Summary
CVE-2026-98432 is a critical use-after-free vulnerability in the Linux kernel’s IPv4 TCP code. It occurs when the kernel frees memory that is still being used during TCP connection handling or socket migration. An attacker can trigger this by sending specially crafted network requests, potentially causing memory corruption, system crashes, or denial of service. Researchers also demonstrated remote code execution without authentication on systems with an MPTCP listener under specific conditions. The vulnerability affects Linux kernel builds containing the vulnerable code, with fixes available through upstream kernel updates and vendor security packages.
Urgent Actions Required
- Update the Linux kernel: Install the fixed version recommended by your Linux vendor.
- Reboot the system: Self-managed AWS EC2 systems need a reboot after the update.
- Disable MPTCP: If patching is delayed, set net.mptcp.enabled=0 to reduce exposure.
- Block IPv4 packets with IP options: Use AWS's recommended firewall rule as a temporary measure. This does not replace the kernel update.
Which Systems Are Vulnerable to CVE-2026-98432?
Technical Overview
- Vulnerability Type: Use-After-Free (CWE-416) in IPv4 TCP socket memory handling
- Affected Software/Versions: Linux kernel versions containing the vulnerable code, including version 4.4 and later affected builds.
- Fixed Versions (Upstream Kernel):
- Linux 5.10: 5.10.272
- Linux 5.15: 5.15.223
- Linux 6.1: 6.1.190
- Linux 6.6: 6.6.159
- Linux 6.12: 6.12.113
- Linux 6.18: 6.18.56
- Linux 7.2: 7.2.10
- Linux 7.3: 7.3-rc7
- CVSS Vector: v3.1
- Attack Vector: Network
- Attack Complexity: Low
- Privileges Required: None
- User Interaction: None
- Scope: Unchanged
- Confidentiality Impact: High
- Integrity Impact: High
- Availability Impact: High
- Patch Availability: Yes, available
How Does the CVE-2026-98432 Exploit Work?
The attack typically follows these steps:
What Causes CVE-2026-98432?
Vulnerability Root Cause:
CVE-2026-98432 is caused by improper memory handling in the Linux kernel’s IPv4 TCP code. The kernel uses kfree() to free socket memory while another CPU may still be using it. This causes a use-after-free issue. The fix replaces kfree() with kfree_rcu() to free the memory only after ongoing operations finish.
How Can You Mitigate CVE-2026-98432?
If immediate patching is delayed or not possible:
- Disable MPTCP: Set net.mptcp.enabled=0 to reduce exposure to the vulnerable conditions.
- Block IPv4 packets with IP options: Use firewall rules to block these incoming packets.
- AWS Network Firewall: Use a stateful rule to drop IPv4 packets containing IP options.
Note: These are temporary measures and do not fully fix the vulnerability. Installing the appropriate kernel security update remains essential.
Which Assets and Systems Are at Risk?
- Asset Types Affected:
- Linux Servers - Systems running kernel versions containing the vulnerable IPv4 TCP code
- Network-Facing Systems - Linux hosts handling incoming IPv4 TCP connection requests
- MPTCP-Enabled Systems - Systems with an MPTCP listener, where researchers demonstrated unauthenticated remote code execution under specific conditions
- Business-Critical Systems at Risk:
- Amazon Linux Instances - Affected Amazon Linux 2 and Amazon Linux 2023 systems requiring the relevant kernel updates
- Debian Systems - Affected Bookworm and Trixie kernel packages
- Red Hat Systems - Affected RHEL kernel packages and listed OpenShift CoreOS components
- SUSE Systems - Linux products with affected kernel packages, including SLES versions
- Exposure Level:
- Network-Accessible Linux Systems - The vulnerability has a network attack vector and does not require authentication
- Systems Handling TCP Requests - Concurrent TCP connection requests can trigger the vulnerable memory-handling condition
- Systems Using Request-Socket Migration - The issue can also occur when request sockets are migrated, and their options are shared between the original socket and its clone
Will Patching CVE-2026-98432 Cause Downtime?
Patch application impact: A kernel update and system reboot are required. This may cause temporary service downtime during the reboot.
How Can You Detect CVE-2026-98432 Exploitation?
Exploitation Signatures:
- Unusual bursts of IPv4 TCP SYN requests to network-facing Linux systems
- Repeated SYN retransmissions that may cause concurrent processing of the same connection request
- Incoming IPv4 TCP packets containing IP options
- Unexpected activity involving TCP request-socket migration
Behavioral Indicators:
- Signs of a kernel use-after-free condition during IPv4 TCP connection handling
- Unexpected kernel crashes or service disruptions
- Memory corruption associated with vulnerable TCP socket handling
Alerting Strategy:
- Priority: Critical
- Monitor for unusual IPv4 TCP traffic, particularly packets containing IP options
- Investigate repeated TCP connection requests and unexpected socket migration activity
- Alert on unexplained kernel crashes or memory corruption affecting network-facing systems
Remediation & Response
- Remediation Timeline:
- Immediate: Apply AWS-recommended temporary measures if kernel patching is delayed, such as disabling MPTCP or blocking incoming IPv4 packets containing IP options.
- As soon as updates are available: Install the security update provided by the relevant Linux distribution.
- After updating: Reboot the system where required to complete remediation. AWS specifically requires a reboot for self-managed EC2 Linux systems.
- Incident Response Considerations:
- Check whether affected Linux systems are running vulnerable kernel packages.
- Review network activity for crafted IPv4 TCP packets containing IP options and unusual TCP connection requests.
- Investigate unexpected kernel crashes or memory corruption that may be associated with the vulnerability.
- Confirm that the appropriate kernel update has been installed and that required reboots are complete.
- Note that AWS reported no evidence of active exploitation at the time of its advisory.
Compliance & Governance Notes
- Audit Trail Requirement:
- Record the affected systems, installed kernel versions, security updates applied, and update dates.
- Maintain evidence that required system reboots were completed after kernel updates.
- Document any temporary measures used while patching was delayed, such as disabling MPTCP or filtering IPv4 packets containing IP options.
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CVSS Breakdown Table
| Metric | Value | Description |
|---|---|---|
| Base Score | 9.8 | Indicates a critical vulnerability with high potential impact and remote exploitability |
| Attack Vector | Network | An attacker can exploit the flaw remotely by sending crafted IPv4 TCP packets to an affected system |
| Attack Complexity | Low | The published CVSS assessment rates exploitation complexity as low |
| Privileges Required | None | The attacker does not need prior authentication or privileges |
| User Interaction | None | Exploitation does not require action from a user |
| Scope | Unchanged | The impact remains within the vulnerable security authority |
| Confidentiality Impact | High | Successful exploitation can have a high impact on system confidentiality |
| Integrity Impact | High | Successful exploitation can have a high impact on system integrity |
| Availability Impact | High | The vulnerability can cause kernel crashes and service disruption |
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