CVE-2026-77558

Executive Summary

CVE-2026-77558 is an Out-of-bounds Read vulnerability (CWE-125) affecting certain UniFi gateway devices. A remote, unauthenticated attacker with network access can trigger the flaw to cause a Denial of Service (DoS) condition on the affected device. The vulnerability carries a CVSS 3.1 base score of 7.5 (High), reflecting a network-reachable, low-complexity attack with no privileges or user interaction required and a high availability impact. No public proof-of-concept, exploit code, or Nuclei template exists at the time of writing, and no vendor patch has been confirmed.

Technical Analysis

The vulnerability is classified as CWE-125: Out-of-bounds Read. This class of defect occurs when software reads data past the end (or before the beginning) of an allocated memory buffer. In network-facing firmware and embedded gateway appliances, out-of-bounds reads typically arise in packet parsing routines, protocol decoders, or memory-handling logic that fails to validate length or boundary fields supplied by the remote peer before indexing into a buffer.

The NVD description states that "a malicious actor with access to the network could exploit an Out-of-bounds Read vulnerability found in certain UniFi gateway devices to execute a Denial of Service (DoS) attack on the device." From this, the following can be derived with confidence:

• The attack surface is network-reachable (CVSS AV:N).
• No authentication is required (PR:N) and no user interaction is needed (UI:N).
• Attack complexity is low (AC:L), implying the malformed input that triggers the read is straightforward to construct once the vulnerable parsing path is identified.
• The sole impact is availability (A:H), with no confidentiality or integrity impact (C:N/I:N). This is consistent with an out-of-bounds read that crashes or hangs the device rather than one that leaks exploitable memory to the attacker in a way that yields confidentiality gain.

What is NOT known and must not be assumed: the exact firmware versions affected (the CVE data provides no affected-version range), the specific protocol or interface (e.g., WAN-side, LAN-side, VPN, or management plane) that exposes the vulnerable code path, and the precise buffer boundary that is mishandled. No function names, packet structures, or code-level identifiers are present in the supplied data. Any deeper claim — such as the specific daemon, parser, or field involved — would be speculation.

The likely mechanism, at the vulnerability-class level, is: an attacker sends a crafted network request or packet whose length/offset metadata causes the gateway's parsing logic to read memory outside an intended buffer. Depending on firmware hardening (e.g., memory protection, watchdog behavior), the result is a crash, a watchdog-initiated reboot, or an unresponsive state — each of which constitutes the DoS. Because out-of-bounds reads can also, under some conditions, produce information disclosure, defenders should not rule out that adjacent behaviors could leak memory; however, the assigned CVSS reflects availability-only impact and no confidentiality impact was scored.

Exploitation Scenarios

No public PoC is available; the attack chain below is inferred at the vulnerability-class level from the NVD description and CVSS vector. It does not reference any specific function, endpoint, or parameter because none are provided in the source data.

1. Reconnaissance / target identification An attacker identifies internet- or network-exposed UniFi gateway devices. This may be done through internet-wide scanning (e.g., Shodan/Censys queries using UniFi product fingerprints and web titles) or through internal network discovery if the attacker already has a foothold on the same network ("access to the network"). The attacker enumerates reachable services on the gateway to determine which network-facing interface may reach the vulnerable code path.

2. Triggering the flaw class The attacker crafts input that is delivered to the gateway's network parsing logic. For an out-of-bounds read, the crafted input manipulates a length, count, or offset field such that the parser reads beyond the bounds of a buffer. Because AC:L and PR:N are assigned, the trigger is expected to be reproducible without credentials and without complex race conditions — a single or small number of malformed requests is likely sufficient.

3. Execution method (vulnerability-class level) No payload is executed in the traditional sense. The "payload" is the malformed network data itself. The attacker transmits it to the vulnerable interface. Success is measured by the device entering a fault state (crash, hang, or reboot) rather than by code execution. No public exploit code exists to model the exact bytes required.

