Require encryption to write the CCCD of an encrypted characteristic (espressif) - #11236
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Require encryption to write the CCCD of an encrypted characteristic (espressif)#11236dhalbert wants to merge 1 commit into
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…espressif) NimBLE derives the auto-generated CCCD's permissions only from the BLE_GATT_CHR_F_NOTIFY_INDICATE_* flags, which were never set, so the CCCD of an encryption-requiring characteristic was writable on an unencrypted link. An unpaired central could subscribe, nothing ever demanded pairing, and its file transfer commands -- Write Commands, which ATT requires the server to silently ignore when unauthorized -- were dropped with no error, hanging the client forever. Now the CCCD write permission follows the characteristic read permission, as in the nordic port. The central's subscribe attempt gets an ATT insufficient-authentication error, which is what makes it initiate pairing. Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>
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Claude found this bug and wrote the fix. I reworked this explanation for clarity.
(Part of the series of small PRs replacing #11178. Independent of the others.)
The problem
NimBLE (used on Espressif) derives the auto-generated CCCD's permissions only from the
BLE_GATT_CHR_F_NOTIFY_INDICATE_ENC/AUTHENflags. Those flags were not set by us, so the CCCD of an encryption-requiring characteristic was writable on an unencrypted link. This meant an unpaired central could subscribe without having to pair.By contrast, the nordic port rejects the unencrypted CCCD write with Insufficient Authentication, and that rejection then makes the OS pair.
Since no pairing happened, subsequent web workflow commands, which sent BLE Write Commands, were dropped with no error. This caused the client to just hang waiting for some file transfer results.
(Note that Write Requests can return an error because the server sends a response, but Write Commands get no response. The BLE ATT protocol requires the server to silently ignore those Write Commands when unauthorized.)
The fix
Set
BLE_GATT_CHR_F_NOTIFY_INDICATE_ENC(or_AUTHEN) whenever the characteristic's read permission requires encryption (or authentication), so the CCCD write permission follows the characteristic read permission, matching the nordic port.Note that upstream NimBLE made this inheritance automatic in apache/mynewt-nimble@bfde0dc (July 2026, first released in NimBLE 1.10.0), calling the old behavior a security bypass. But the NimBLE fork pinned by ESP-IDF 6.0.x is 1.6.0, so it doesn't include this fix.
Testing
Tested on a Metro ESP32-S3 with the web editor on Linux, verified with btmon: the CCCD Write Request is now rejected with Insufficient Authentication (0x05), which triggers SMP pairing and encryption, after which the retried write succeeds and file transfer works. Open characteristics still work without pairing, and a bonded reconnect resumes without a new pairing prompt.