network, the request can be delivered to the device's web server without physical access. The CVSS vector (AV:N) confirms the network attack vector, while PR:L indicates that some level of authorization or authenticated session is expected to reach the handler. S:C (Changed) indicates that successful exploitation impacts resources beyond the vulnerable component's own security scope — consistent with router firmware compromise granting control over the device and the network segment it serves.

The NVD entry does not disclose the exact buffer size, the precise vulnerable code, or the specific overflow offset. No public PoC code is referenced in the provided data, and no CVE-assigned PoC/exploit artifact is listed. Accordingly, the precise byte-level offset and any ROP chain details cannot be stated and must not be assumed.

Key facts supported by the data:

• Affected product: Linksys E1700, firmware up to 1.0.0.4.003.
• Vulnerable function: setVlan.
• Vulnerable file/endpoint: /goform/setVlan.
• Vulnerable argument: vlan_set.
• Vulnerability class: stack-based buffer overflow (CWE-119 / CWE-121-adjacent, reported under CWE-119).
• Disclosure status: vendor contacted, no response; exploit publicly available.

Exploitation Scenarios

The attack chain below is described at the vulnerability-class level. No public PoC is available in the supplied data; where the description does not specify implementation details, the chain is inferred strictly from the stated vulnerability class.

1. Reconnaissance / target identification. An attacker scans for internet- or LAN-exposed Linksys E1700 management interfaces. Identification relies on the device's HTTP banner/title and the presence of goform handler paths. Shodan-style queries such as product:"Linksys E1700" or http.html:"goform" can enumerate candidate targets. Because the endpoint is a goform handler, the attacker needs an authorized session (PR:L), so reconnaissance also includes determining whether default, weak, or reused administrative credentials grant access, or whether an existing session can be leveraged.

2. Triggering the flaw. The attacker sends an HTTP request to /goform/setVlan containing an oversized value in the vlan_set argument. The setVlan handler copies this argument into a fixed-size stack buffer without bounds checking. When the supplied value exceeds the buffer capacity, the overflow is triggered.

3. Payload / execution method. For a stack-based buffer overflow, the payload length and structure must satisfy two goals: overwrite the saved return address (or other control data on the stack) and place attacker-controlled data — typically a payload or shellcode — in a deterministic location. In embedded MIPS/ARM router firmware, this commonly involves overwriting the return address with a gadget or controlled address and pointing execution at injected data. The exact offsets, bad bytes, and target addresses depend on the specific firmware build and binary and are not disclosed in the available data. The attacker must accommodate the value-processing that setVlan performs on vlan_set (e.g., encoding or delimiter handling) when crafting the payload.

4. Post-exploitation impact. Because the overflow occurs inside the router's web service, successful control-flow hijack yields code execution as that process — typically root or a highly privileged account on embedded firmware. From there the attacker can modify device configuration, alter VLAN/network segmentation settings, redirect or intercept traffic, persist across reboots, or use the device as a foothold into the internal network. Given the Changed scope in the CVSS vector, impact extends beyond the router itself to connected systems and network traffic.

Note: No public PoC is available; attack chain is inferred from the vulnerability class (stack-based buffer overflow via an unvalidated web handler argument), and no specific code patterns, offsets, or hypothetical function names beyond those stated in the NVD description should be assumed.

Impact Assessment

CVSS v3.1 base score: 9.9 (Critical). Vector: CVSS:3.1/AV:N/AC:L/PR:L/UI:N/S:C/C:H/I:H/A:H.

Subscore interpretation:

• Attack Vector (Network): exploitable remotely; no physical or adjacent-only restriction.
• Attack Complexity (Low): no special conditions or race windows required.
• Privileges Required (Low): an authorized session is needed to reach the handler, but this is readily obtainable via default or weak credentials, or session reuse.
• User Interaction (None): the attack is automated; no victim action is required.
• Scope (Changed): exploitation affects resources beyond the vulnerable component — the router's role as a network gateway means compromise can impact downstream systems and traffic.
• Confidentiality/Integrity/Availability (High/High/High): full loss of confidentiality, integrity, and availability of the device and potentially the network it protects.

Business impact: The E1700 is a consumer/SOHO router often deployed at branch offices, home offices, and small business edge. A compromised router can be used for traffic interception/MITM, DNS manipulation, credential harvesting, botnet enrollment, and as a pivot into protected internal networks. Because the device is a network boundary component, scope-changed impact means the blast radius extends to all systems that trust the router's routing, DNS, and segmentation functions. The unpatched status and publicly available exploit elevate this to a high-priority exposure for any environment still operating the device. CISA KEV: no — this CVE is not currently listed.

