high CVSS 7.1 EPSS <0.1%

CVE-2021-46954·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: net/sched: sch_frag: fix stack OOB read while fragmenting IPv4 packets when 'act_mirred' tries to fragment IPv4 packets that had been previously re-assembled using 'act_ct', splats like the following can be observed on kernels built with KASAN: BUG: KASAN: stack-out-of-bounds in ip_do_fragment+0x1b03/0x1f60 Read of size 1 at addr ffff888147009574 by task ping/947 CPU: 0 PID: 947 Comm: ping Not tainted 5.12.0-rc6+ #418 Hardware name: Red Hat KVM, BIOS 1.11.1-4.module+el8.1.0+4066+0f1aadab 04/01/2014 Call Trace: <IRQ> dump_stack+0x92/0xc1 print_address_description.constprop.7+0x1a/0x150 kasan_report.cold.13+0x7f/0x111 ip_do_fragment+0x1b03/0x1f60 sch_fragment+0x4bf/0xe40 tcf_mirred_act+0xc3d/0x11a0 [act_mirred] tcf_action_exec+0x104/0x3e0 fl_classify+0x49a/0x5e0 [cls_flower] tcf_classify_ingress+0x18a/0x820 __netif_receive_skb_core+0xae7/0x3340 __netif_receive_skb_one_core+0xb6/0x1b0 process_backlog+0x1ef/0x6c0 __napi_poll+0xaa/0x500 net_rx_action+0x702/0xac0 __do_softirq+0x1e4/0x97f do_softirq+0x71/0x90 </IRQ> __local_bh_enable_ip+0xdb/0xf0 ip_finish_output2+0x760/0x2120 ip_do_fragment+0x15a5/0x1f60 __ip_finish_output+0x4c2/0xea0 ip_output+0x1ca/0x4d0 ip_send_skb+0x37/0xa0 raw_sendmsg+0x1c4b/0x2d00 sock_sendmsg+0xdb/0x110 __sys_sendto+0x1d7/0x2b0 __x64_sys_sendto+0xdd/0x1b0 do_syscall_64+0x33/0x40 entry_SYSCALL_64_after_hwframe+0x44/0xae RIP: 0033:0x7f82e13853eb Code: 66 2e 0f 1f 84 00 00 00 00 00 0f 1f 44 00 00 f3 0f 1e fa 48 8d 05 75 42 2c 00 41 89 ca 8b 00 85 c0 75 14 b8 2c 00 00 00 0f 05 <48> 3d 00 f0 ff ff 77 75 c3 0f 1f 40 00 41 57 4d 89 c7 41 56 41 89 RSP: 002b:00007ffe01fad888 EFLAGS: 00000246 ORIG_RAX: 000000000000002c RAX: ffffffffffffffda RBX: 00005571aac13700 RCX: 00007f82e13853eb RDX: 0000000000002330 RSI: 00005571aac13700 RDI: 0000000000000003 RBP: 0000000000002330 R08: 00005571aac10500 R09: 0000000000000010 R10: 0000000000000000 R11: 0000000000000246 R12: 00007ffe01faefb0 R13: 00007ffe01fad890 R14: 00007ffe01fad980 R15: 00005571aac0f0a0 The buggy address belongs to the page: page:000000001dff2e03 refcount:1 mapcount:0 mapping:0000000000000000 index:0x0 pfn:0x147009 flags: 0x17ffffc0001000(reserved) raw: 0017ffffc0001000 ffffea00051c0248 ffffea00051c0248 0000000000000000 raw: 0000000000000000 0000000000000000 00000001ffffffff 0000000000000000 page dumped because: kasan: bad access detected Memory state around the buggy address: ffff888147009400: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff888147009480: f1 f1 f1 f1 04 f2 f2 f2 f2 f2 f2 f2 00 00 00 00 >ffff888147009500: 00 00 00 00 00 00 00 00 00 00 f2 f2 f2 f2 f2 f2 ^ ffff888147009580: 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 ffff888147009600: 00 00 00 00 00 00 00 00 00 00 00 00 00 f2 f2 f2 for IPv4 packets, sch_fragment() uses a temporary struct dst_entry. Then, in the following call graph: ip_do_fragment() ip_skb_dst_mtu() ip_dst_mtu_maybe_forward() ip_mtu_locked() the pointer to struct dst_entry is used as pointer to struct rtable: this turns the access to struct members like rt_mtu_locked into an OOB read in the stack. Fix this changing the temporary variable used for IPv4 packets in sch_fragment(), similarly to what is done for IPv6 few lines below.

Severity
high
Software
Kernel
Fixed in
5.12.3
Published

How likely is exploitation

FIRST's Exploit Prediction Scoring System puts the probability of this CVE being exploited in the next 30 days at <0.1% — higher than 5% of all published CVEs. EPSS is an independent forecast of exploitation, not a severity rating: it says nothing about how much damage a successful attack does, which is what a CVSS score measures.

Read the two together. CVE-2021-46954 scores 7.1 on CVSS, so a successful attack would be serious — but at <0.1%, almost nobody is currently attempting it. That is a different situation from a flaw under mass exploitation, and it is usually the difference between patching on your next maintenance window and patching tonight.

Affected versions

From: 5.12.0

Until: 5.12.3

Fixed in: 5.12.3

How to fix this CVE

Update your Linux kernel to version 5.12.3 or later to resolve this stack buffer over-read vulnerability in the packet fragmentation path. The vulnerability occurs when traffic control modules attempt to fragment previously reassembled IPv4 packets, causing out-of-bounds memory access. Kernel upgrades should be applied promptly followed by a system reboot to activate the patched version.

sudo dnf update kernel kernel-devel && sudo reboot

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check kernel version with `uname -r` and compare against 5.12.3 — versions 5.12.0 through 5.12.2 are vulnerable
  2. Step 2: Verify if traffic control (tc) modules are loaded: `lsmod | grep sch_frag` and `lsmod | grep act_mirred` — both modules must be present for exploitation
  3. Step 3: Search kernel logs for KASAN stack-out-of-bounds errors related to ip_do_fragment: `sudo grep -i 'KASAN.*ip_do_fragment' /var/log/kern.log` or `sudo journalctl -k | grep -i 'KASAN.*ip_do_fragment'`
  4. Step 4: After patching, verify new kernel is active with `uname -r` and confirm version is 5.12.3 or later

FAQ

What is CVE-2021-46954?

CVE-2021-46954 is a stack buffer over-read vulnerability in the Linux kernel's packet fragmentation code that occurs when traffic control act_mirred attempts to fragment IPv4 packets previously reassembled by act_ct, resulting in out-of-bounds memory access and potential system crash.

Is CVE-2021-46954 being actively exploited?

No, CVE-2021-46954 is not listed on the CISA Known Exploited Vulnerabilities (KEV) catalog and no public exploits are currently available.

What versions of Kernel are affected by CVE-2021-46954?

Linux kernel versions 5.12.0 through 5.12.2 are affected; the vulnerability is fixed in kernel 5.12.3 and later.

How do I check if my server is vulnerable to CVE-2021-46954?

Run `uname -r` to check your kernel version — if it shows 5.12.0, 5.12.1, or 5.12.2, your system is vulnerable. Additionally, confirm the vulnerable code path is present with `grep -r 'sch_fragment' /lib/modules/$(uname -r)/kernel/net/`

Does Defensia detect CVE-2021-46954?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2021-46954 will appear in your dashboard with remediation steps.

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References

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