4. Post-exploitation impact The direct effect is loss of availability: the gateway becomes unresponsive or restarts, disrupting routing/firewalling, VPN, and any network traffic that depends on it. Repeated triggering could produce a sustained or intermittent outage. There is no scored confidentiality or integrity impact, so data theft or configuration tampering is not an expected outcome of this specific CVE based on the provided data.

Because details of the exact vulnerable interface and versions are absent, defenders should treat any network-reachable UniFi gateway as potentially in scope until the vendor clarifies affected models and firmware.

Impact Assessment

• CVSS 3.1: 7.5 (High) — AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H.
• Availability: High impact. The device can be rendered unavailable (DoS). For organizations where the UniFi gateway is the edge router/firewall, an outage can cascade to all downstream connectivity, VPN access, and dependent services.
• Confidentiality / Integrity: No impact scored. Out-of-bounds reads can in principle disclose memory, but the assigned vector does not credit confidentiality loss.
• Scope: Unchanged (S:U) — impact is confined to the vulnerable device.
• Blast radius: Every network that relies on an affected UniFi gateway for routing or security enforcement. Branch offices, small-to-mid-size deployments, and remote sites with a single gateway are especially exposed because there is no redundant path.
• Authentication requirement: None. Any party with network reachability to the vulnerable interface can attempt exploitation, lowering the barrier for both targeted and opportunistic attacks.

Business impact centers on operational disruption: connectivity loss, downtime for dependent services, remediation labor, and potential SLA violations. Because no patch exists (0/4 confidence), the window of exposure remains open.

Detection Guidance

Sigma Rule

This rule targets device-side fault conditions surfaced via syslog (gateway crash, kernel fault, watchdog restart) that may indicate a triggered out-of-bounds read. It is heuristic, since exploit packets are not observable at the host log layer.

title: Potential UniFi Gateway DoS via Out-of-Bounds Read (CVE-2026-77558)
id: 6f2b1a9e-uni-2026-7755-cve-obr-dos
status: experimental
description: Detects gateway crash, kernel memory fault, or watchdog restart events consistent with a remote out-of-bounds read DoS on UniFi gateway devices.
references:
  - https://nvd.nist.gov/vuln/detail/CVE-2026-77558
author: valtersit.com
date: 2026/01/01
logsource:
  product: unifi
  category: gateway
detection:
  selection_fault:
    - message|contains:
        - 'out of bounds'
        - 'Out-of-bounds'
        - 'general protection fault'
        - 'kernel panic'
        - 'sg_read'
  selection_restart:
    - message|contains:
        - 'watchdog'
        - 'unexpected restart'
        - 'process crashed'
        - 'segfault'
  condition: selection_fault or selection_restart
falsepositives:
  - Legitimate firmware updates or manual reboots
  - Power events or hardware failures
level: medium
tags:
  - attack.t1190
  - cve.2026.77558

YARA Rule

No exploit sample or memory artifact is available, so this rule is a conservative heuristic for firmware binaries or artifacts referencing the vulnerable product. It does not assert an exact code signature.

rule CVE_2026_77558_UniFi_Gateway_OBR_DoS
{
    meta:
        description = "Heuristic detection for UniFi gateway out-of-bounds read DoS context (CVE-2026-77558)"
        author = "valtersit.com"
        date = "2026-01-01"
        reference = "https://nvd.nist.gov/vuln/detail/CVE-2026-77558"
        cve = "CVE-2026-77558"

    strings:
        $u1 = "UniFi" ascii nocase
        $u2 = "UniFiOS" ascii nocase
        $v1 = "CWE-125" ascii nocase

    condition:
        (uint32be(0) == 0x7F454C46 or uint32be(0) == 0xFEEDFACE) and
        any of ($u*) and $v1
}

Note: The $v1 string is rarely present in real binaries and should be treated as an optional correlator. The rule is intentionally broad because no concrete artifact is available.

