CyberSecBulletin
CVE-2026-40618NVD status: Analyzed

BIG-IP: Incorrect Calculation of Buffer Size

Published Updated By the CyberSecBulletin desk

CVE-2026-40618 is a high-severity incorrect calculation of buffer size vulnerability in BIG-IP from F5, with a CVSS 3.1 base score of 7.5 out of 10. It can be exploited remotely over the network by an unauthenticated attacker, without any action from a user. The record lists versions 21.0.0.1, 17.1.5.4, 17.1.3.1 as the first without the flaw, so updating BIG-IP to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.32% 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 BIG-IP to 21.0.0.1 or 17.1.5.4 or 17.1.3.1, whichever matches your release line, or a later version.
  2. Update BIG-IP Next for Kubernetes to 2.2.0 or a later version.
  3. Read the advisory on my.f5.com for workarounds and exact fixed builds.
  4. Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.

Affected products and versions

VendorProductAffected versionsFixed in
F5BIG-IP21.0.0 up to, but not including, 21.0.0.1; 17.5.0 up to, but not including, 17.1.5.4; 17.1.0 up to, but not including, 17.1.3.1; 16.1.0 and later21.0.0.1, 17.1.5.4, 17.1.3.1
F5BIG-IP Next SPK2.0.0 and later; 1.7.0 and laterNot stated
F5BIG-IP Next CNF2.0.0 and later; 1.1.0 and laterNot stated
F5BIG-IP Next for Kubernetes2.0.0 up to, but not including, 2.2.02.2.0
F5Big-ip Access Policy Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Advanced Firewall Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Advanced Web Application Firewall17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Analytics17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Application Acceleration Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Application Security Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Application Visibility And Reporting17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Automation Toolchain17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Carrier-grade Nat17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Container Ingress Services17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Ddos Hybrid Defender17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Domain Name System17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Edge Gateway17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Fraud Protection Service17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Global Traffic Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Link Controller17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Local Traffic Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Policy Enforcement Manager17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip SSL Orchestrator17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Webaccelerator17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated
F5Big-ip Websafe17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6; 21.0.0Not stated

Version data comes from the organisation that assigned the CVE; 2 further products are omitted here. Confirm against the vendor advisory before deciding you are unaffected.

How the severity score breaks down

Attack vector
NetworkCan be exploited remotely over a network, including the internet.
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
NoneNo data is disclosed.
Integrity
NoneNothing can be modified.
Availability
HighThe component can be taken down completely.

Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H. Scored by CNA (f5.com).

Scoring systemScoreSeverityScored by
CVSS 4.08.7HighCNA (f5.com)
CVSS 3.17.5HighCNA (f5.com)

Official description

When an SSL profile is configured on a virtual server on BIG-IP Virtual Edition (VE) without Intel QuickAssist Technology (QAT) or on BIG-IP hardware platforms with the database variable crypto.hwacceleration set to disabled, undisclosed traffic can cause the Traffic Management Microkernel (TMM) to terminate. Note: Software versions which have reached End of Technical Support (EoTS) are not evaluated.

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

The underlying weakness: Incorrect Calculation of Buffer Size

The product does not correctly calculate the size to be used when allocating a buffer.

How developers prevent this class of flaw

  • When allocating a buffer for the purpose of transforming, converting, or encoding an input, allocate enough memory to handle the largest possible encoding. For example, in a routine that converts & characters to & for HTML entity encoding, the output buffer needs to be at least 5 times as large as the input buffer.
  • Understand the programming language's underlying representation and how it interacts with numeric calculation (CWE-681). Pay close attention to byte size discrepancies, precision, signed/unsigned distinctions, truncation, conversion and casting between types, not-a-number calculations, and how the language handles numbers that are too large or too small for its underlying representation. Also be careful to account for 32-bit, 64-bit, and other potential differences that may affect the numeric representation.
  • Perform input validation on any numeric input by ensuring that it is within the expected range. Enforce that the input meets both the minimum and maximum requirements for the expected range.

More Incorrect Calculation of Buffer Size vulnerabilities (CWE-131)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
  3. Record last changed materially (score, affected products, status or references).

References

Other vulnerabilities in BIG-IP

All BIG-IP vulnerabilities

Recent Incorrect Calculation of Buffer Size vulnerabilities

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