CVE-2026-80586 in Linux
Summary
by MITRE • 08/26/2026
In the Linux kernel, the following vulnerability has been resolved:
mptcp: options: reset DSS fields in case of unexpected size
A remote peer could send a malformed DSS with a wrong size, followed by another DSS or MPC + Data. In this case, the first suboption will be ignored, but leaving some fields written, which could lead to inconsistency or access uninitialized data.
Explicitly reset the fields that could have been modified in case of unexpected size.
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Analysis
by VulDB Data Team • 08/26/2026
The Linux kernel's Multipath TCP implementation contains a vulnerability related to the handling of Data Sequence Signal options during connection establishment and data transfer phases. This issue arises when a remote peer transmits a malformed DSS option with an incorrect length field. In standard TCP operations, suboptions are processed sequentially, and parsers typically rely on the length field to determine how many bytes to consume for that specific suboption. However, in this scenario, while the kernel correctly identifies the size mismatch and ignores the invalid first suboption as per protocol specifications, it fails to clear or reset certain internal state variables that were partially updated during the initial parsing attempt. This oversight creates a window where subsequent processing of either another DSS option or an MPC plus Data packet operates on stale or uninitialized memory values rather than clean default states.
The technical flaw lies in the lack of defensive programming practices within the MPTCP suboption parser. When the kernel encounters a suboption with an unexpected size, it aborts further parsing of that specific element but does not revert any temporary modifications made to local variables before detecting the error condition. Consequently, if the next packet contains valid data or control information, these residual values from the malformed option may be interpreted as legitimate configuration parameters. This can lead to logical inconsistencies in how subflows are managed or how sequence numbers are tracked across multiple paths. The primary risk is not necessarily a direct remote code execution but rather a denial of service through connection instability or potential information leakage if uninitialized kernel memory contents are inadvertently exposed during subsequent operations.
From an operational perspective, this vulnerability affects the stability and security posture of systems running affected versions of the Linux kernel that utilize Multipath TCP functionality. An attacker capable of injecting network packets into the communication stream could exploit this inconsistency to disrupt ongoing MPTCP sessions or potentially trigger undefined behavior within the networking stack. While the immediate impact is often limited to connection drops or performance degradation due to state desynchronization, such memory inconsistencies are frequently precursors to more severe exploits in complex kernel subsystems. The presence of uninitialized data access aligns with common vulnerability classes found in network protocol implementations where input validation is incomplete regarding side effects on internal state variables.
This issue maps directly to CWE-908, which describes the use of uninitialized resource, and CWE-20, improper input validation, as the parser fails to ensure that partial processing does not leave the system in a compromised state. In terms of offensive security frameworks, this type of vulnerability is relevant to MITRE ATT&CK technique T1498, Network Denial of Service, specifically through resource exhaustion or logic errors caused by malformed inputs. It also touches upon aspects of privilege escalation if the uninitialized data leads to kernel memory corruption that can be leveraged for control flow hijacking, although such an outcome would require additional exploitation steps beyond this specific flaw.
To mitigate this vulnerability, system administrators and developers should apply the latest security patches provided by their Linux distribution vendors. These updates include code changes that explicitly reset DSS fields before processing begins or ensure that any modifications are rolled back upon detection of invalid suboption sizes. Organizations relying on high-availability network configurations involving MPTCP must prioritize these kernel updates to maintain connection integrity and prevent potential disruptions caused by malformed packet injection attacks. Continuous monitoring for anomalous network traffic patterns indicative of fuzzing attempts can also help in detecting exploitation efforts targeting this class of vulnerabilities before they cause significant service impact.