UBUNTU-CVE-2026-53208: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: Bluetooth: L2CAP: reject BR/EDR signaling packets over MTUsig net/bluetooth/l2capcore.c:l2capsigchannel() accepts BR/EDR signaling packets up to the channel MTU and dispatches each command without enforcing the signaling MTU (MTUsig). A Bluetooth BR/EDR peer within radio range can send a fixed-channel CID 0x0001 packet that is larger than MTUsig and contains many L2CAPECHOREQ commands before pairing. In a real-radio stock-kernel run, one 681-byte signaling packet containing 168 zero-length ECHOREQ commands made the target transmit 168 ECHORSP frames over about 220 ms. Impact: a Bluetooth BR/EDR peer within radio range, before pairing, can force 168 ECHORSP frames from one 681-byte fixed-channel signaling packet containing packed ECHOREQ commands. Define Linux's BR/EDR signaling MTU as the spec minimum of 48 bytes and reject any larger signaling packet with one L2CAPCOMMANDREJECTRSP carrying L2CAPREJMTUEXCEEDED before any command is dispatched. The Bluetooth Core spec wording for MTUExceeded says the reject identifier shall match the first request command in the packet, and that packets containing only responses shall be silently discarded. Linux intentionally deviates from that prescription: silently discarding desynchronizes the peer because the remote stack never learns its responses were dropped, and locating the first request command requires walking command headers past MTUsig, i.e. processing bytes from a packet we have already decided is too large to process. We therefore always emit one reject and use the identifier from the first command header, a single fixed-offset byte read. The unrestricted BR/EDR signaling parser and ECHOREQ response path both trace to the initial git import; no later introducing commit is available for a Fixes tag.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-nvidiato a version that resolves this vulnerability.Fixed in 7.0.0-1018.18 - Upgrade
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-31.31.1 - Configuration
Enforce a BR/EDR signaling MTU of 48 bytes and reject any larger signaling packet with one L2CAP_COMMAND_REJECT_RSP using L2CAP_REJ_MTU_EXCEEDED before dispatching commands.
Linux Bluetooth L2CAP BR/EDR signaling signaling MTU (MTUsig) = 48 bytes
Event History
Frequently Asked Questions
Who can exploit this issue?
A Bluetooth BR/EDR peer within radio range can exploit it before pairing. No privileges or user interaction are required according to the supplied severity vector.
What does exploitation allow an attacker to do?
An attacker can send an oversized fixed-channel L2CAP signaling packet containing many ECHO_REQ commands, causing the target to transmit many ECHO_RSP frames. The documented example caused 168 response frames from a single 681-byte packet over about 220 ms.
Are systems affected before a Bluetooth device is paired?
Yes. The described packet can be sent by a nearby BR/EDR peer before pairing, so pairing is not a prerequisite for exposure.
Which Ubuntu kernel packages are listed as affected?
The listed software includes ubuntu/linux, ubuntu/linux-fips, ubuntu/linux-aws, ubuntu/linux-azure, ubuntu/linux-gcp, ubuntu/linux-oracle, ubuntu/linux-aws-fips, and ubuntu/linux-azure-fips.