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In the Linux kernel, a critical vulnerability has been identified in the IPv6 networking stack, specifically within the `__ip6_append_data()` function. This flaw, designated CVE-2026-53362, stems from an accounting error that can lead to an out-of-bounds write, allowing an unprivileged local attacker to escalate privileges to root.
The vulnerability resides in the kernel’s handling of UDPv6 packets when specific conditions are met. When the `__ip6_append_data()` function processes data and takes the “paged-allocation” path—which occurs with the `MSG_MORE` flag, `NETIF_F_SG` (scatter-gather) support, and a large fragment length—it miscalculates memory sizes.
Normally, when data is fragmented, leftover bytes from a previous packet, known as fraggap, are carried over. The calculation for the new buffer is intended to account for this. However, on this specific paged-allocation path, the `alloclen` (linear area size) is set to fragheaderlen + transhdrlen. When `fraggap` is non-zero, it signifies this is not the first packet, and `transhdrlen` is zero. The code fails to add the `fraggap` bytes to alloclen, making the linear buffer undersized. Concurrently, `pagedlen` (data for paged memory) is miscalculated as datalen - transhdrlen, which overstates the required paged memory by the `fraggap` amount.
This miscalculation causes a critical memory corruption. The `fraggap` bytes from the previous packet are copied directly past the fragment headers in the new, undersized linear area. This copy operation writes past the legitimate `skb->end` boundary and into the adjacent `skb_shared_info` structure. An unprivileged user can trigger this by opening a UDPv6 socket and using the `MSG_MORE` flag in conjunction with MSG_SPLICE_PAGES. The issue was introduced by commit `773ba4fe9104` (“ipv6: avoid partial copy for zc”), and was made exploitable by commit `ce650a166335` (“udp6: Fix __ip6_append_data()’s handling of MSG_SPLICE_PAGES”), which removed an earlier `-EINVAL` return that would have prevented the corruption.
DailyCVE Form:
Platform: Linux Kernel
Version: All vulnerable versions
Vulnerability: Out-of-Bounds Write
Severity: Critical (CVSS 7.8)
date: July 4, 2026
Prediction: Patches available July 2026
What Undercode Say:
To verify if your system is affected, check your kernel version against the patched commits.
uname -r
To check if the vulnerable code path exists, review the kernel source for the presence of the flawed accounting logic in net/ipv6/ip6_output.c.
grep -n "alloclen = fragheaderlen + transhdrlen" net/ipv6/ip6_output.c grep -n "pagedlen = datalen - transhdrlen" net/ipv6/ip6_output.c
Exploit: (Educational Purposes!)
A proof-of-concept (PoC) to trigger this vulnerability involves creating a UDPv6 socket and sending fragmented packets with the `MSG_MORE` and `MSG_SPLICE_PAGES` flags. The exact exploit code is complex and requires precise control over packet sizes to manipulate the `fraggap` value and cause the out-of-bounds write.
A simplified conceptual example in C:
int sock = socket(AF_INET6, SOCK_DGRAM, 0); send(sock, buffer, size, MSG_MORE | MSG_SPLICE_PAGES);
By crafting the size of the data to be sent, an attacker can force a specific `fraggap` value. The resulting memory corruption can be used to overwrite function pointers in the `skb_shared_info` structure, leading to arbitrary code execution with kernel privileges.
Protection:
The primary protection is to apply the official kernel patch that corrects the accounting in __ip6_append_data(). The fix involves adding `fraggap` to `alloclen` and subtracting it from pagedlen.
alloclen = fragheaderlen + transhdrlen + fraggap; pagedlen = datalen - transhdrlen - fraggap;
System administrators should:
- Update the kernel to the latest stable version that includes the patch. Refer to the following stable commits:
– `14200d435af9a9eeb444f529fc2f689a236b7962`
– `65fb14cbebb0cd0eff903a22d33537ddc8b95769`
– `46f201f8b4c39633a1fa3dc12459f506d470993d`
– `6374fb9edf72c67a118a2c214a0dddd04c921e0a`
– `e9eacf19281ea2498b36291b56c9606118c2d74e`
– `736b380e28d0480c7bc3e022f1950f31fe53a7c5`
2. Monitor for security advisories from your Linux distribution vendor for specific backported patches. - Enable Linux Kernel Live Patching solutions (e.g., Kpatch, Kgraft) if available, to apply the fix without rebooting the system.
Impact:
A successful exploitation of this vulnerability allows an unprivileged local attacker to:
– Gain root-level privileges on the affected system.
– Completely compromise the confidentiality, integrity, and availability of the system.
– Execute arbitrary code with kernel-level permissions, bypassing all security restrictions.
– Pivot to other systems on the network if the attacker can establish further access.
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Sources:
Reported By: www.cve.org
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