Detection Guidance

Sigma Rule

title: Linksys E1700 setVlan Stack Buffer Overflow Attempt
id: 8f3c1b2e-4a7d-4c9e-9b1a-linksys-e1700-setvlan
status: experimental
description: Detects HTTP requests targeting the Linksys E1700 /goform/setVlan endpoint with an oversized vlan_set argument indicative of a stack-based buffer overflow attempt (CVE-2026-108893).
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-108893
author: valtersit.com
logsource:
  category: webserver
  product: linksys
detection:
  selection_endpoint:
    cs-uri-stem|contains: '/goform/setVlan'
  selection_arg:
    cs-uri-query|contains: 'vlan_set='
  selection_length:
    cs-uri-query|re: 'vlan_set=.{512,}'
  condition: selection_endpoint and selection_arg and selection_length
fields:
  - c-ip
  - cs-uri-stem
  - cs-uri-query
level: critical
falsepositives:
  - Legitimate administrative configuration of VLAN settings with unusually long values (unlikely)

YARA Rule

rule CVE_2026_108893_Linksys_E1700_setVlan_Overflow
{
    meta:
        description = "Detects references to the vulnerable Linksys E1700 setVlan handler / goform endpoint associated with CVE-2026-108893"
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-108893"
        date = "2025-01-01"
        severity = "critical"

    strings:
        $s1 = "/goform/setVlan" ascii wide nocase
        $s2 = "setVlan" ascii wide nocase
        $s3 = "vlan_set" ascii wide nocase
        $s4 = "goform" ascii wide nocase

    condition:
        (uint16(0) == 0x5A4D or uint32(0) == 0x464C457F) and
        any of ($s1, $s2) and any of ($s3, $s4)
}

Suricata IDS Rule

alert http any any -> any any (msg:"CVE-2026-108893 Linksys E1700 setVlan Stack Buffer Overflow Attempt"; flow:established,to_server; http.uri; content:"/goform/setVlan"; nocase; http.uri; content:"vlan_set="; nocase; pcre:"/vlan_set=[^&]{512,}/i"; classtype:attempted-admin; reference:cve,2026-108893; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-108893; sid:2026108893; rev:1;)

Elastic Detection

sequence by source.ip with maxspan=10s
  [network where http.request.method == "POST" and url.path == "/goform/setVlan"]
  [network where url.path == "/goform/setVlan" and
     length(http.request.body) > 512 and
     wildcard(http.request.body, "*vlan_set=*")]

Splunk SPL Query

index=web OR index=network sourcetype=*access* OR sourcetype=*stream*http*
| eval uri=coalesce(uri_path, cs_uri_stem, url)
| eval query=coalesce(uri_query, cs_uri_query, url)
| where match(uri, "(?i)/goform/setVlan") AND match(query, "(?i)vlan_set=")
| eval arglen=len(query)
| where arglen > 512
| stats count min(_time) as first_seen max(_time) as last_seen values(arglen) as arg_lengths by src_ip, uri
| convert ctime(first_seen) ctime(last_seen)
| sort - count

Wazuh OSSEC Rule

<group name="linksys,e1700,web,">
  <rule id="100893" level="12">
    <decoded_as>web-accesslog</decoded_as>
    <match>/goform/setVlan</match>
    <regex>vlan_set=[^&amp;]{512,}</regex>
    <description>CVE-2026-108893: Possible Linksys E1700 setVlan stack buffer overflow attempt (oversized vlan_set argument)</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>attack,cve_2026_108893,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). The vendor was contacted about the disclosure and did not respond; no fixed firmware version exists.

1. Immediate mitigations:

   • Discontinue or replace the Linksys E1700 in any production, branch, or remote-work deployment. Given the unpatched, remotely exploitable, publicly exploited status, the device should be treated as end-of-life and untrusted.
   • If removal is not immediately possible, remove the device's management interface from any untrusted network. Block external access to the HTTP management port at the edge firewall and restrict management to an isolated, trusted administrative VLAN.
   • Change all default administrative credentials and enforce unique, strong passwords to eliminate the low-privilege foothold the attacker relies on to reach the handler (PR:L).
   • Disable remote management / WAN-side administration if the feature is enabled on the device.
   • Monitor the device for anomalous configuration changes, unexpected reboots, DNS changes, or outbound connections.

2. Patch / upgrade steps:

   • No vendor patch is available. Monitor the Linksys E1700 support/download page and the vendor security advisory feed for any future firmware release addressing /goform/setVlan.
   • Because the vendor did not respond to disclosure, do not expect a timely fix; migration to a supported, actively maintained router platform should be treated as the remediation path.

3. Long-term hardening:

   • Maintain a hardware/software inventory to identify unsupported SOHO/edge devices before they become liabilities.
   • Apply an end-of-life policy that mandates replacement of network edge devices that no longer receive security updates.
   • Segment IoT/SOHO routing gear away from critical internal networks to reduce scope-changed impact.
   • Deploy network and web-log detection (rules above) at the perimeter to catch overflow attempts against goform handlers.
   • Subscribe to CVE/NVD feeds and CISA KEV monitoring for early warning on edge-device vulnerabilities.

References

• NVD — CVE-2026-108893: https://nvd.nist.gov/vuln/detail/CVE-2026-108893
• 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/
• Linksys E1700 product/support page: https://www.linksys.com/support/
• FIRST EPSS: not yet scored for this CVE as of the provided data.