- by x32x01 ||
If your Mac suddenly restarts and the panic log contains:
the problem is likely happening inside macOS's network stack, not because your Mac is simply "running out of memory."
I spent a lot of time tracing this kind of crash through panic logs, kernel code, disassembly, and repeated crash reports. The interesting part is that
There is an important update to the original investigation, though: this panic signature is not unique to one Mac. Other reports from 2026 show the same
The kernel uses data structures called
Think of it like a train:
Each train car contains part of the data, and the cars are connected together to form the complete packet.
When the kernel wants to copy part of that packet,
The panic happens when the kernel discovers that the requested copy would go beyond the data that actually exists in the chain.
The XNU source contains
So:
copy overflow does not simply mean "the Mac has too much data."
It means that the kernel encountered an inconsistency between the amount of data it believes should be available and the amount of data actually present in the mbuf chain.
A kernel panic is a protective failure mechanism. Once the kernel detects an internal consistency or bounds violation that it cannot safely recover from, continuing execution could make the situation worse.
In this case, the kernel detects that the requested copy would exceed the available mbuf data and triggers the panic rather than blindly continuing.
That is why an automatic restart does not necessarily mean that your Mac has a failing SSD, bad RAM, or damaged hardware.
A recurring panic with the same kernel function and the same stack can instead point toward a repeatable software path.
However, it would be incorrect to conclude from the panic alone that memory corruption would definitely have occurred without the panic. The panic proves that the kernel detected an invalid condition; it does not by itself prove what would have happened if that check were removed.
The
The networking stack has many paths where packets can be received, processed, transformed, routed, and transmitted.
Recent independent reports are especially interesting here.
One Tailscale report describes repeated crashes with the exact same:
signature. Another report involving AmneziaWG describes the same panic while large TCP traffic was flowing through a packet tunnel.
This suggests that the underlying problem may involve the interaction between XNU's mbuf/networking code and a particular packet-processing path.
It does not prove that Tailscale, AmneziaWG, or another network application is the root cause in every case.
The presence of
For example, one public report with the same panic signature describes the crash inside the outbound TCP path, while another describes the panic during traffic passing through a packet tunnel.
So the safer conclusion is:
The crash is strongly associated with network packet processing, but the exact trigger depends on the surrounding call stack and network configuration.
If you want to determine whether your particular crash happens during upload, download, VPN traffic, local routing, or another network operation, you need to examine the complete panic stack rather than relying on
The name
A message indicating that the calculated value cannot be negative means the kernel encountered an unexpected state while calculating that TCP state.
By itself, this message does not prove that it caused the panic.
But if you repeatedly see it before crashes, and the messages consistently point to the same networking code path, it becomes useful evidence when investigating a recurring kernel problem.
The important distinction is:
A correlated log message is evidence of a suspicious condition, not automatically proof of the root cause.
Compare the complete reports.
Look for:
panic and the same relative call-chain structure, that is much stronger evidence of a repeatable software path than simply seeing the word "network" in both reports.
Recent public reports have used this exact type of comparison and found highly similar crash paths across multiple occurrences.
This counts matching log entries from the last hour.
For example, if the command returns a large number, you can investigate the actual messages instead of looking only at the count:
Keep in mind that the number of messages alone does not establish that your Mac has the same bug.
A count of zero does not prove that your system is unaffected either. Logging behavior can vary between macOS versions, configurations, and circumstances.
For example, reports from 2026 include macOS 26.5.x systems using Tailscale and AmneziaWG, as well as other network-heavy configurations.
This is important because it shows that the problem should not automatically be dismissed as a single machine's hardware failure.
At the same time, the available reports do not establish one universal trigger.
Different machines may reach the same vulnerable kernel code through different network paths.
VPNs, packet tunnels, firewalls, network filters, and other Network Extension components can sit directly in network traffic paths.
Several recent reports with this exact panic signature involved network-tunnel software. One Tailscale report specifically describes the crash occurring with its macOS network extension active, while another report links the same signature to an AmneziaWG packet tunnel during large TCP traffic.
But there is an important difference between:
"The network extension was present when the panic happened."
and:
"The network extension caused the kernel bug."
The first can be established from a panic report. The second requires controlled reproduction or stronger evidence.
The goal is not simply to "try random fixes."
The goal is to determine whether changing one variable changes the crash behavior.
You can start with a single line:
and then work backward through:
That distinction matters.
If you are debugging your own Mac, the strongest evidence is a reproducible pattern across multiple panic reports, especially when the same kernel path, offsets, and surrounding network conditions repeat.
share the following information:
and report the number you get.
