BIG-IP: Loop with Unreachable Exit Condition
CVE-2026-42920 is a high-severity loop with unreachable exit condition 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, 17.5.1.4, 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 0.26% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update BIG-IP to 21.0.0.1 or 17.5.1.4 or 17.1.3.1, whichever matches your release line, or a later version.
- Read the advisory on my.f5.com for workarounds and exact fixed builds.
- Until you can update, restrict network access to the affected service so it is not reachable from untrusted networks.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| F5 | BIG-IP | 21.0.0 up to, but not including, 21.0.0.1; 17.5.0 up to, but not including, 17.5.1.4; 17.1.0 up to, but not including, 17.1.3.1; 16.1.0 and later | 21.0.0.1, 17.5.1.4, 17.1.3.1 |
| F5 | Big-ip Access Policy Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Advanced Firewall Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Advanced Web Application Firewall | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Analytics | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Application Acceleration Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Application Security Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Application Visibility And Reporting | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Automation Toolchain | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Carrier-grade Nat | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Container Ingress Services | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Ddos Hybrid Defender | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Domain Name System | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Edge Gateway | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Fraud Protection Service | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Global Traffic Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Link Controller | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Local Traffic Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Policy Enforcement Manager | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip SSL Orchestrator | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Webaccelerator | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
| F5 | Big-ip Websafe | 21.0.0; 17.1.0 through 17.1.3; 17.5.0 through 17.5.1; 16.1.0 through 16.1.6 | Not stated |
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
- 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 system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 4.0 | 8.7 | High | CNA (f5.com) |
| CVSS 3.1 | 7.5 | High | CNA (f5.com) |
Official description
When a Client SSL profile is configured with Allow Dynamic Record Sizing on a UDP virtual server, 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: Loop with Unreachable Exit Condition
The product contains an iteration or loop with an exit condition that cannot be reached, i.e., an infinite loop.
More Loop with Unreachable Exit Condition vulnerabilities (CWE-835)
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: yes, technical impact: partial).
- Record last changed materially (score, affected products, status or references).
References
Advisories
- my.f5.com/manage/s/article/K000160901Mitigation
Other vulnerabilities in BIG-IP
- 9.8 Critical severity
F5 BIG-IP APM Heap-based Buffer Overflow Exploited
- 8.8 High severity
- 7.5 High severity
- 8.7 High severity
- 8.7 High severity
- 7.5 High severity
Recent Loop with Unreachable Exit Condition vulnerabilities
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 9.4 Critical severity
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