CVE-2026-77555

Executive Summary

CVE-2026-77555 is an out-of-bounds write vulnerability (CWE-787) affecting certain UniFi gateway devices. A remote, unauthenticated attacker with network access to a vulnerable device can trigger a memory corruption condition that results in a Denial of Service (DoS), rendering the device unavailable. With a CVSS base score of 7.5 and no patch, workaround, or public exploit currently available, affected organizations must rely on network-level compensating controls to reduce exposure.

Technical Analysis

CVE-2026-77555 is classified as CWE-787: Out-of-bounds Write. This weakness class occurs when software writes data past the end of an allocated memory buffer. In network-facing device firmware, out-of-bounds writes most commonly arise from improper bounds checking on attacker-supplied input — for example, when parsing protocol headers, packet payloads, or management request fields where a length or index value is trusted without validation. When the write exceeds the buffer boundary, it corrupts adjacent memory, which typically results in a crash (Denial of Service) or, depending on memory layout, potential code execution.

The CVSS vector CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H establishes the following characteristics of this vulnerability:

• Attack Vector: Network (AV:N) — The vulnerability is exploitable remotely over a network. The attacker does not require local or adjacent access; a routable network path to the affected device is sufficient.
• Attack Complexity: Low (AC:L) — No specialized conditions, race windows, or elevated attacker knowledge are required. Exploitation is repeatable and straightforward once the attack surface is reached.
• Privileges Required: None (PR:N) — No authentication or prior authorization is needed. The attacker does not need valid credentials.
• User Interaction: None (UI:N) — No victim action (such as clicking a link) is required. The attack can proceed unattended against the device.
• Scope: Unchanged (S:U) — The impact is confined to the vulnerable component itself.
• Confidentiality: None (C:N) — No data disclosure is expected as a direct result of this specific flaw.
• Integrity: None (I:N) — No modification of data is expected as a direct result.
• Availability: High (A:H) — Successful exploitation causes a full or near-full loss of availability of the affected device.

The attack surface is the network-exposed interface of the UniFi gateway device. Because the vulnerability is an out-of-bounds write reachable without authentication, the relevant exposure is any network path that reaches the vulnerable code path on the device. The NVD description does not specify which protocol, port, or service houses the vulnerable parser, nor does it identify the specific vendor firmware versions affected. Insufficient technical detail is available to name specific functions, endpoints, or parameters; any such detail would be speculative and is deliberately omitted.

The practical consequence of CWE-787 in this context is memory corruption in the device process handling the crafted input. The most reliable outcome of such corruption is an abnormal process termination or device reboot — a Denial of Service. While out-of-bounds writes can theoretically enable code execution, the CVSS vector explicitly scores Confidentiality and Integrity impacts as None, indicating that the assessed primary and realistic impact is availability loss, not arbitrary code execution.

Exploitation Scenarios

No public proof-of-concept, exploit code, or Nuclei template exists for CVE-2026-77555. The following attack chain is described at the vulnerability-class level and is inferred from the CWE-787 classification and the NVD description. It does not rely on fabricated function names, parameters, or endpoints.

1) Reconnaissance / Target Identification An adversary first identifies reachable UniFi gateway devices. This is achieved through internet-wide scanning services or by scanning internal address ranges for UniFi management or gateway interfaces. Devices with management or gateway interfaces exposed to untrusted networks (directly or via port forwarding) represent the highest-risk targets. Public scan engines frequently index such devices by product fingerprint and exposed management ports.

2) Triggering the Flaw Class The attacker crafts network input designed to manipulate a length, count, or index field consumed by the device's parsing logic, such that the device writes data beyond the bounds of an allocated buffer. Because the vulnerability requires no authentication (PR:N) and no user interaction (UI:N), the crafted input can be sent directly to the vulnerable network interface without any prior session establishment. The NVD description does not disclose which protocol or field is involved; the precise trigger therefore cannot be enumerated without vendor-provided detail.

3) Execution Method (Vulnerability-Class Level) At the class level, the attacker transmits the malformed input to the vulnerable service on the device. The device's parser processes the input and performs an out-of-bounds write, corrupting adjacent in-memory structures. No specific payload bytes, function names, or request formats are documented for this CVE, and none are asserted here.

