NFC app refuses to write NDEF after tag was password-protected, misinterprets AUTH0 / lock bytes for NTAG21x

Describe the bug.

Firmware: Momentum & latest Official (latest build as of Oct 2025)
App: Flipper NFC → NTAG21x / Advanced → Write / Read / Detect

Cross posting as I submitted on the flipper firmware repo and got no traction.

Hardware tested:

• Genuine NXP NTAG216
• Dangerous Things NExT implant (NTAG216 core)

Summary

After setting a password on a genuine NTAG216 or NExT implant (using standard NXP authentication scheme), Flipper detects the tag as "password protected" even after the password is cleared and the tag is reset to default (AUTH0=FF).

Other NFC apps (e.g. NFC Tools, TagInfo) correctly detect the tag as unprotected and can read/write NDEF data normally.

The issue appears to be that Flipper’s NFC app incorrectly interprets bits in the AUTH0 or lock byte pages (0x28–0x2B) as indicating password protection when it should not.

Hypothesis

Flipper NFC’s NTAG21x detection logic likely interprets:

• AUTH0 incorrectly (checking for non-0xFF or reserved bits)
• or ACCESS / LOCK bytes as indicating protection even when cleared.

The NTAG216 spec defines:

• AUTH0 = 0xFF → no protection
• ACCESS bits define protection granularity but do not indicate password presence directly.

Flipper might be reading stale or misaligned config bytes

Reproduction

Reproducible command sets (raw Type-2 frames)

Use these command blocks in NFC Tools → Advanced → Raw / Advanced Commands (each comma-separated string is one run/sequence). They are the exact sequences we used to reproduce.

1) Check for a password / protection (diagnostic)

3029,302B,302A,3003,1B4E457854

What this does

• 3029 — READ page 0x29 (contains AUTH0; byte 3 = AUTH0). If AUTH0 != FF protection begins at that page.
• 302B — READ page 0x2B (PWD page). Shows the 4-byte PWD if present.
• 302A — READ page 0x2A (ACCESS byte + flags). Bit flags here (PROT, AUTHLIM, etc.) affect password behaviour.
• 3003 — READ page 0x03 (Capability Container / CC) to confirm NDEF presence and memory sizing.
• 1B4E457854 — PWD_AUTH using the lowercase NExT password (4E 45 78 54). If authentication succeeds the tag returns PACK (2 bytes). This proves whether NExT is accepted.

2) Set password to NExT and protect configuration pages only (leave user/NDEF area writable)

3029,302B,1B4E457854,A22B4E457854,A22CA55A0000,A22900002B00,1B4E457854,3029,302B

What this does

• 3029,302B — Read current config & PWD pages (safety check).
• 1B4E457854 — Attempt auth with NExT (safe; NAK if not set). If tag already protected it lets you proceed.
• A22B4E457854 — WRITE page 0x2B = 4E 45 78 54 (set PWD = NExT).
• A22CA55A0000 — WRITE page 0x2C = A5 5A 00 00 (set PACK = A5 5A, padding zeroes).
• A22900002B00 — WRITE page 0x29 = 00 00 2B 00 (set AUTH0 = 0x2B so protection starts at page 0x2B — this protects config pages but keeps NDEF/user memory writable).
• 1B4E457854 — Authenticate again to verify the password returns PACK.
• 3029,302B — Read back 0x29 / 0x2B to confirm changes.

3) Clear password and return tag to default/unprotected state

1B4E457854,A22B00000000,A22C00000000,A2290000FF00,3029,302B,3003,3004

What this does

• 1B4E457854 — Authenticate with current password NExT (required if config pages are currently protected).
• A22B00000000 — WRITE page 0x2B = 00 00 00 00 (clear PWD).
• A22C00000000 — WRITE page 0x2C = 00 00 00 00 (clear PACK).
• A2290000FF00 — WRITE page 0x29 = 00 00 FF 00 (set AUTH0 = 0xFF which disables password protection).
• 3029,302B — Read back 0x29 and 0x2B to verify AUTH0 = FF and PWD cleared.
• 3003,3004 — Read CC and first NDEF page to confirm the user memory and TLV are intact and writable.

Observed vs expected behaviour

Observed

• After running the “clear password” block, 3029 shows AUTH0 = 0xFF, 302A shows ACCESS = 0x00, and 302B/302C show zeros — tag is unprotected and writable by phone tools.
• Flipper NFC helper still reports “card protected by password AUTH0 or lock bits” and refuses to write saved NDEF via the helper. Raw page writes via Flipper CLI/UI may succeed, indicating the helper (not the hardware) is making the decision to abort.

Expected

• When AUTH0 == 0xFF and ACCESS == 0x00 and PWD/PACK cleared, Flipper should treat the tag as unprotected and allow NDEF writes via the NFC helper.

Suggested fixes for Flipper NFC app

1. Correct AUTH0 interpretation — treat AUTH0 == 0xFF as unprotected and allow writes. Do not conservatively block writes just because lock bytes are nonzero unless CFGLCK/permanent lock bits indicate irreversible config lock.
2. Distinguish permanent vs non-permanent lock bits — interpret page 0x02 lock bytes according to the NTAG datasheet; only treat CFGLCK/irreversible bits as absolute blockers.
3. Prompt for PWD_AUTH when required — if AUTH0 <= first NDEF page or ACCESS indicates protection that will affect the operation, prompt the user to supply a password and attempt PWD_AUTH rather than failing immediately.
4. Add an “Advanced/Force write (unsafe)” option to allow expert users to bypass the helper checks (with clear warnings).
5. Always re-read config immediately before a write — do not rely on cached/paranoid heuristics from earlier reads.

References

• NXP NTAG213/215/216 Data Sheet — Section 8.8.2 AUTH0 definition
• Dangerous Things NExT Documentation

Logs

## Commands & tag dumps (exact used during testing)

### Auth succeeded with `NExT`
1B4E457854
<< 00 00

---

### After clearing & setting `AUTH0 = FF`
A2 29 00 00 FF 00
<< 0A

30 29
<< 00 00 FF 00 00 00 00 00 00 00 00 00 00 00 00 00

30 2B
<< 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 0

---

### CC and NDEF TLV
30 03
<< E1 10 6D 00 03 0B D1 01 07 54 02 65 6E 54 65 73

30 04
<< 03 0B D1 01 07 54 02 65 6E 54 65 73 74 FE 00 00

---

### UID and lock bytes
30 00
<< 04 A7 6E 45

30 01
<< 1A BD 39 80

30 02
<< 1E 48 00 00 // lock/OTP area (non-zero; normal for many tags)


> **Note:** tag is a genuine NXP NTAG216 sticker used for testing.

### Target

Flipper Zero

### Logs

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Anything else?

No response