CVE-2026-90088 in Linux
Summary
by MITRE • 09/17/2026
In the Linux kernel, the following vulnerability has been resolved:
Bluetooth: RFCOMM: Validate MTU in rfcomm_apply_pn() to prevent infinite loop
rfcomm_apply_pn() accepts the MTU value from a remote PN (Parameter Negotiation) frame without checking for zero. When the remote peer sends an MTU of zero, d->mtu is set to 0. This causes the sendmsg path to enter an infinite loop when fragmenting data, as each fragment has size == min_t(size_t, len, 0) == 0, so the remaining length never decreases. The infinite allocation of zero-length skbs exhausts all system memory.
Fix by clamping d->mtu to RFCOMM_DEFAULT_MTU when the negotiated value is zero, consistent with the initial value assigned in rfcomm_dlc_alloc().
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Analysis
by VulDB Data Team • 09/17/2026
The vulnerability identified within the Linux kernel's Bluetooth subsystem specifically targets the Radio Link Control and Adaptation Protocol (RFCOMM) implementation. This flaw resides in the function rfcomm_apply_pn(), which is responsible for processing Parameter Negotiation frames exchanged between local and remote devices during connection setup or reconfiguration. The core technical deficiency lies in the absence of input validation regarding the Maximum Transmission Unit value provided by a peer device. When a remote endpoint sends an MTU parameter set to zero, the kernel blindly accepts this invalid configuration without performing any sanity checks. Consequently, the internal data structure representing the RFCOMM connection is updated with an MTU size of zero, which fundamentally breaks the assumptions made by subsequent network stack operations that rely on positive integer values for packet sizing and fragmentation logic.
The operational impact of this misconfiguration manifests as a denial-of-service condition triggered through resource exhaustion. When the system attempts to transmit data over this compromised connection, it enters the sendmsg path where packets are fragmented according to the negotiated MTU size. Because the MTU is zero, every attempt to calculate fragment sizes results in a value of zero due to the min_t logic used for determining chunk lengths. This leads to an infinite loop wherein the kernel continuously allocates socket buffers with zero length without ever reducing the remaining data length or advancing the transmission pointer. The rapid and uncontrolled allocation of these empty skbs consumes all available system memory, effectively freezing the host machine or causing a kernel panic due to out-of-memory conditions.
From a classification perspective, this vulnerability aligns with CWE-20 Improper Input Validation, as the application fails to verify that user-controlled input from a network peer falls within expected bounds before processing it. Furthermore, in the context of attack tactics, this flaw facilitates Denial of Service via resource exhaustion, which can be mapped to MITRE ATT&CK techniques related to impact and availability disruption. The lack of boundary checking allows an attacker with access to the Bluetooth interface or capable of spoofing RFCOMM frames to trigger this state remotely if proper isolation is not in place between network-facing components and critical system resources.
The resolution involves implementing a defensive programming practice by clamping the negotiated MTU value to RFCOMM_DEFAULT_MTU whenever it is detected as zero. This ensures that d->mtu retains a valid, non-zero size consistent with its initial allocation state in rfcomm_dlc_alloc(). By enforcing this lower bound on input values, the kernel prevents the division-by-zero-like behavior inherent in the fragmentation loop and guarantees that data transmission proceeds normally even if a malicious or buggy peer attempts to negotiate an invalid MTU. This fix restores system stability by ensuring that memory allocations are tied to actual data sizes rather than infinite iterations of zero-byte buffers.