high CVSS 7.8

CVE-2021-47355·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: atm: nicstar: Fix possible use-after-free in nicstar_cleanup() This module's remove path calls del_timer(). However, that function does not wait until the timer handler finishes. This means that the timer handler may still be running after the driver's remove function has finished, which would result in a use-after-free. Fix by calling del_timer_sync(), which makes sure the timer handler has finished, and unable to re-schedule itself.

Severity
high
Software
Kernel
Fixed in
5.13.3
Published
2024-05-21

Affected versions

From: 5.13

Until: 5.13.3

Fixed in: 5.13.3

How to fix this CVE

Update your Linux kernel to version 5.13.3 or later to resolve this use-after-free vulnerability in the ATM nicstar driver. The vulnerability stems from improper timer cleanup during module removal, allowing the timer handler to execute after driver resources are freed. Apply security patches immediately to all systems running affected kernel versions between 5.13 and 5.13.2.

sudo dnf update kernel kernel-devel && sudo reboot

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check current kernel version with: uname -r | grep -E '5\.13\.[0-2]'
  2. Step 2: Verify if ATM nicstar driver is loaded: lsmod | grep nicstar
  3. Step 3: Search kernel logs for timer-related crashes or use-after-free warnings: dmesg | grep -i 'use-after-free\|timer\|nicstar'
  4. Step 4: After patching, confirm kernel version: uname -r and verify it shows 5.13.3 or higher

FAQ

What is CVE-2021-47355?

CVE-2021-47355 is a use-after-free vulnerability in the Linux kernel's ATM nicstar driver caused by improper timer cleanup during module removal. The driver's remove function uses del_timer() instead of del_timer_sync(), allowing the timer handler to access freed memory.

Is CVE-2021-47355 being actively exploited?

No, CVE-2021-47355 is not actively exploited in the wild according to CISA KEV data, and no public exploits are currently available. However, it remains a high-severity vulnerability requiring immediate patching.

What versions of Kernel are affected by CVE-2021-47355?

Linux kernel versions 5.13 through 5.13.2 are affected. The vulnerability was patched in kernel 5.13.3 and later releases.

How do I check if my server is vulnerable to CVE-2021-47355?

Run: uname -r and check if output matches 5.13.[0-2]. Additionally, verify ATM nicstar driver presence with: lsmod | grep nicstar. If both conditions are true, your system is vulnerable.

Does Defensia detect CVE-2021-47355?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Kernel is installed on a monitored server, CVE-2021-47355 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2026-64303CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: spi: fsl-lpspi: terminate the RX channel on TX prepare failure path When dmaengine_prep_slave_sg() fails for the TX channel, the error path terminates the TX DMA channel but leaves the RX channel running. Since the RX channel was already submitted and issued prior to preparing the TX descriptor, returning -EINVAL causes the SPI core to unmap the DMA buffers while the RX DMA engine continues writing to them, leading to potential memory corruption or use-after-free. Terminate the RX channel before returning on the TX prepare failure path.
CVE-2026-64355CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: bpf: Reject fragmented frames in devmap Devmap broadcast redirects clone the packet for all but the last destination. For native XDP, that clone path copies only the linear xdp_frame data, while fragmented frames keep skb_shared_info in tailroom outside the linear area. Cloning such a frame leaves XDP_FLAGS_HAS_FRAGS set but without valid frag metadata, and the later free path can interpret uninitialized tail data as skb_shared_info, leading to an out-of-bounds access during frame return. Reject fragmented native XDP frames in dev_map_enqueue_clone(). Add the same restriction to the generic XDP clone path in dev_map_redirect_clone(). Generic XDP represents fragmented packets as nonlinear skbs, and rejecting them here keeps clone-based broadcast support aligned between native and generic XDP.
CVE-2026-63993CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: vxlan: do not reuse cached ip_hdr() value after skb_tunnel_check_pmtu() skb_tunnel_check_pmtu() can change skb->head. Reusing old_iph afer skb_tunnel_check_pmtu() can cause an UAF. Use instead ip_hdr(skb) as done in drivers/net/bareudp.c and drivers/net/geneve.c. Found by Sashiko.
CVE-2026-43501CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ipv6: rpl: reserve mac_len headroom when recompressed SRH grows ipv6_rpl_srh_rcv() decompresses an RFC 6554 Source Routing Header, swaps the next segment into ipv6_hdr->daddr, recompresses, then pulls the old header and pushes the new one plus the IPv6 header back. The recompressed header can be larger than the received one when the swap reduces the common-prefix length the segments share with daddr (CmprI=0, CmprE>0, seg[0][0] != daddr[0] gives the maximum +8 bytes). pskb_expand_head() was gated on segments_left == 0, so on earlier segments the push consumed unchecked headroom. Once skb_push() leaves fewer than skb->mac_len bytes in front of data, skb_mac_header_rebuild()'s call to: skb_set_mac_header(skb, -skb->mac_len); will store (data - head) - mac_len into the u16 mac_header field, which wraps to ~65530, and the following memmove() writes mac_len bytes ~64KiB past skb->head. A single AF_INET6/SOCK_RAW/IPV6_HDRINCL packet over lo with a two segment type-3 SRH (CmprI=0, CmprE=15) reaches headroom 8 after one pass; KASAN reports a 14-byte OOB write in ipv6_rthdr_rcv. Fix this by expanding the head whenever the remaining room is less than the push size plus mac_len, and request that much extra so the rebuilt MAC header fits afterwards.
CVE-2026-64216CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: netfs: Fix potential UAF in netfs_unlock_abandoned_read_pages() netfs_unlock_abandoned_read_pages(rreq) accesses the index of the folios it is wanting to unlock and compares that to rreq->no_unlock_folio so that it doesn't unlock a folio being read for netfs_perform_write() or netfs_write_begin(). However, given that netfs_unlock_abandoned_read_pages() is called _after_ NETFS_RREQ_IN_PROGRESS is cleared, the one folio that it's not allowed to dereference is the one specified by ->no_unlock_folio as ownership immediately reverts to the caller. Fix this by storing the folio pointer instead and using that rather than the index. Also fix netfs_unlock_read_folio() where the same applies.

References

Track CVEs across your fleet automatically

Defensia scans your Linux servers and tells you exactly which ones are running vulnerable versions — including CVE-2021-47355. Free for 1 server.

Get started free