CVE-2026-64114 in Linux
Summary
by MITRE • 07/19/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv4: raw: reject IP_HDRINCL packets with ihl < 5
raw_send_hdrinc() validates that the caller-supplied IPv4 header fits within the message length:
iphlen = iph->ihl * 4; err = -EINVAL; if (iphlen > length) goto error_free;
if (iphlen >= sizeof(*iph)) {
/* fix up saddr, tot_len, id, csum, transport_header */ }
It does not, however, reject ihl < 5. For such a packet the "if (iphlen >= sizeof(*iph))" branch is skipped, leaving the crafted iphdr untouched, but the packet is still handed to __ip_local_out() and onward. Downstream consumers that read iph->ihl assume a sane value: net/ipv4/ah4.c:ah_output() in particular subtracts sizeof(struct iphdr) from top_iph->ihl * 4 and passes the (signed-int-negative, then cast to size_t) result to memcpy(), producing an OOB access of length close to SIZE_MAX and a host kernel panic.
An IPv4 header with ihl < 5 is malformed by definition (RFC 791: "Internet Header Length is the length of the internet header in 32 bit words ... Note that the minimum value for a correct header is 5."). The kernel should not be willing to inject such a packet into its own output path.
Reject "iphlen < sizeof(*iph)" alongside the existing "iphlen > length" check. This matches the principle that locally constructed packets that re-enter the IP stack must pass the same basic sanity tests that a foreign packet would be subjected to.
Once this lands, the "if (iphlen >= sizeof(*iph))" wrapper around the fixup branch becomes redundant; left in place to keep the patch minimal and backport-friendly. A follow-up can unwrap it.
Note that commit 86f4c90a1c5c ("ipv4, ipv6: ensure raw socket message is big enough to hold an IP header") ensures the message buffer is large enough to hold an iphdr, but does not constrain the self-reported iph->ihl.
Reachability: the malformed packet source is any caller with CAP_NET_RAW, including an unprivileged process in a user+net namespace on a kernel with CONFIG_USER_NS=y. The reproduced AH crash also requires a matching xfrm AH policy on the outgoing route; a container granted CAP_NET_ADMIN can install that state and policy in its netns. Loopback bypasses xfrm_output, so the trigger uses a real netdev.
Reproduced on UML + KASAN: kernel-mode fault at addr 0x0 with memcpy_orig at the crash site. Same shape reproduces inside a rootless Docker container with --cap-add NET_ADMIN on a stock distro kernel.
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Analysis
by VulDB Data Team • 07/20/2026
The vulnerability under discussion involves a critical flaw in the Linux kernel's IPv4 raw socket implementation that allows for improper handling of malformed IP headers. Specifically, the raw_send_hdrinc() function performs validation checks on caller-supplied IPv4 headers to ensure they fit within the message length but fails to reject packets with an Internet Header Length (ihl) field value less than five. This oversight creates a path where malformed packets can be processed and passed through the kernel's IP output pipeline, leading to potential memory corruption and system instability. The issue stems from the fact that the validation logic only checks if iphlen exceeds the message length but neglects to verify that the ihl field meets minimum requirements defined by RFC 791, which mandates a minimum value of five for valid IPv4 headers.
The technical flaw manifests when an IPv4 header with ihl < 5 bypasses the initial validation check and enters the kernel's IP processing path. While the condition iphlen >= sizeof(iph) is skipped for such malformed headers, the packet still proceeds to __ip_local_out() and beyond. Subsequent kernel components that read iph->ihl assume a sane value, causing critical functions like ah_output() in net/ipv4/ah4.c to perform calculations based on negative values derived from iph->ihl 4 minus sizeof(struct iphdr). This results in memcpy operations with extremely large size parameters approaching SIZE_MAX, leading to out-of-bounds memory access and ultimately a kernel panic. The vulnerability directly violates the principle that locally constructed packets must undergo the same basic sanity checks as externally received packets, aligning with common security practices outlined in various cybersecurity frameworks.
The operational impact of this vulnerability extends beyond simple denial-of-service conditions to encompass potential privilege escalation and system compromise. An attacker with CAP_NET_RAW capability can exploit this flaw, which includes unprivileged processes running within user namespaces when CONFIG_USER_NS=y is enabled on the kernel. The attack requires an additional xfrm AH policy to be installed on the outgoing route, which can be accomplished by a container granted CAP_NET_ADMIN privileges. This makes the vulnerability particularly concerning in containerized environments where such capabilities might be available to less privileged users. The reproduction mechanism uses real network devices rather than loopback interfaces, as the loopback path bypasses xfrm_output processing and requires a more complex attack vector involving actual network interface manipulation.
Security mitigations for this vulnerability involve implementing proper validation of the ihl field value during packet processing, rejecting packets with ihl < 5 similar to how the existing iphlen > length check operates. This approach ensures that all packets entering the kernel's IP stack undergo consistent validation regardless of their origin or construction method. The fix should be applied at the raw_send_hdrinc() function level where the validation currently occurs, extending the existing checks to include a minimum value constraint on the ihl field. Additionally, the redundant wrapper around the fixup branch can be removed in follow-up patches to clean up the code structure while maintaining backward compatibility for backporting purposes.
This vulnerability aligns with CWE-129, which addresses improper validation of input ranges, and demonstrates characteristics consistent with ATT&CK technique T1068, involving privilege escalation through local exploitation. The issue also reflects broader concerns around kernel memory safety and the importance of defensive programming practices in operating system components. The fix addresses a fundamental principle of secure coding that emphasizes the need for comprehensive input validation and consistent security checks across all code paths within kernel subsystems. This particular vulnerability represents a significant risk to systems running Linux kernels with user namespaces enabled and demonstrates how seemingly minor oversights in input validation can lead to severe security implications affecting system stability and potentially enabling further exploitation techniques.