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CVE-2026-63968
ipv6: fix possible infinite loop in fib6_select_path()
Published 2026-07-19 (3 months ago) by the Linux kernel CNA. Subsystem net/ipv6. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-63968 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixed | 6.1.176-1 | Update the kernel and reboot. Fixed in 6.1.176-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixed | 6.12.94-1 | Update the kernel and reboot. Fixed in 6.12.94-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.0.12-1 | Update the kernel and reboot. Fixed in 7.0.12-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
Found while auditing the same pattern Sashiko reported in rt6_fill_node() [1]. Apply the same fix as commit f8d8ce1b515a ("ipv6: fix possible infinite loop in fib6_info_uses_dev()"). Writers holding tb6_lock can list_del_rcu(&first->fib6_siblings) without waiting for RCU readers; first->fib6_siblings.next then still points into the old ring and this softirq-side walker never reaches &first->fib6_siblings as its terminator. fib6_purge_rt() always WRITE_ONCE()s first->fib6_nsiblings to 0 before list_del_rcu(), so an inside-loop check is a reliable detach signal. [1] https://sashiko.dev/#/patchset/20260526020227.4857-1-jiayuan.chen%40linux.dev
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/ipv6/route.c
- What this is
- NETWORKING [IPv4/IPv6]MAINTAINERS section matched on net/ipv6/route.c · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_IPV6 /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sidy
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_IPV6
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-07-19 (3 months ago)source
- CVSS
- 7.5 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.66%, higher than 50.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv6
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 6.1.128 | 6.1.176 | 3948a7d92f76 |
| 6.6.75 | 6.6.143 | c87cd3cb3096 |
| 6.12.2 | 6.12.93 | 0f7b73c34526 |
| 6.13 | 6.18.35 | ab9a10969a90 |
| 6.13 | 7.0.12 | 9b9d5bd6e3d4 |
| 6.13 | 7.1 | 9c7da87c2dc8 |
| 6.11.11 | 0 | 0 |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The vulnerable function is reached from the IPv6 packet receive path (ipv6_rcv → ip6_route_input → ip6_pol_route → fib6_select_path) in softirq context, and production lockups were observed during processing of incoming IPv6 traffic on internet-facing edge routers.
- AC:LAttack complexity, low
- The race is reliably triggerable when an attacker influences both packet flow and route churn (e.g., user-namespace ECMP setup plus traffic, or remote traffic coinciding with BGP nexthop updates); a dedicated selftest reproduces soft lockups in under a minute under controlled conditions.
- PR:NPrivileges required, none
- Triggering fib6_select_path() during packet processing requires only the ability to send IPv6 traffic to destinations using multipath routes; concurrent route-table updates can occur from infrastructure (BGP/dynamic routing) without the attacker holding any local privileges.
- UI:NUser interaction, none
- Exploitation requires no victim interaction beyond normal network traffic delivery to a host or router processing IPv6 packets.
- S:UScope, unchanged
- The impact is kernel CPU soft lockup and watchdog panic within the same kernel security boundary; there is no sandbox, VM, or IOMMU boundary crossing.
- C:NConfidentiality impact, none
- The bug is an infinite loop over stale sibling-list pointers during RCU read-side traversal; it does not read or disclose attacker-controlled memory or kernel secrets.
- I:NIntegrity impact, none
- No data modification or memory corruption occurs; the failure mode is CPU hang from unbounded list iteration, not arbitrary write or code execution.
- A:HAvailability impact, high
- The infinite loop in softirq context causes CPU soft lockup, and documented production and test cases show this leads to watchdog-triggered kernel panic, a complete loss of system availability.