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CVE Record

CVE-2023-53821: ip6_vti: fix slab-use-after-free in decode_session6

In the Linux kernel, the following vulnerability has been resolved: ip6_vti: fix slab-use-after-free in decode_session6 When ipv6_vti device is set to the qdisc of the sfb type, the cb field of the sent skb may be modified during enqueuing. Then, slab-use-after-free may occur when ipv6_vti device sends IPv6 packets. The stack information is as follows: BUG: KASAN: slab-use-after-free in decode_session6+0x103f/0x1890 Read of size 1 at addr ffff88802e08edc2 by task swapper/0/0 CPU: 0 PID: 0 Comm: swapper/0 Not tainted 6.4.0-next-20230707-00001-g84e2cad7f979 #410 Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS 1.14.0-1.fc33 04/01/2014 Call Trace: <IRQ> dump_stack_lvl+0xd9/0x150 print_address_description.constprop.0+0x2c/0x3c0 kasan_report+0x11d/0x130 decode_session6+0x103f/0x1890 __xfrm_decode_session+0x54/0xb0 vti6_tnl_xmit+0x3e6/0x1ee0 dev_hard_start_xmit+0x187/0x700 sch_direct_xmit+0x1a3/0xc30 __qdisc_run+0x510/0x17a0 __dev_queue_xmit+0x2215/0x3b10 neigh_connected_output+0x3c2/0x550 ip6_finish_output2+0x55a/0x1550 ip6_finish_output+0x6b9/0x1270 ip6_output+0x1f1/0x540 ndisc_send_skb+0xa63/0x1890 ndisc_send_rs+0x132/0x6f0 addrconf_rs_timer+0x3f1/0x870 call_timer_fn+0x1a0/0x580 expire_timers+0x29b/0x4b0 run_timer_softirq+0x326/0x910 __do_softirq+0x1d4/0x905 irq_exit_rcu+0xb7/0x120 sysvec_apic_timer_interrupt+0x97/0xc0 </IRQ> Allocated by task 9176: kasan_save_stack+0x22/0x40 kasan_set_track+0x25/0x30 __kasan_slab_alloc+0x7f/0x90 kmem_cache_alloc_node+0x1cd/0x410 kmalloc_reserve+0x165/0x270 __alloc_skb+0x129/0x330 netlink_sendmsg+0x9b1/0xe30 sock_sendmsg+0xde/0x190 ____sys_sendmsg+0x739/0x920 ___sys_sendmsg+0x110/0x1b0 __sys_sendmsg+0xf7/0x1c0 do_syscall_64+0x39/0xb0 entry_SYSCALL_64_after_hwframe+0x63/0xcd Freed by task 9176: kasan_save_stack+0x22/0x40 kasan_set_track+0x25/0x30 kasan_save_free_info+0x2b/0x40 ____kasan_slab_free+0x160/0x1c0 slab_free_freelist_hook+0x11b/0x220 kmem_cache_free+0xf0/0x490 skb_free_head+0x17f/0x1b0 skb_release_data+0x59c/0x850 consume_skb+0xd2/0x170 netlink_unicast+0x54f/0x7f0 netlink_sendmsg+0x926/0xe30 sock_sendmsg+0xde/0x190 ____sys_sendmsg+0x739/0x920 ___sys_sendmsg+0x110/0x1b0 __sys_sendmsg+0xf7/0x1c0 do_syscall_64+0x39/0xb0 entry_SYSCALL_64_after_hwframe+0x63/0xcd The buggy address belongs to the object at ffff88802e08ed00 which belongs to the cache skbuff_small_head of size 640 The buggy address is located 194 bytes inside of freed 640-byte region [ffff88802e08ed00, ffff88802e08ef80) As commit f855691975bb ("xfrm6: Fix the nexthdr offset in _decode_session6.") showed, xfrm_decode_session was originally intended only for the receive path. IP6CB(skb)->nhoff is not set during transmission. Therefore, set the cb field in the skb to 0 before sending packets.

UnknownCVSS not scoredNot KEV-listedUpdated
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Glexia's TakeAutomated analysismoderate

Security readout for executives and security teams

Plain-English summary

This is a Linux kernel memory safety bug in IPv6 virtual tunnel handling. It can occur when an ipv6_vti device uses the sfb queue discipline and sends IPv6 packets. The public record does not provide a CVSS score or confirmed real-world exploitation.

Executive priority

Treat this as a targeted Linux kernel patching item, not an emergency internet-wide issue. Prioritize network infrastructure, VPN, tunnel, and security appliances that may use IPv6 VTI. Escalate if those systems match the described configuration.

Technical view

The bug is a slab-use-after-free in decode_session6 during vti6_tnl_xmit. The source says sfb enqueueing may modify skb control-buffer data, while transmit-side IPv6 VTI calls xfrm_decode_session without IP6CB(skb)->nhoff being set. The fix clears the skb cb field before sending.

Likely exposure

Exposure is likely limited to Linux systems using IPv6 VTI tunnel devices with sfb qdisc. Generic Linux hosts without that networking configuration are less likely to be exposed based on the provided description.

Exploitation context

The bundle states KEV is false and provides no evidence of active exploitation or public exploit use. It documents a KASAN-detected kernel use-after-free in a specific transmit path, but not practical exploitability or attacker prerequisites.

Researcher notes

The record lacks CVSS, CWE, and exploitability detail. Analysis should focus on the ipv6_vti transmit path, sfb enqueue behavior, skb cb mutation, and the fix that zeroes skb control-buffer state before packet transmission.

Mitigation direction

  • Apply vendor kernel updates containing the referenced stable fixes.
  • Prioritize systems using IPv6 VTI tunnels and non-default qdisc configurations.
  • Avoid ipv6_vti plus sfb qdisc where operationally feasible until patched.
  • Check Linux distribution advisories for backported fixed kernel packages.
  • Reboot into the fixed kernel after patch installation.

Validation and detection

  • Inventory Linux kernel versions across exposed hosts.
  • Identify systems using ipv6_vti tunnel interfaces.
  • Check whether affected systems configure sfb qdisc on those interfaces.
  • Confirm installed kernels include the relevant stable fix or distro backport.
  • Monitor kernel logs for related KASAN or networking crash reports.
Prepared
Confidence
medium
Sources
10

Public sources used

Generated from the cited source records. This long-tail analysis has not been individually reviewed by a named human.

Potential ATT&CK relevance

Conservative CVE-to-ATT&CK context

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Vulnerability profileCVE Program record
Severity
Unknown
CVSS
Not scored
Known Exploited
No
Published
Official CVE source material

CNA and ADP enrichment extracted from CVE v5

These fields come from the CVE record and ADP containers, not from Glexia's Take. They preserve time-varying source decisions such as CISA SSVC, KEV status, CVSS metrics, and provider references.

0CVSS vectors
3Timeline events
0ADP providers
9Source links

Vulnerability timeline

Timeline events are normalized from CVE metadata, CNA source timelines, ADP timelines, and KEV metadata when present.

  1. CVE reservedCVE Program

    The CVE ID was reserved by the assigning CNA.

  2. CVE publishedCVE Program

    The CVE record was published.

  3. CVE updatedCVE Program

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Source materials

Affected products

Products and packages named in the record

VendorProductVersion / packageStatus
LinuxLinuxf855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536, f855691975bb06373a98711e4cfe2c224244b536unaffected
LinuxLinux3.19, 0, 4.14.324, 4.19.293, 5.4.255, 5.10.192, 5.15.128, 6.1.47, 6.4.12, 6.5affected
Weakness

CWE details

No CWE listed

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