UBUNTU-CVE-2026-81038: High severity ubuntu/zephyr vulnerability

The Object Transfer Service server builds its Object Action Control Point (OACP) response indication in oacpindsend() (subsys/bluetooth/services/ots/otsoacp.c). Before this fix the four-byte Checksum Value field was appended whenever the requested op code was BTGATTOTSOACPPROCCHECKSUMCALC, regardless of the result code. The response payload is a bare, uninitialized four-byte stack buffer declared with NETBUFSIMPLEDEFINE(respparam, sizeof(uint32t)) in btgattotsoacpwrite(), and it is filled only on the success path of oacpchecksumprocvalidate(). On every failure path the buffer is still empty, so netbufsimplepullle32() was invoked on a zero-length buffer. The OACP characteristic and its client characteristic configuration descriptor are declared with BTGATTPERMWRITE and BTGATTPERMREAD | BTGATTPERMWRITE, so no pairing, encryption, or authentication is required. Any peer within radio range that can connect to a device running the OTS server can enable OACP indications and write a nine-byte Calculate Checksum command that fails validation — for example by issuing it with no object selected, which returns BTGATTOTSOACPRESINVOBJ. Because oacpcommanddecode() accepts the Calculate Checksum op code unconditionally (it is not guarded by CONFIGBTOTSOACPCHECKSUMSUPPORT), builds without checksum support are reachable too: they answer BTGATTOTSOACPRESOPCODENOTSUP and take the same bad path. On builds with CONFIGASSERT enabled, the zero-length pull trips the assertion inside netbufsimplepullmem() and panics the device, giving a remotely triggered denial of service. On builds with assertions disabled there is no check: the pull returns the untouched stack array and four bytes of uninitialized stack memory are copied into the indication and transmitted to the peer, repeatable on every request. The disclosure is bounded to four bytes per request and the write stays inside the seven-byte res indication buffer, so there is no memory corruption; the impact is uninitialized-memory disclosure plus an availability loss in assertion-enabled configurations.