CVE-2026-46193: Linux Kernel IPsec AH ESN Async Callback Offset Vulnerability

Technical summary

CVE-2026-46193 addresses a memory layout mismatch in the Linux kernel's xfrm AH subsystem. When ESN is enabled, the async ahash setup prepends a 4-byte seqhi (sequence number high bits) slot before the ICV or auth_data area. However, the async completion callbacks reconstruct the temporary buffer layout without accounting for this offset, causing them to read and compare authentication data from incorrect positions. This affects both IPv4 (ah4) and IPv6 (ah6) code paths. Per RFC 4302, the ESN high-order 32 bits participate in AH ICV computation, so the async path must preserve the seqhi slot position throughout the authentication lifecycle.

Business impact

Systems relying on IPsec AH with ESN and async hardware crypto engines (or forced async implementations) will experience authentication failures on outbound and inbound traffic. This manifests as 100% packet loss in affected traffic flows, effectively disabling IPsec protection for those connections. Organizations using AH for high-assurance network security or VPN endpoints may face complete loss of encrypted tunnel functionality until patched.

Affected systems

The vulnerability affects Linux kernels that implement AH with ESN support when async cryptographic operations are selected or available. This includes systems using hardware-accelerated AH engines and configurations where async hmac-sha1 or similar algorithms are forced. Both IPv4 and IPv6 AH implementations are vulnerable, though IPv6 AH+ESN has not been extensively tested. The issue does not affect synchronous AH implementations or AH without ESN enabled.

Exploitability

Exploitability is limited by several factors. The vulnerability requires local access (AV:L) and specific kernel configuration combining AH with ESN and async crypto paths. It is not remotely triggerable and does not grant privilege escalation or data exfiltration. An attacker cannot weaponize this for confidentiality breaches; the impact is availability-only (denial of IPsec service). However, once the conditions are met, the flaw is trivial to trigger—any AH-protected packet with ESN will fail.

Remediation

Apply the Linux kernel patch that corrects the async callback layout reconstruction to account for the ESN seqhi slot. The fix modifies net/ipv4/ah4.o and net/ipv6/ah6.o to skip the seqhi offset before locating saved auth_data or ICV during callback processing, aligning the callback-side logic with the setup path. Verify the patch against the upstream Linux kernel advisory for exact version applicability.

Patch guidance

Identify kernel versions and distributions affected by checking if AH+ESN with async crypto is in use (verify via kernel config and IPsec policy). Apply the upstream fix from the Linux kernel stable trees covering the xfrm/ah subsystem. Test patched kernels in a staging environment with AH+ESN traffic before production rollout. Organizations should consult their distribution maintainers (Red Hat, Canonical, SUSE, etc.) for backported patches to supported kernel versions. Verify build cleanly at W=1 warnings level post-patch.

Detection guidance

Monitor IPsec AH state machine logs for callback offset mismatches (e.g., 'ah4 output_done: icv_off=20 expected_off=24'). Enable kernel debugging or audit logs for xfrm operations when AH+ESN is active. Test connectivity on AH+ESN tunnels; 100% packet loss is a strong indicator. Correlate IPsec policy audit logs with traffic loss during peak usage. Check kernel version and config: if CONFIG_XFRM and CONFIG_XFRM_AH are enabled with ESN policies in place, assume vulnerability unless patched.

Why prioritize this

This is a medium-severity availability issue affecting a niche but critical subsystem (IPsec AH+ESN). The low CVSS score reflects local-only access and lack of escalation, but organizations actively using AH for VPN or site-to-site encryption should prioritize patching. The flaw is simple to trigger once conditions align, making it a reliability concern for infrastructure that depends on async AH engines. Not a KEV entry, so no active exploitation pressure, but the impact on affected deployments is total.

Risk score, explained

CVSS 5.5 (Medium) reflects AV:L (local attack vector), AC:L (low complexity once conditions met), PR:L (low privilege required), and A:H (high availability impact). The score does not account for the rarity of the vulnerable configuration (AH+ESN+async crypto), which in practice may reduce real-world risk. However, for organizations meeting all three conditions, this is a critical availability issue despite the medium rating.

Frequently asked questions