CyberSecBulletin
CVE-2026-21639NVD status: Modified

airMAX AC: Buffer Copy without Checking Size of Input

Published Updated By the CyberSecBulletin desk

CVE-2026-21639 is a high-severity buffer copy without checking size of input vulnerability in airMAX AC from Ubiquiti, with a CVSS 3.1 base score of 8.8 out of 10. It can be exploited from the same local network by an unauthenticated attacker, without any action from a user. The record lists version 8.7.21 as the first without the flaw, so updating airMAX AC to that release or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.39% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update airMAX AC to 8.7.21 or a later version.
  2. Update airMAX M to 6.3.24 or a later version.
  3. Update airFiber AF60-XG to 1.2.3 or a later version.
  4. Read the advisory on community.ui.com for workarounds and exact fixed builds.

Affected products and versions

VendorProductAffected versionsFixed in
UbiquitiairMAX ACbefore 8.7.218.7.21
UbiquitiairMAX Mbefore 6.3.246.3.24
UbiquitiairFiber AF60-XGbefore 1.2.31.2.3
UbiquitiairFiber AF60before 2.6.82.6.8
UIAirmax Ac Firmwarebefore 8.7.218.7.21
UIAirmax M Firmwarebefore 6.3.246.3.24
UIAirfiber Af60-xg Firmwarebefore 1.2.31.2.3
UIAirfiber Af60 Firmwarebefore 2.6.82.6.8

Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
AdjacentThe attacker must be on the same local network segment.
Attack complexity
LowNo special conditions are needed; the attack works reliably.
Privileges required
NoneNo account or login is needed.
User interaction
NoneNo action by a victim is required.
Scope
UnchangedImpact stays within the vulnerable component.
Confidentiality
HighAll data handled by the component can be read.
Integrity
HighData or code can be modified at will.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H. Scored by CNA (hackerone.com).

Scoring systemScoreSeverityScored by
CVSS 3.18.8HighCNA (hackerone.com)
CVSS 3.15.4MediumCISA (ADP)

Official description

A malicious actor in Wi-Fi range of the affected product could leverage a vulnerability in the airMAX Wireless Protocol to achieve a remote code execution (RCE) within the affected product.

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Buffer Copy without Checking Size of Input

The product copies an input buffer to an output buffer without verifying that the size of the input buffer is less than the size of the output buffer.

How developers prevent this class of flaw

  • Use a language that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. For example, many languages that perform their own memory management, such as Java and Perl, are not subject to buffer overflows. Other languages, such as Ada and C#, typically provide overflow protection, but the protection can be disabled by the programmer. Be wary that a language's interface to native code may still be subject to overflows, even if the language itself is theoretically safe.
  • Use a vetted library or framework that does not allow this weakness to occur or provides constructs that make this weakness easier to avoid. Examples include the Safe C String Library (SafeStr) by Messier and Viega, and the Strsafe.h library from Microsoft. These libraries provide safer versions of overflow-prone string-handling functions.
  • Use automatic buffer overflow detection mechanisms that are offered by certain compilers or compiler extensions. Examples include: the Microsoft Visual Studio /GS flag, Fedora/Red Hat FORTIFY_SOURCE GCC flag, StackGuard, and ProPolice, which provide various mechanisms including canary-based detection and range/index checking. D3-SFCV (Stack Frame Canary Validation) from D3FEND discusses canary-based detection in detail.

More Buffer Copy without Checking Size of Input vulnerabilities (CWE-120)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Recent Buffer Copy without Checking Size of Input vulnerabilities

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