CVE-2026-53249: Linux Kernel IPv4 Source Routing Privilege Escalation Fix

Technical summary

The vulnerability exists in the Linux kernel's IPv4 options handling. LSRR and SSRR are IP header options defined in RFC 791 that allow a packet sender to specify or influence the route packets take through a network. Historically, these options posed a security concern because an unprivileged application could set them without restriction, enabling a local user to redirect their own traffic (or traffic they can influence) through attacker-controlled intermediaries. This can be weaponized to perform TCP sequence number prediction attacks or to intercept and analyze protocol handshake information. The patch implements a capability check, restricting IPOPT_SSRR and IPOPT_LSRR setting to processes holding the CAP_NET_RAW capability. This aligns with industry best practices documented in RFC 7126, which recommends filtering or rejecting packets carrying these options at network boundaries.

Business impact

For most organizations, the direct business impact is limited because this vulnerability requires local system access to exploit. However, it can compound other risks in multi-tenant or containerized environments where unprivileged users or containers may coexist. In such scenarios, an attacker with local access could use this to reconnaissance network topology, predict TCP handshake parameters for session hijacking, or exfiltrate protocol metadata. The medium CVSS score reflects the local-only attack vector but acknowledges the potential for availability impact in performance-sensitive environments. Organizations running kernel versions prior to the patch on systems where untrusted local users have shell access should prioritize remediation.

Affected systems

All versions of the Linux kernel prior to the patch are affected. The vulnerability impacts any Linux distribution running an unpatched kernel, regardless of architecture. This includes cloud instances, on-premises servers, embedded Linux systems, and containerized workloads. The risk is primarily relevant on systems where multiple users or unprivileged processes can execute code (shared hosting, multi-user servers, Kubernetes clusters with local container access). Systems with strict user isolation or single-user deployments face lower practical risk, though patch application remains recommended for defense-in-depth.

Exploitability

Exploitability requires local code execution as an unprivileged user on an affected system. The attack is not particularly difficult to execute once local access is achieved—setting IP options is straightforward via standard socket APIs. However, the attack is not remotely exploitable and does not affect systems where local user access is tightly controlled. No known public exploits or active threat actor campaigns leveraging this specific weakness have been reported at the time of this analysis. The vulnerability would likely be chained with other attack vectors in a real-world compromise scenario.

Remediation

Apply the Linux kernel patch that restricts IPOPT_SSRR and IPOPT_LSRR to CAP_NET_RAW-capable processes. Most distributions have backported this fix into stable and long-term support kernel versions. Verify the specific patch version against your vendor's security advisory or kernel changelog. As an interim measure, organizations can restrict local user access via SSH and shell accounts, enforce seccomp policies to block raw socket creation, or use network-level filtering to drop packets with source routing options at ingress points.

Patch guidance

Check your Linux distribution's security update channels for a kernel version incorporating this fix. Major distributions (Red Hat, Ubuntu, Debian, SUSE) have released patched kernel versions; consult your vendor's security bulletin for specific version numbers and timelines. Apply updates using your distribution's package manager (yum, apt, zypper, etc.). A system restart is typically required for kernel updates to take effect. Test patches in a non-production environment first to ensure compatibility with your workloads and third-party kernel modules.

Detection guidance

Detection at the application level is difficult because the vulnerability is kernel-internal and does not produce obvious malicious signatures. Monitor system logs for attempts to set socket options IP_OPTIONS or IP_RETOPTS by unprivileged processes, though such activity is uncommon in legitimate applications. Implement network-level filtering to drop or log IPv4 packets with LSRR or SSRR options at your network boundary. Host-based intrusion detection tools may flag unusual raw socket creation by non-root processes. Behavioral monitoring for TCP sequence number prediction attacks (rapid connection attempts with varying sequence numbers) can indicate exploitation attempts, though these are difficult to distinguish from benign network issues.

Why prioritize this

This vulnerability should be prioritized for environments where unprivileged local access is possible, particularly multi-tenant platforms and container orchestration systems. While the CVSS score of 5.5 is medium, the ease of exploitation post-compromise and the potential to extract cryptographic or session state information warrant timely patching. Organizations with strict access controls and single-user systems can defer patching slightly but should include it in standard kernel maintenance cycles. The lack of active exploits in the wild reduces urgency for low-risk deployments but does not eliminate the need for eventual remediation.

Risk score, explained

The CVSS 3.1 score of 5.5 (MEDIUM) reflects an attack vector that is Local (not remote), requires Low privilege (an unprivileged user), has Low complexity, requires No user interaction, and produces a High availability impact. The score emphasizes that an attacker must first gain local system access, which is the primary barrier to exploitation. Once local access is achieved, the low complexity and ease of triggering the vulnerability increase the risk. The availability component reflects potential kernel-level disruption or denial of service, though confidentiality is the more likely primary concern in practice due to the information disclosure nature of the attack.

Frequently asked questions