If several independent users report the same panic with similar conditions, that can provide much stronger evidence that the issue belongs to a repeatable software path rather than an isolated hardware failure.
The interesting part of debugging is often not the restart itself.
It's asking "why did the kernel decide it had to stop?" and then following the evidence until the answer starts to make sense.
m_copym_with_hdrs ... copy overflow @uipc_mbuf.c:3268the problem is likely happening inside macOS's network stack, not because your Mac is simply "running out of memory."
I spent a lot of time tracing this kind of crash through panic logs, kernel code, disassembly, and repeated crash reports. The interesting part is that
m_copym_with_hdrs is involved in copying data from an mbuf chain, and macOS can deliberately trigger a kernel panic when the lengths it expects do not match the data actually available.There is an important update to the original investigation, though: this panic signature is not unique to one Mac. Other reports from 2026 show the same
m_copym_with_hdrs ... copy overflow @uipc_mbuf.c:3268 signature under different network configurations, including Tailscale and AmneziaWG. That means it is safer to describe this as a macOS/XNU networking bug or kernel-stack failure that can potentially be triggered by different network paths, rather than assuming one specific application is always responsible.What does m_copym_with_hdrs copy overflow actually mean?
The easiest way to understand it is to think about how the macOS kernel stores network packets.The kernel uses data structures called
mbufs to hold network data. An mbuf can be part of a chain, with each element pointing to another part of the packet data.Think of it like a train:
Each train car contains part of the data, and the cars are connected together to form the complete packet.
When the kernel wants to copy part of that packet,
m_copym_with_hdrs calculates how much data it expects to copy and walks through the mbuf chain.The panic happens when the kernel discovers that the requested copy would go beyond the data that actually exists in the chain.
The XNU source contains
m_copym_with_hdrs specifically for copying mbuf data while also handling the required headers.So:
copy overflow does not simply mean "the Mac has too much data."
It means that the kernel encountered an inconsistency between the amount of data it believes should be available and the amount of data actually present in the mbuf chain.
Why does macOS restart instead of continuing?
This is the important part.A kernel panic is a protective failure mechanism. Once the kernel detects an internal consistency or bounds violation that it cannot safely recover from, continuing execution could make the situation worse.
In this case, the kernel detects that the requested copy would exceed the available mbuf data and triggers the panic rather than blindly continuing.
That is why an automatic restart does not necessarily mean that your Mac has a failing SSD, bad RAM, or damaged hardware.
A recurring panic with the same kernel function and the same stack can instead point toward a repeatable software path.
However, it would be incorrect to conclude from the panic alone that memory corruption would definitely have occurred without the panic. The panic proves that the kernel detected an invalid condition; it does not by itself prove what would have happened if that check were removed.
Why the network stack is an important clue
Another useful clue is a message such as:dlil_input_en0The
en0 interface is commonly associated with the primary network interface on a Mac, but seeing dlil_input_en0 in a panic log does not automatically prove that the crash was caused by downloading data or by a weak internet connection.The networking stack has many paths where packets can be received, processed, transformed, routed, and transmitted.
Recent independent reports are especially interesting here.
One Tailscale report describes repeated crashes with the exact same:
m_copym_with_hdrs ... copy overflow @uipc_mbuf.c:3268signature. Another report involving AmneziaWG describes the same panic while large TCP traffic was flowing through a packet tunnel.
This suggests that the underlying problem may involve the interaction between XNU's mbuf/networking code and a particular packet-processing path.
It does not prove that Tailscale, AmneziaWG, or another network application is the root cause in every case.
Is this caused by uploading or downloading?
This is where things get more complicated.The presence of
m_copym_with_hdrs in a TCP-related stack can provide clues about the path being processed, but the panic signature alone is not enough to confidently say: "This is definitely an upload problem."For example, one public report with the same panic signature describes the crash inside the outbound TCP path, while another describes the panic during traffic passing through a packet tunnel.
So the safer conclusion is:
The crash is strongly associated with network packet processing, but the exact trigger depends on the surrounding call stack and network configuration.
If you want to determine whether your particular crash happens during upload, download, VPN traffic, local routing, or another network operation, you need to examine the complete panic stack rather than relying on
dlil_input_en0 alone.What is tcp_flight_size and why is it interesting?
Another useful clue can be messages such as:tcp_flight_size ... can't be negativeThe name
tcp_flight_size relates to TCP's accounting for data that has been sent but has not yet been acknowledged.A message indicating that the calculated value cannot be negative means the kernel encountered an unexpected state while calculating that TCP state.
By itself, this message does not prove that it caused the panic.
