CVE-2026-52943: Linux Kernel Zero-Copy Buffer Use-After-Free Privilege Escalation

Technical summary

The vulnerability resides in pskb_carve_inside_header() and pskb_carve_inside_nonlinear(), two kernel functions that fragment socket buffers (skbs) during network packet processing. Both functions use memcpy() to duplicate skb_shared_info structures, including the destructor_arg pointer used for MSG_ZEROCOPY operations. The functions fail to increment the reference count via net_zcopy_get() for the copied zero-copy metadata. When the new skb is freed, skb_zcopy_clear() decrements the reference count on the ubuf_info_msgzc structure, driving it to zero prematurely even though other skbs still hold pointers to it. Subsequent access to the freed ubuf_info_msgzc triggers a use-after-free condition exploitable for privilege escalation. The fix mirrors the existing pattern in pskb_expand_head() by calling net_zcopy_get() at appropriate points post-validation.

Business impact

This vulnerability allows any unprivileged local user to escalate to root privileges on affected Linux systems running vulnerable kernel versions. The exploitation is reliable and reproducible. Organizations relying on kernel privilege isolation for container security, multi-tenant systems, or sandbox protections face complete compromise. Affected services may experience unintended system shutdowns, data corruption, or persistent unauthorized access if exploited.

Affected systems

The Linux kernel is affected. The vulnerability impacts all distributions shipping the vulnerable kernel versions prior to the fix. Users of shared hosting, container platforms, multi-user systems, and any Linux deployment where unprivileged users have local access are at immediate risk. Kernel versions between the introduction of the vulnerable code and the fix date (verify against vendor advisories for specific version ranges) are affected.

Exploitability

Exploitability is high. The attack requires only local user access—no special capabilities or system configuration changes are needed. The vulnerability has been validated with a working proof-of-concept achieving full root escalation from an unprivileged context on default kernel configurations. No user interaction or network access is required. The attack is deterministic and does not require spraying or probabilistic techniques.

Remediation

Apply the kernel patch that adds net_zcopy_get() calls to pskb_carve_inside_header() and pskb_carve_inside_nonlinear() following the pattern established in pskb_expand_head(). This fix ensures reference counts for zero-copy buffers remain accurate across packet fragmentation operations. Verify the specific patched kernel version number against your Linux distribution's security advisory, then reboot systems to load the patched kernel.

Patch guidance

Identify your current kernel version using 'uname -r'. Cross-reference with your distribution's (Red Hat, Debian, Ubuntu, SUSE, etc.) security advisory to locate the patched kernel version containing the net_zcopy_get() fix for these two pskb_carve functions. Use your package manager (yum, apt, zypper) to update the kernel package to the fixed version, then reboot. If you maintain a custom or vendor-supplied kernel, verify the presence of the fix in the kernel source changelog or patch files before deployment.

Detection guidance

Monitor for KASAN (Kernel Address Sanitizer) error messages indicating use-after-free on ubuf_info_msgzc structures in kernel logs, particularly in skb_release_data() or related skb cleanup paths. Audit for unprivileged user activity initiating MSG_ZEROCOPY operations followed by packet fragmentation patterns (TCP sends with MSG_ZEROCOPY followed by RDS operations or similar). Privilege escalation monitoring tools should flag unexpected transitions from unprivileged to root context originating from network-related syscalls. Systems without KASAN enabled may show general kernel crashes or hangs; enable KASAN on test systems to improve detection visibility.

Why prioritize this

CVSS 7.8 (HIGH) combined with reliable local privilege escalation from an unprivileged user makes this a critical priority. The vulnerability requires no user interaction, affects the kernel directly, and is not yet in the Known Exploited Vulnerabilities (KEV) catalog but has demonstrated weaponizable proof-of-concept code. Any multi-user or containerized Linux deployment should treat this as urgent. Prioritize patching systems where unprivileged users have shell access, development/build servers, and container host kernels immediately.

Risk score, explained

The CVSS 3.1 score of 7.8 reflects: local attack vector (user already has system access), low complexity (reliable exploitation), low privilege requirement (unprivileged user sufficient), and high impact across confidentiality, integrity, and availability (full root access). The score underrepresents risk in multi-user and container scenarios where local access is a common baseline; real-world impact is closer to critical. The use-after-free nature and proven privilege escalation path elevate operational severity beyond the base score.

Frequently asked questions