CVE-2026-46254: Linux Kernel Privilege Escalation Flaw

CVE-2026-46254 Overview

CVE-2026-46254 affects the Linux kernel AppArmor subsystem. The vulnerability stems from improper handling of unaligned deterministic finite automaton (DFA) tables in security/apparmor/match.c. DFA tables can originate from kernel or userspace, and 8-byte alignment is not guaranteed. Loading an unaligned DFA blob triggers unaligned memory accesses on architectures such as SPARC, producing kernel warnings from aa_dfa_unpack(). The fix replaces direct dereferences with the get_unaligned_xx() helpers so AppArmor can safely process unaligned table data.

Critical Impact

Loading a crafted or naturally unaligned AppArmor policy can trigger unaligned memory access warnings and undefined behavior on architectures that do not transparently handle misaligned reads.

Affected Products

• Linux kernel AppArmor subsystem (security/apparmor/match.c)
• Linux kernel 6.18.0-rc6 and prior versions containing the unfixed aa_dfa_unpack() code path
• Architectures sensitive to unaligned access, including SPARC

Discovery Timeline

• 2026-06-03 - CVE-2026-46254 published to NVD
• 2026-06-03 - Last updated in NVD database

Technical Details for CVE-2026-46254

Vulnerability Analysis

The issue resides in aa_dfa_unpack() within security/apparmor/match.c. AppArmor uses DFA tables to evaluate policy matches against file paths and other inputs. When the kernel unpacks a profile via the apparmor_parser userspace tool, it reads a serialized DFA blob streamed into the kernel through writes to the policy interface. The unpack routine assumes 8-byte alignment for table fields, then performs natively aligned loads. When the blob is not aligned, the CPU raises an unaligned access trap. On SPARC and similar architectures, this produces a kernel warning and a stack trace originating from aa_dfa_unpack+0x6cc/0x720. The reported call chain traverses unpack_pdb, unpack_profile, aa_unpack, aa_replace_profiles, policy_update, and profile_replace before reaching vfs_write from the userspace policy loader.

Root Cause

The root cause is an implicit alignment assumption in DFA blob parsing. The serialized format does not guarantee that table headers and arrays begin on 8-byte boundaries, yet the parser dereferences pointers directly as if they did. This is a kernel input validation and architecture-portability defect rather than a remotely reachable memory corruption.

Attack Vector

Loading an AppArmor policy requires the CAP_MAC_ADMIN capability, so triggering the condition is restricted to privileged local processes such as apparmor_parser. A privileged user supplying a malformed or naturally misaligned policy blob can induce the warning and unaligned access on affected architectures. There is no indication of remote exploitability and no public proof-of-concept is referenced in the advisory.

// No verified exploitation code is available for CVE-2026-46254.
// The vulnerability is described in prose based on the upstream commit
// messages and kernel warning trace published with the advisory.

Detection Methods for CVE-2026-46254

Indicators of Compromise

• Kernel log entries containing dfa blob stream followed by not aligned
• WARNING traces referencing aa_dfa_unpack+ in security/apparmor/match.c:316
• Kernel unaligned access messages of the form Kernel unaligned access at TPC[...] aa_dfa_unpack
• Process name apparmor_parser appearing in the warning context during policy load

Detection Strategies

• Monitor dmesg and /var/log/kern.log for AppArmor warnings tied to aa_dfa_unpack and unaligned blob streams.
• Alert on unexpected apparmor_parser invocations outside of package management, container startup, or policy deployment workflows.
• Track writes to AppArmor policy interfaces under /sys/kernel/security/apparmor/ to detect anomalous policy replacements.

Monitoring Recommendations

• Forward kernel ring buffer logs to a centralized log platform and create rules matching aa_dfa_unpack warnings.
• Establish a baseline of legitimate policy update events per host and flag deviations.
• Audit which accounts and services hold CAP_MAC_ADMIN, since only those can load AppArmor policies.

How to Mitigate CVE-2026-46254

Immediate Actions Required

• Apply the upstream Linux kernel patches that introduce get_unaligned_xx() usage in aa_dfa_unpack().
• Restrict CAP_MAC_ADMIN to trusted system components responsible for AppArmor policy management.
• Validate any third-party AppArmor policies before loading them on production hosts.

Patch Information

The fix is available in the following upstream commits referenced by the advisory: Linux Kernel Commit 23f112bd, Linux Kernel Commit 64802f73, Linux Kernel Commit cded6360, and Linux Kernel Commit ec737e7f. Rebuild and deploy a kernel that incorporates these changes, or install a distribution kernel update that backports them.

Workarounds

• Disable AppArmor policy reloads on alignment-sensitive architectures until a patched kernel is deployed.
• Limit which users and services can invoke apparmor_parser and write to AppArmor policy sysfs interfaces.
• Use only AppArmor policy blobs produced by trusted tooling that emits 8-byte aligned DFA tables.

# Restrict access to AppArmor policy interfaces to root only
chmod 600 /sys/kernel/security/apparmor/.load
chmod 600 /sys/kernel/security/apparmor/.replace
chmod 600 /sys/kernel/security/apparmor/.remove

# Verify the running kernel includes the fix
dmesg | grep -i "aa_dfa_unpack"
uname -r