4) Post-Exploitation Impact Successful triggering results in memory corruption leading to a Denial of Service — the affected gateway process crashes or the device becomes unresponsive and may reboot. The CVSS impact subscores (C:N/I:N/A:H) indicate that the outcome is loss of availability. Repeated transmission of the triggering input can sustain the outage (a persistent DoS), disrupting network connectivity for all users and systems behind the gateway.

Constraints and Caveats Because no public exploit exists and the description does not name the affected protocol or firmware, this chain is a class-level reconstruction. It should not be treated as an operational exploit recipe. Actual triggering conditions require vendor confirmation.

Impact Assessment

Business Impact. Compromise or failure of a UniFi gateway device directly affects network connectivity. A gateway functions as the boundary between internal networks and external networks; a sustained DoS condition can interrupt internet access, inter-VLAN routing, VPN connectivity, and remote access for the entire site served by the device. For distributed organizations with branch offices, each exposed gateway represents an independent single point of failure.

Affected Systems. Certain UniFi gateway devices, per the NVD description. The specific model list and firmware version ranges are not enumerated in available data (affected version range: not specified; affected packages: not specified). Organizations running UniFi gateways should treat their entire deployed fleet as potentially affected until the vendor clarifies the scope.

Blast Radius. The CVSS scope is Unchanged (S:U), meaning impact is confined to the vulnerable component. However, because the component is a network gateway, the operational blast radius extends to every user and service dependent on that gateway for connectivity, even though the technical CVSS scope is limited to the device.

Severity. CVSS base score 7.5 (High), driven almost entirely by the High availability impact combined with the Network/Low/None/None exploitability profile. The absence of a public exploit or KEV listing tempers immediate urgency for well-segmented environments, but the unauthenticated, low-complexity, remote nature of the flaw means any internet-exposed device should be remediated or isolated promptly. No EPSS score is currently available from FIRST.org; no CISA KEV listing exists for this CVE.

Detection Guidance

The rules below are heuristic and behavior-based, because no public exploit signature exists. They target anomalous or malformed traffic toward UniFi gateway interfaces, repeated device restarts, and availability anomalies. They should be tuned to the specific firmware and traffic profile of each environment, and some rules will require tailoring before production use.

Sigma Rule

title: Suspected Exploitation Attempt Against UniFi Gateway Out-of-Bounds Write (CVE-2026-77555)
id: 8f2c1a74-3b6e-4d2f-9a11-77555cve2026
status: experimental
description: Detects repeated connection attempts or malformed traffic directed at UniFi gateway management/gateway interfaces that may indicate exploitation of CVE-2026-77555.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-77555
author: valtersit.com
date: 2026/01/01
logsource:
  product: suricata
  category: network_flow
detection:
  selection_dst_port:
    dst_port:
      - 80
      - 443
      - 8443
      - 8080
      - 3478
  selection_uniFi_src:
    src_ip|cidr:
      - '0.0.0.0/0'
  timeframe: 5m
  condition: selection_dst_port | count() by src_ip > 100
falsepositives:
  - Legitimate management traffic from authorized administrative hosts
  - Vulnerability scanners and monitoring systems
level: medium
tags:
  - attack.t1499
  - cve.2026-77555

YARA Rule

rule CVE_2026_77555_UniFi_OOB_Write_Indicator
{
    meta:
        description = "Heuristic indicator for traffic/payloads associated with CVE-2026-77555 UniFi gateway out-of-bounds write. No known public PoC; rule targets anomalous oversized fields consistent with CWE-787 exploitation patterns."
        author = "valtersit.com"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77555"
        date = "2026-01-01"
        severity = "high"

    strings:
        $uniFi = "UniFi" ascii nocase
        $mgmt   = "unifi" ascii nocase

    condition:
        $uniFi or $mgmt
}

Note: With no public exploit sample, this YARA rule is a placeholder indicator matcher for UniFi-related artifacts and must be replaced with a content-specific rule once a sample or vendor detail is available. It is intentionally non-specific to avoid false confidence.

Suricata IDS Rule

alert tcp any any -> $HOME_NET [80,443,8443,8080] (msg:"CVE-2026-77555 Possible UniFi Gateway Out-of-Bounds Write Exploitation Attempt"; flow:to_server,established; threshold:type both, track by_src, count 50, seconds 60; classtype:attempted-dos; reference:cve,2026-77555; reference:url,nvd.nist.gov/vuln/detail/CVE-2026-77555; sid:202677555; rev:1;)

This rule flags volumetric connection bursts toward UniFi gateway service ports, a heuristic proxy for brute-force or malformed-input DoS activity. It does not identify the specific trigger, which is not publicly documented.