Suricata IDS Rule

This rule detects large or malformed network input toward a gateway service that could be consistent with an OBR trigger. It is behavioral and will require tuning; no specific vulnerable protocol/port is known.

alert tcp any any -> any any (msg:"CVE-2026-77558 Possible UniFi Gateway Out-of-Bounds Read DoS Attempt"; flow:to_server,established; content:"|00|"; depth:1; byte_test:2,>,1400,0,relative; threshold: type both, track by_src, count 20, seconds 10; classtype:attempted-dos; reference:cve,2026-77558; sid:202677558; rev:1;)

Because the exact vulnerable protocol is unknown, this rule is a placeholder heuristic and should be adapted once the vendor or researchers confirm the affected interface.

Elastic Detection

Detects a high rate of connection resets or service-unavailability events from a single source toward a gateway, which may accompany a DoS attempt.

sequence by source.ip with maxspan=1m
  [ network where destination.port in (80,443,8080,8443) and network.transport == "tcp" ]
  [ network where event.outcome == "failure" or network.bytes == 0 ]

Splunk SPL Query

Identifies spikes in dropped resets or fault logs on gateway hosts.

index=network (sourcetype=unifi:* OR sourcetype=firewall) ("out of bounds" OR "watchdog" OR "crash" OR "segfault" OR "kernel panic")
| stats count values(message) as events by host src_ip
| where count > 5
| sort - count

Wazuh OSSEC Rule

Monitors gateway syslog for crash/fault indicators consistent with the DoS.

<group name="unifi,gateway,cve-2026-77558,">
  <rule id="2027755" level="10">
    <decoded_as>syslog</decoded_as>
    <match>out of bounds|Out-of-bounds|general protection fault|kernel panic|watchdog|segfault</match>
    <description>CVE-2026-77558 - Possible UniFi gateway out-of-bounds read / DoS condition detected</description>
    <mitre>
      <id>T1190</id>
    </mitre>
    <group>availability_impact,</group>
  </rule>
</group>

Mitigation & Patch Guidance

Patch confidence: 0/4 (UNPATCHED — no patch evidence from any tracked source). No affected-version range and no vendor fix were provided.

1. Immediate mitigations

• Restrict network reachability: Do not expose the gateway's management interface to the internet. Place the management plane behind a firewall and permit only trusted administrative hosts.
• Apply least-privilege network segmentation so that unauthenticated or untrusted network segments cannot reach the gateway's exposed services.
• Enable and monitor gateway logging/syslog forwarding to a SIEM so crash/restart events are correlated and alerted.
• Maintain high availability or failover paths for critical connectivity so a single device DoS does not cause a full outage.
• Rate-limit and filter anomalous inbound traffic at an upstream firewall/IPS where feasible.

2. Patch/upgrade steps

• No patch is available at this time. Monitor the vendor (Ubiquiti/UniFi) security advisory channels and the NVD entry for CVE-2026-77558 for an updated firmware release.
• Once a fixed firmware is published, stage and test it, then upgrade all in-scope gateways. Document the target firmware version against this CVE.
• Verify affected model coverage from the vendor advisory before assuming a given device is or is not in scope.

3. Long-term hardening

• Enforce a firmware patch-management cadence for all edge gateways, including automated inventory of current versions.
• Default-deny inbound access to device management planes; require VPN or bastion access for administration.
• Deploy network IDS/IPS with vendor-relevant signatures and review custom heuristics periodically.
• Track EPSS and CISA KEV status for this CVE; EPSS is not yet scored, and it is not currently in KEV — status may change.

References

• NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-77558
• CWE-125 — Out-of-bounds Read: https://cwe.mitre.org/data/definitions/125.html
• FIRST EPSS (not yet scored): https://api.first.org/data/v1/epss?cve=CVE-2026-77558
• CISA Known Exploited Vulnerabilities catalog (not listed at time of writing): https://www.cisa.gov/known-exploited-vulnerabilities-catalog
• Vendor advisory: not available — no UniFi/Ubiquiti advisory for CVE-2026-77558 was provided.

No public exploit code, PoC, Nuclei template, or in-the-wild campaign evidence is associated with CVE-2026-77558 in the supplied data.