CVE-2026-89476 in Linux
Summary
by MITRE • 09/12/2026
In the Linux kernel, the following vulnerability has been resolved:
sctp: fix stream->outcnt underflow on duplicate RECONF responses
A cached RECONF chunk may contain more than one request parameter. A duplicate response can therefore find and process the same ADD_OUT request again while another parameter is still outstanding, rolling back outcnt twice and possibly underflowing it.
Track outstanding request types as bits and clear each bit after its first response. Later responses for the same request are then ignored.
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Analysis
by VulDB Data Team • 09/12/2026
The Stream Control Transmission Protocol (SCTP) implementation in the Linux kernel contains a logic error within the handling of stream reconfiguration requests, specifically concerning duplicate Response to Confirm REquest chunks. This vulnerability arises from an inadequate tracking mechanism for outstanding configuration changes requested by SCTP endpoints. When an endpoint initiates a change to its outbound streams using the ADD_OUT parameter type, it expects a corresponding response to confirm the modification. The kernel maintains a counter, outcnt, which tracks the number of pending or active outbound stream configurations. However, the current implementation fails to distinguish between distinct requests when multiple parameters are bundled into a single cached RECONF chunk. This oversight allows for a scenario where duplicate responses trigger redundant processing logic that incorrectly decrements this counter twice for the same logical request, leading to an integer underflow condition.
The technical root cause lies in how the kernel processes incoming confirmation chunks during stream reconfiguration operations. The code path responsible for handling these responses does not maintain state regarding which specific requests have already been acknowledged. Consequently, if a network anomaly or protocol implementation quirk results in duplicate RECONF response packets being received, the handler iterates through the parameters and applies the decrement operation to outcnt without verifying whether that particular request has previously been processed. This lack of idempotency means that subsequent processing of identical requests effectively rolls back the counter beyond its valid range. Since outcnt is an unsigned integer in many contexts or a signed integer with strict bounds, underflowing it results in either wrapping around to a large positive value if treated as unsigned, or entering negative territory which can cause severe logic errors in downstream functions that rely on this count for resource allocation and state management decisions.
The operational impact of this vulnerability is significant due to the potential for memory corruption and denial of service conditions within the kernel space. An underflowed counter may lead to incorrect assumptions about available stream slots, potentially allowing the system to allocate resources for streams that do not exist or fail to properly release resources associated with closed connections. In severe cases, if this corrupted state propagates to other parts of the networking stack, it could result in use-after-free vulnerabilities or kernel panics, effectively crashing the host machine. Furthermore, because SCTP is often used in critical infrastructure and high-availability applications where reliability is paramount, such instability can disrupt service continuity for dependent services relying on stable network connections. This flaw represents a classic example of improper input validation and state management failure within protocol handling code.
From a classification perspective, this vulnerability aligns with CWE-190 Integer Overflow or Wraparound, as the core issue involves an arithmetic operation that exceeds the representable range of the data type due to repeated decrements without proper bounds checking or state tracking. It also relates closely to CWE-362 Concurrent Execution using Shared Resource with Improper Synchronization if one considers the race-like nature of processing duplicate packets concurrently, although it is primarily a logical flaw in sequential packet handling rather than true concurrency. In terms of attack vectors and techniques, this falls under ATT&CK T1498 Network Denial of Service, as an attacker could potentially trigger this condition by sending crafted SCTP traffic that induces the reception of duplicate RECONF responses. While exploitation requires specific network conditions or a malicious peer capable of injecting such packets, it highlights weaknesses in protocol state machine robustness.
Mitigation for this issue involves applying the upstream kernel patch that modifies the stream reconfiguration logic to track outstanding request types using bitmasks rather than relying solely on counter arithmetic without context. By assigning each distinct ADD_OUT request type a unique bit position and clearing that specific bit only after its first corresponding response is processed, subsequent duplicate responses are effectively ignored. This ensures that outcnt is decremented exactly once per actual configuration change, preserving the integrity of the stream state machine. System administrators should ensure their Linux kernels are updated to versions containing this fix. Additionally, network security monitoring tools can be configured to detect anomalous SCTP traffic patterns involving repeated reconfiguration requests and responses, which may indicate attempts to exploit similar logic flaws in other implementations or serve as a precursor to more sophisticated attacks targeting protocol state machines.