Elastic Detection

sequence by destination.ip with maxspan=2m
  [network where destination.port in (80, 443, 8080, 8443) and event.action == "network_flow" and process.name == null]
  [network where destination.port in (80, 443, 8080, 8443)] by source.ip

Purpose: Correlate high-frequency connection sequences from a single source toward UniFi gateway management/service ports, which may precede or accompany exploitation attempts. Requires network flow or packet metadata ingestion.

Splunk SPL Query

index=network sourcetype=firewall OR sourcetype=suricata
| search dest_port IN (80,443,8443,8080,3478)
| bucket _time span=1m
| stats count AS conn_count, dc(dest_ip) AS unique_targets BY src_ip, _time
| where conn_count > 100 AND unique_targets >= 1
| sort - conn_count

Purpose: Identify sources generating abnormal connection volumes toward UniFi gateway service ports within a one-minute window. Tune the threshold to baseline traffic.

Wazuh OSSEC Rule

<group name="cve_2026_77555,unifi,ddos,">
  <rule id="100755" level="10">
    <decoded_as>json</decoded_as>
    <field name="dest_port">^(80|443|8080|8443)$</field>
    <description>CVE-2026-77555: Possible UniFi gateway DoS attempt - excessive connections to gateway service port</description>
    <options>no_full_log</options>
    <mitre>
      <id>T1499</id>
    </mitre>
    <group>availability_impact,</group>
  </rule>
</group>

Requires the Wazuh agent to ingest firewall or network flow logs; adjust the field and decoded_as directives to match the local decoder configuration.

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No vendor fix, workaround, or affected-version enumeration is currently available.

1) Immediate Mitigations

• Remove internet exposure. Restrict UniFi gateway management and service interfaces so they are not reachable from untrusted networks. This is the single most effective compensating control given the Network/Low/None/None exploitability profile.
• Network segmentation. Place management interfaces on a dedicated management VLAN accessible only to authorized administrative hosts.
• Access control. Enforce firewall rules limiting inbound access to gateway service ports to known, trusted source addresses.
• Monitoring. Deploy the detection rules above to alert on anomalous connection patterns and unexpected device reboots or process restarts.
• Log collection. Ensure device logs are forwarded off-box so that a DoS-induced crash does not destroy the evidence trail.

2) Patch / Upgrade Steps

• Monitor the vendor for an official advisory and firmware release addressing CVE-2026-77555. No patch reference exists at this time; do not assume any current firmware version is fixed.
• When a vendor fix is published, verify the integrity of the firmware image, apply it during a maintenance window, and confirm the device version post-upgrade.
• Because affected version ranges are unknown, treat all deployed UniFi gateway devices as candidates for upgrade once a fix is available.

3) Long-Term Hardening

• Establish a firmware lifecycle process: track vendor advisories, maintain an inventory of UniFi gateway devices and firmware versions, and apply security updates on a defined cadence.
• Adopt a "no management plane on the internet" policy for all network infrastructure.
• Enable centralized logging and availability monitoring on gateway devices to detect recurring crashes or disconnects.
• Conduct periodic attack-surface reviews to identify any re-exposed management interfaces, particularly after changes to NAT or firewall configuration.
• Where operationally feasible, deploy resilient gateway designs (redundancy/failover) to reduce the availability impact of a single-device DoS.

References

1. NVD — CVE-2026-77555: https://nvd.nist.gov/vuln/detail/CVE-2026-77555
2. CWE-787: Out-of-bounds Write — MITRE: https://cwe.mitre.org/data/definitions/787.html
3. MITRE ATT&CK T1499 — Endpoint Denial of Service: https://attack.mitre.org/techniques/T1499/
4. FIRST.org EPSS (score not yet available for this CVE): https://www.first.org/epss/
5. CISA Known Exploited Vulnerabilities Catalog (CVE-2026-77555 not listed): https://www.cisa.gov/known-exploited-vulnerabilities-catalog

Exploit-in-the-wild status: No public PoC, no exploit code, no Nuclei template, and no CISA KEV listing exist for CVE-2026-77555 at the time of writing. No threat actor campaign has been associated with this CVE. Organizations should monitor vendor and CISA channels for changes to this status.