CVE-2025-39889

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: l2cap: Check encryption key size on incoming connection

This is required for passing GAP/SEC/SEM/BI-04-C PTS test case: Security Mode 4 Level 4, Responder - Invalid Encryption Key Size

• 128 bit

This tests the security key with size from 1 to 15 bytes while the Security Mode 4 Level 4 requests 16 bytes key size.

Currently PTS fails with the following logs:

• expected:Connection Response: Code: [3 (0x03)] Code Identifier: (lt)WildCard: Exists(gt) Length: [8 (0x0008)] Destination CID: (lt)WildCard: Exists(gt) Source CID: [64 (0x0040)] Result: [3 (0x0003)] Connection refused - Security block Status: (lt)WildCard: Exists(gt), but received:Connection Response: Code: [3 (0x03)] Code Identifier: [1 (0x01)] Length: [8 (0x0008)] Destination CID: [64 (0x0040)] Source CID: [64 (0x0040)] Result: [0 (0x0000)] Connection Successful Status: [0 (0x0000)] No further information available

And HCI logs: < HCI Command: Read Encrypti.. (0x05|0x0008) plen 2 Handle: 14 Address: 00:1B:DC:F2:24:10 (Vencer Co., Ltd.)

HCI Event: Command Complete (0x0e) plen 7 Read Encryption Key Size (0x05|0x0008) ncmd 1 Status: Success (0x00) Handle: 14 Address: 00:1B:DC:F2:24:10 (Vencer Co., Ltd.) Key size: 7 ACL Data RX: Handle 14 flags 0x02 dlen 12 L2CAP: Connection Request (0x02) ident 1 len 4 PSM: 4097 (0x1001) Source CID: 64 < ACL Data TX: Handle 14 flags 0x00 dlen 16 L2CAP: Connection Response (0x03) ident 1 len 8 Destination CID: 64 Source CID: 64 Result: Connection successful (0x0000) Status: No further information available (0x0000)

The Bluetooth L2CAP stack accepted incoming connections even when the negotiated BR/EDR encryption key size was below the policy requirement (e.g., <16 bytes for Security Mode 4 Level 4), effectively allowing a downgrade to weaker encryption. The fix adds an explicit key-size check for non-SDP PSMs and rejects such connections with SECURITY BLOCK. Practically, a nearby device could trigger this only over an established encrypted link with a too-short key, resulting in policy bypass and weaker protection rather than a kernel crash.

The CVSS score(s) listed for this vulnerability do not reflect the associated product's status, and are included for informational purposes to better understand the severity of this vulnerability.