# CVE-2026-45913: net: bridge: mcast: always update mdb_n_entries for vlan contexts

> A logic error in the Linux kernel bridge multicast code allows triggering a kernel warning and potential denial of service via specific VLAN and multicast snooping configurations.

- **Severity:** Medium 5.5 (`CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Any signed-in user
- **Attackable:** Local or same network only
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 27 May 2026
- **Updated:** 27 May 2026
- **Also known as:** BELL-CVE-2026-45913, DEBIAN-CVE-2026-45913, GHSA-wpm7-9hcr-ppjf, ROOT-OS-UBUNTU-2404-CVE-2026-45913, UBUNTU-CVE-2026-45913

## What is CVE-2026-45913?

CVE-2026-45913 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.

### Official description

In the Linux kernel, the following vulnerability has been resolved:

net: bridge: mcast: always update mdb_n_entries for vlan contexts

syzbot triggered a warning[1] about the number of mdb entries in a context.
It turned out that there are multiple ways to trigger that warning today
(some got added during the years), the root cause of the problem is that
the increase is done conditionally, and over the years these different
conditions increased so there were new ways to trigger the warning, that is
to do a decrease which wasn't paired with a previous increase.

For example one way to trigger it is with flush:
 $ ip l add br0 up type bridge vlan_filtering 1 mcast_snooping 1
 $ ip l add dumdum up master br0 type dummy
 $ bridge mdb add dev br0 port dumdum grp 239.0.0.1 permanent vid 1
 $ ip link set dev br0 down
 $ ip link set dev br0 type bridge mcast_vlan_snooping 1
   ^^^^ this will enable snooping, but will not update mdb_n_entries
        because in __br_multicast_enable_port_ctx() we check !netif_running
 $ bridge mdb flush dev br0
   ^^^ this will trigger the warning because it will delete the pg which
       we added above, which will try to decrease mdb_n_entries

Fix the problem by removing the conditional increase and always keep the
count up-to-date while the vlan exists. In order to do that we have to
first initialize it on port-vlan context creation, and then always increase
or decrease the value regardless of mcast options. To keep the current
behaviour we have to enforce the mdb limit only if the context is port's or
if the port-vlan's mcast snooping is enabled.

[1]
 ------------[ cut here ]------------
 n == 0
 WARNING: net/bridge/br_multicast.c:718 at br_multicast_port_ngroups_dec_one net/bridge/br_multicast.c:718 [inline], CPU#0: syz.4.4607/22043
 WARNING: net/bridge/br_multicast.c:718 at br_multicast_port_ngroups_dec net/bridge/br_multicast.c:771 [inline], CPU#0: syz.4.4607/22043
 WARNING: net/bridge/br_multicast.c:718 at br_multicast_del_pg+0x1bbe/0x1e20 net/bridge/br_multicast.c:825, CPU#0: syz.4.4607/22043
 Modules linked in:
 CPU: 0 UID: 0 PID: 22043 Comm: syz.4.4607 Not tainted syzkaller #0 PREEMPT(full)
 Hardware name: Google Google Compute Engine/Google Compute Engine, BIOS Google 01/24/2026
 RIP: 0010:br_multicast_port_ngroups_dec_one net/bridge/br_multicast.c:718 [inline]
 RIP: 0010:br_multicast_port_ngroups_dec net/bridge/br_multicast.c:771 [inline]
 RIP: 0010:br_multicast_del_pg+0x1bbe/0x1e20 net/bridge/br_multicast.c:825
 Code: 41 5f 5d e9 04 7a 48 f7 e8 3f 73 5c f7 90 0f 0b 90 e9 cf fd ff ff e8 31 73 5c f7 90 0f 0b 90 e9 16 fd ff ff e8 23 73 5c f7 90 <0f> 0b 90 e9 60 fd ff ff e8 15 73 5c f7 eb 05 e8 0e 73 5c f7 48 8b
 RSP: 0018:ffffc9000c207220 EFLAGS: 00010293
 RAX: ffffffff8a68042d RBX: ffff88807c6f1800 RCX: ffff888066e90000
 RDX: 0000000000000000 RSI: 0000000000000000 RDI: 0000000000000000
 RBP: 0000000000000000 R08: ffff888066e90000 R09: 000000000000000c
 R10: 000000000000000c R11: 0000000000000000 R12: ffff8880303ef800
 R13: dffffc0000000000 R14: ffff888050eb11c4 R15: 1ffff1100a1d6238
 FS:  00007fa45921b6c0(0000) GS:ffff8881256f5000(0000) knlGS:0000000000000000
 CS:  0010 DS: 0000 ES: 0000 CR0: 0000000080050033
 CR2: 00007fa4591f9ff8 CR3: 0000000081df2000 CR4: 00000000003526f0
 Call Trace:
  <TASK>
  br_mdb_flush_pgs net/bridge/br_mdb.c:1525 [inline]
  br_mdb_flush net/bridge/br_mdb.c:1544 [inline]
  br_mdb_del_bulk+0x5e2/0xb20 net/bridge/br_mdb.c:1561
  rtnl_mdb_del+0x48a/0x640 net/core/rtnetlink.c:-1
  rtnetlink_rcv_msg+0x77e/0xbe0 net/core/rtnetlink.c:6967
  netlink_rcv_skb+0x232/0x4b0 net/netlink/af_netlink.c:2550
  netlink_unicast_kernel net/netlink/af_netlink.c:1318 [inline]
  netlink_unicast+0x80f/0x9b0 net/netlink/af_netlink.c:1344
  netlink_sendmsg+0x813/0xb40 net/netlink/af_netlink.c:1894
  sock_sendmsg_nosec net/socket.c:727 [inline]
  __sock_sendmsg net/socket.c:742 [inline]
  ____sys_sendmsg+0xa68/0xad0 net/socket.c:2592
  ___sys_sendmsg+0x2a5/0x360 net/socke
---truncated---

## Which versions are affected by CVE-2026-45913?

It affects 1 product.


Products: Linux kernel.

## How do I fix CVE-2026-45913?

Apply the vendor's patch.

## Is CVE-2026-45913 being exploited?

There is no known exploitation of CVE-2026-45913 in the wild and no public exploit code.

## Other vulnerabilities in Linux kernel

- [CVE-2026-53266](https://insights.sbomify.com/CVE-2026-53266/): Linux Kernel Out-of-Bounds Write (high 8.8, exploited in the wild)
- [CVE-2013-6282](https://insights.sbomify.com/CVE-2013-6282/): Linux Kernel Improper Input Validation (high 8.8, exploited in the wild)
- [CVE-2022-0185](https://insights.sbomify.com/CVE-2022-0185/): Linux Kernel Heap-Based Buffer Overflow (high 8.4, exploited in the wild)
- [CVE-2013-2094](https://insights.sbomify.com/CVE-2013-2094/): Linux Kernel Privilege Escalation (high 8.4, exploited in the wild)
- [CVE-2026-53362](https://insights.sbomify.com/CVE-2026-53362/): Linux Kernel Unspecified (high 7.8, exploited in the wild)

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-45913 and fix it if it is affected.

What it is: CVE-2026-45913 is a medium-severity denial of service vulnerability in Linux kernel, published on 27 May 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Exploitation: There is no known exploitation of CVE-2026-45913 in the wild and no public exploit code.

Affected product: Linux kernel. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-45913/ (Markdown: https://insights.sbomify.com/CVE-2026-45913/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-45913
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-45913
- OSV: https://osv.dev/vulnerability/CVE-2026-45913

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-45913/
Summaries are generated and may contain mistakes; the official description above is the record.