CVE-2026-98794 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
ipvs: filter some flags received in the backup server
While the IPVS SYNC protocol is not secure by design we can still protect the backup server from messages that can wreak havoc.
This commit addresses problems from received connection flags or their combinations. We now drop messages as follows:
1. the NO_CPORT+TEMPLATE combination allows lookups for normal connections to hit template which can break in many ways. While the master does not sync connections with NO_CPORT flag, i.e. before they are established, we still accept NO_CPORT without TEMPLATE.
2. ONE_PACKET: it is not sent by master, so we do not expect it in backup. Before now it was ignored by IP_VS_CONN_F_BACKUP_MASK for protocol v1 while protocol v0 created connections that are not hashed and dropped immediately. Better to apply the IP_VS_CONN_F_BACKUP_MASK also to the flags from v0 messages for consistency with v1.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's implementation of the Virtual Server over IP (IPVS) module contains a synchronization protocol vulnerability that allows backup servers to process malformed or unexpected connection state flags, potentially leading to service disruption or inconsistent load balancing behavior. The IPVS SYNC protocol is inherently designed for high-availability clustering where master and backup nodes synchronize connection tables. However, this design lacks robust authentication or integrity checks by default, making it susceptible to manipulation if an attacker can inject packets into the synchronization channel. While the primary risk often involves active exploitation of network access, improper handling of specific flag combinations received from a compromised or malfunctioning master server can cause significant operational instability on backup nodes that are expected to take over traffic seamlessly during failover events.
The core technical flaw lies in how the kernel processes incoming connection flags within the synchronization messages. Specifically, the combination of NO_CPORT and TEMPLATE flags creates a dangerous state where lookups for normal client connections may incorrectly match against template entries rather than established real server connections. This mismatch can break various aspects of traffic forwarding logic because templates are intended to define rules for new connections, not serve as targets for existing ones. Although the master node is designed not to synchronize connections marked with NO_CPORT before they are fully established, the backup side must still validate incoming data rigorously. The vulnerability arises when these invalid flag combinations are accepted and processed, leading to incorrect connection table entries that disrupt normal packet forwarding paths.
Additionally, the handling of the ONE_PACKET flag presents another vector for instability. This flag is not transmitted by master nodes under standard operation, meaning its presence in a synchronization message indicates either corruption or malicious injection. Previously, protocol version one ignored this flag due to masking logic, while protocol version zero created unhashed connections that were immediately dropped but still consumed resources and caused transient state inconsistencies. The fix enforces the application of IP_VS_CONN_F_BACKUP_MASK to flags from both protocol versions, ensuring consistent behavior across different sync message formats. This prevents the creation of orphaned or improperly hashed connection entries that do not align with expected cluster states.
From a threat modeling perspective, this vulnerability maps to CWE-20 Improper Input Validation and CWE-787 Out-of-bounds Write if memory corruption occurs during flag processing, though primarily it represents CWE-94 Improper Control of Generation of Code (Code Injection) in the context of logic manipulation leading to denial of service. In terms of MITRE ATT&CK techniques, this relates to T1053 Scheduled Task/Job which could be used if an attacker automates injection of malformed sync packets, or more accurately T1498 Network Denial of Service via resource exhaustion through invalid state creation. The impact is primarily a denial of service condition where backup servers fail to properly assume traffic load due to corrupted connection tables, resulting in dropped packets and interrupted services for end users relying on the high-availability cluster.
Mitigation strategies should focus on network segmentation and access control lists that restrict synchronization traffic between master and backup nodes to trusted interfaces only. Since IPVS sync is typically UDP-based, implementing firewall rules to allow only specific source IPs associated with known masters can prevent external actors from injecting malicious flags. Furthermore, keeping the Linux kernel updated ensures that the patched logic correctly filters out NO_CPORT+TEMPLATE combinations and applies consistent masking for ONE_PACKET across protocol versions. Administrators should also monitor cluster health metrics for signs of connection table anomalies or unexpected drops in throughput during failover scenarios to detect potential exploitation attempts early.