high CVSS 7.8

CVE-2025-21764·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: ndisc: use RCU protection in ndisc_alloc_skb() ndisc_alloc_skb() can be called without RTNL or RCU being held. Add RCU protection to avoid possible UAF.

Severity
high
Software
Kernel
Fixed in
6.13.4
Published
2025-02-27

Affected versions

From: 6.13

Until: 6.13.4

Fixed in: 6.13.4

How to fix this CVE

Update the Linux kernel to version 6.13.4 or later to resolve a use-after-free vulnerability in the NDISC (Neighbor Discovery) subsystem. This vulnerability allows local attackers with standard user privileges to trigger memory corruption through unsafe RCU access patterns. Ensure your system receives kernel updates promptly and schedule a reboot to load the patched kernel.

sudo dnf update kernel

Defensia detects this vulnerability

How to check if you are affected

  1. Check installed kernel version with: uname -r (should be 6.13.4 or later if patched)
  2. Verify if ndisc module is loaded: lsmod | grep ndisc
  3. Check kernel compilation flags for RCU support: grep -i rcu /boot/config-$(uname -r)
  4. Confirm patch application by reviewing kernel commit hashes: git log --oneline | grep -i ndisc

FAQ

What is CVE-2025-21764?

CVE-2025-21764 is a use-after-free vulnerability in the Linux kernel's NDISC (IPv6 Neighbor Discovery) module where ndisc_alloc_skb() can be invoked without proper RCU locking, allowing local attackers to cause memory corruption and potential privilege escalation.

Is CVE-2025-21764 being actively exploited?

No, CVE-2025-21764 is not currently listed in CISA's Known Exploited Vulnerabilities catalog and no public exploits have been disclosed.

What versions of Kernel are affected by CVE-2025-21764?

Linux kernel versions 6.13.0 through 6.13.3 are vulnerable. The fix is available in kernel 6.13.4 and later.

How do I check if my server is vulnerable to CVE-2025-21764?

Run: uname -r — if the output shows kernel version 6.13.0 to 6.13.3, your system is vulnerable. Update to 6.13.4 or later.

Does Defensia detect CVE-2025-21764?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If Linux kernel is installed on a monitored server, CVE-2025-21764 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

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