BugZero | F5 BugID 521556 - Assertion "valid pcb" in TCP4 with ICAP adaptation

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

Last updated on July 18th, 2026

BugZero Risk Score
8.5 High

Overall: 8.5

Severity: 9.1

Community: 4.2

Lifecycle: 7.8

What is the BugZero Risk Score?

F5 Integration

Learn more about where this data comes from

F5 Integration

Learn more

BugZero Plan

Streamline upgrades with automated vendor bug scrubs

BugZero Plan

Learn more

BugZero Prevent

Wish you caught this bug sooner? Get proactive today.

BugZero Prevent

Learn more

Bug Details

Priority: 2-Critical
Status: Not Verified
Impact Category: Service Provider

Description

Symptoms

TMM crashes with assertion "valid pcb" in tcp4.c

Impact

Intermittent crash under load. Traffic disrupted while tmm restarts.

Conditions

Virtual server with request-adapt or response-adapt profile. Congested client or TCP small window (flow-control is active). Multiple HTTP requests in a single client connection. More likely with iRules that park.

Workaround

None

Fix Information

Assertion "valid pcb" does not occur.

Change history

Links

Original Vendor Announcement

Relevant Products

Click on a version to see all relevant bugs

Affected versions:11.5.1, 11.5.1 hf1, 11.5.1 hf2, 11.5.1 hf3, 11.5.1 hf4, 11.5.1 hf5, 11.5.1 hf6, 11.5.1 hf7, 11.5.1 hf8, 11.5.1 hf9, 11.5.1 hf10, 11.5.1 hf11, 11.5.2, 11.5.2 hf1, 11.5.3, 11.5.3 hf1, 11.5.3 hf2, 11.5.4, 11.5.4 hf1, 11.5.4 hf2, 11.5.4 hf3, 11.5.4 hf4, 11.5.5, 11.5.6, 11.5.7, 11.5.8, 11.5.9, 11.5.10, 11.6.0, 11.6.0 hf1, 11.6.0 hf2, 11.6.0 hf3, 11.6.0 hf4, 11.6.0 hf5, 11.6.0 hf6, 11.6.0 hf7, 11.6.0 hf8, 11.6.1, 11.6.1 hf1, 11.6.1 hf2, 11.6.2, 11.6.2 hf1, 11.6.3, 11.6.3.1, 11.6.3.2, 11.6.3.3, 11.6.3.4, 11.6.4, 11.6.5, 11.6.5.1, 11.6.5.2, 11.6.5.3

Fixed versions: 12.0.0

Relevant Products

Click on a version to see all relevant bugs

Fixed versions: 12.0.0

Top F5 Defects

9.6Defect ID: 546260
TMM can crash if using the v6rd profile
9.6Defect ID: 934133
Unable to Create/delete ltm virtual server via CLI transaction when destination is not specified on the ltm virtual object
9.6Defect ID: 1154417
Profile based attack is not mitigated at hardware on L2 wire setup
9.6Defect ID: 913713
Rebooting a blade causes MCPd to core as it rejoins the cluster
9.6Defect ID: 2141125
Multicast traffic is dropped with incorrect VLAN tagging

F5 Integration

Learn more about where this data comes from

F5 Integration

Learn more

BugZero Plan

Streamline upgrades with automated vendor bug scrubs

BugZero Plan

Learn more

BugZero Prevent

Wish you caught this bug sooner? Get proactive today.

BugZero Prevent

Learn more

Ready to prevent the next vendor outage?

Get a demo

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

Last updated on July 18th, 2026

BugZero Risk Score8.5 High

Bug Details

Symptoms

Impact

Conditions

Workaround

Fix Information

Links

Top F5 Defects

Ready to prevent the next vendor outage?

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

F5 - Defect ID: 521556

Assertion "valid pcb" in TCP4 with ICAP adaptation

Last updated on July 18th, 2026

BugZero Risk Score8.5 High

Bug Details

Symptoms

Impact

Conditions

Workaround

Fix Information

Links

Top F5 Defects

Ready to prevent the next vendor outage?

BugZero Risk Score
8.5 High

BugZero Risk Score
8.5 High