But if you repeatedly see it before crashes, and the messages consistently point to the same networking code path, it becomes useful evidence when investigating a recurring kernel problem.
The important distinction is:
A correlated log message is evidence of a suspicious condition, not automatically proof of the root cause.
How I would verify whether multiple crashes have the same cause
If your Mac has restarted several times, don't just compare the first line of each panic report.Compare the complete reports.
Look for:
- The same panic string.
- The same kernel function.
- The same source-file location.
- The same calling functions.
- The same relative offsets after accounting for ASLR/KASLR.
- The same network interfaces or extensions.
- The same VPN, firewall, packet tunnel, or network-filtering software.
- Similar activity immediately before the panic.
m_copym_with_hdrspanic and the same relative call-chain structure, that is much stronger evidence of a repeatable software path than simply seeing the word "network" in both reports.
Recent public reports have used this exact type of comparison and found highly similar crash paths across multiple occurrences.
How to check your Mac for tcp_flight_size messages
If you want to investigate whether your Mac is producing these messages, you can search the unified log with: Bash:
/usr/bin/log show --last 1h --predicate 'eventMessage CONTAINS "flight_size"' | wc -lFor example, if the command returns a large number, you can investigate the actual messages instead of looking only at the count:
Bash:
/usr/bin/log show --last 1h --predicate 'eventMessage CONTAINS "flight_size"'A count of zero does not prove that your system is unaffected either. Logging behavior can vary between macOS versions, configurations, and circumstances.
What about macOS 26?
The samem_copym_with_hdrs panic signature has been publicly reported on macOS 26.x in multiple configurations.For example, reports from 2026 include macOS 26.5.x systems using Tailscale and AmneziaWG, as well as other network-heavy configurations.
This is important because it shows that the problem should not automatically be dismissed as a single machine's hardware failure.
At the same time, the available reports do not establish one universal trigger.
Different machines may reach the same vulnerable kernel code through different network paths.
Could a VPN or network extension be involved?
Yes, it is a reasonable thing to investigate.VPNs, packet tunnels, firewalls, network filters, and other Network Extension components can sit directly in network traffic paths.
Several recent reports with this exact panic signature involved network-tunnel software. One Tailscale report specifically describes the crash occurring with its macOS network extension active, while another report links the same signature to an AmneziaWG packet tunnel during large TCP traffic.
But there is an important difference between:
"The network extension was present when the panic happened."
and:
"The network extension caused the kernel bug."
The first can be established from a panic report. The second requires controlled reproduction or stronger evidence.
What should you do if your Mac keeps crashing?
If you are seeing this panic repeatedly, collect evidence before changing too many things.- Save several panic reports.
- Record your exact macOS version and build.
- Record the Mac model and chip.
- Note whether a VPN or network filter was active.
- Note whether the crash happened during heavy network traffic.
- Compare the panic strings and call stacks.
- Check whether the same network extension appears in every report.
- Install macOS updates when available.
- If the crash continues, submit the panic reports and a sysdiagnose to Apple.
The goal is not simply to "try random fixes."
The goal is to determine whether changing one variable changes the crash behavior.
What this investigation actually tells us
The most interesting lesson here is that a kernel panic can be investigated like any other software failure.You can start with a single line:
m_copym_with_hdrs ... copy overflow @uipc_mbuf.c:3268and then work backward through:
- Panic logs.
- Call stacks.
- Kernel symbols.
- Disassembly.
- Relative offsets.
- Network interfaces.
- TCP state.
- mbuf structures.
- Repeated log messages.
- Third-party network extensions.
That distinction matters.
If you are debugging your own Mac, the strongest evidence is a reproducible pattern across multiple panic reports, especially when the same kernel path, offsets, and surrounding network conditions repeat.
Have you seen the same macOS kernel panic?
If your Mac has produced:m_copym_with_hdrs ... copy overflow @uipc_mbuf.c:3268share the following information:
- Mac model.
- Apple Silicon or Intel.
- macOS version and build.
- Whether a VPN or network extension was active.
- What the Mac was doing immediately before the panic.
- Whether the panic happened during heavy upload or download traffic.
- Whether the same panic appeared more than once.
Bash:
/usr/bin/log show --last 1h --predicate 'eventMessage CONTAINS "flight_size"' | wc -lIf several independent users report the same panic with similar conditions, that can provide much stronger evidence that the issue belongs to a repeatable software path rather than an isolated hardware failure.
The interesting part of debugging is often not the restart itself.
It's asking "why did the kernel decide it had to stop?" and then following the evidence until the answer starts to make sense.