CVE-2026-98438 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
mptcp: options: fix uninit-value in mptcp_write_data_fin
When sending a DATA_FIN without data, and because the DATA_FIN occupies 1 octet of the connection-level sequence space [1], it is then required
to add a DSS mapping with specific values.
If the checksum has been negotiated, it also needs to be computed, and included in the outgoing packet, and thus the initial csum data needs to be reset to 0 as well. This is no longer the case since commit cfcceb7a39fc ("tcp: shrink per-packet memset in __tcp_transmit_skb()"), because the whole ext_copy structure is no longer zeroed by default.
This seems to be the only case where use_map is changed and set afterwards, so initialising the csum field only in this case, along with other fields for this specific case.
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Analysis
by VulDB Data Team • 10/18/2026
The vulnerability identified as CVE-2024-something related to Multipath TCP (MPTCP) implementation within the Linux kernel involves an uninitialized value usage during the transmission of a DATA_FIN packet without accompanying data payload. This issue stems from changes in how network packets are prepared for transmission, specifically affecting the handling of checksum calculations and sequence space mappings when no actual user data is being sent alongside control signals like FINs.
The root cause lies in commit cfcceb7a39fc which optimized TCP socket buffer operations by shrinking per-packet memset calls within __tcp_transmit_skb(). This optimization meant that the ext_copy structure, used to hold extended options for packet transmission, was no longer automatically zeroed out before use. In normal data transmission scenarios this is not problematic because fields are typically initialized during standard processing paths. However in MPTCP when sending a DATA_FIN without data specific initialization steps were missed due to reliance on previous zeroing behavior that had been removed.
When an endpoint sends a DATA_FIN it signals the end of its side of the connection while still allowing receipt of remaining data from the peer according to RFC 8684 specifications for MPTCP protocol operation. A critical aspect here is that even though no user data follows the FIN marker in this specific case the DATA_FIN itself consumes one octet within the connection-level sequence space requiring proper mapping via a Data Sequence Signal DSS option with precise values to maintain synchronization between peers.
If checksumming has been negotiated as part of MPTCP session setup then computing and including accurate checksums becomes mandatory for protocol compliance and integrity verification purposes prior commit cfcceb7a39fc the ext_copy structure would have been fully zeroed ensuring that any uninitialized fields did not contain stale or random memory contents. After this change however since memset operations were reduced certain initialization steps needed explicit handling particularly concerning sum data which must be reset to zero before computation begins otherwise garbage values could propagate into transmitted packets leading potentially to incorrect checksums being sent across the network affecting peer validation mechanisms and possibly causing connection drops or security issues if attackers exploit malformed packet structures.
From a technical perspective this represents an uninitialized memory read vulnerability classified under CWE-457 which describes use of an uninitialized variable that can lead to unpredictable behavior depending on what residual data exists in allocated memory regions at runtime context where such bugs occur often involves complex state machines like those found inside kernel networking stacks making them harder to detect through static analysis alone without thorough dynamic testing covering edge cases involving control plane operations rather than just bulk data transfers.
Operationally this flaw could result in several adverse outcomes including but not limited to incorrect checksum values being embedded into outgoing packets causing receiving endpoints reject valid traffic due failed integrity checks potentially leading denial-of-service conditions where legitimate communications fail silently or explicitly depending on implementation specifics also there exists theoretical risk if attacker somehow influences surrounding memory layout they might craft scenarios leveraging these uninitialized bytes influencing decision logic downstream though practical exploitation paths remain limited given kernel isolation protections present in modern systems.
Mitigation strategies primarily involve applying upstream Linux kernel patches addressing this specific regression introduced by optimization efforts aimed at improving performance metrics around socket buffer management developers responsible for maintaining MPTCP subsystems should ensure all relevant code paths initializing structures involved in packet construction explicitly set required fields especially those related to cryptographic operations such as checksum initialization before any computation takes place additionally thorough testing covering edge cases involving minimal payload transmissions alongside control flags helps catch similar oversights early during development cycles preventing them from reaching production environments.
This incident highlights importance of carefully reviewing impacts resulting from low-level optimizations within critical subsystems like networking stack where assumptions about default states may no longer hold true post-modification requiring comprehensive audit trails tracking every instance relying implicitly on prior zeroing behaviors ensuring consistency across all execution paths regardless whether dealing with high throughput data flows or sparse control messages carrying essential signaling information maintaining protocol correctness remains paramount especially in multi-path environments adding complexity around sequence number management and state synchronization requirements.
References to industry standards include Common Weakness Enumeration CWE-457 for uninitialized variable usage which directly maps onto observed behavior described above while ATT&CK framework does not have specific technique mapping since this falls more towards general software defect category rather than adversarial tactic though understanding such flaws aids defenders in building better detection rules around anomalous network behaviors potentially stemming from malformed packets generated by compromised systems exploiting similar weaknesses elsewhere within infrastructure.