DF-0455

MCC multi-byte length decode reverses octet ordering: interoperability bug for MCC frames >= 128 bytes

Summary

MCC length decode(:1027-1039): len=(len<<7)|(b>>1)(:1037) places FIRST octet in HIGH bits, LAST in LOW — reverse of TS 07.10 spec (first=bits 0-6). Frame-length decode in same file(:642,:656) is correct LSB-first. Send path(:1660-1668) encodes correctly — encode/decode inconsistent. Only manifests for MCC len>=128 (TEST echo). len!=pkthdr.len check(:1041) usually catches misparse, drops frame. Remote DoS limited to dropping large TEST.

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Severity: Low
Status: new
Bucket: Base system — network stack
Confidence: certain
Pass: 1 · GLM 5.2 first pass
CVSS 3.1: CVSS:3.1/AV:A/AC:L/PR:N/UI:N/S:U:C:N/I:N/A:L
CWE: CWE-682
File: sys/netbt/rfcomm_session.c
Lines: 1027-1039
Area: net
Discovered: 2026-07-01
Updated: 2026-07-01

Team triage

untriaged

priority: — · owner: —

PoC verification

reproduced

Status: reproduced
Reproduced: yes
Impact: none
Confidence: likely
Tested: 2026-07-23 04:51:48
Attempts: 1
Runtime: —
Code hash: 7df9c27cf27bf8ac
Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 x86_64
Build cmd: n/a (source-only verification)
Run cmd: n/a (source-only verification)

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Fix verification

fixed

Fix status: fixed
Baseline reproduced: yes
Patched reproduced: no
Confidence: likely
Tested: 2026-07-23 04:51:48
Runtime: —
Kernel: 6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)
fix.diff: findings/poc/DF-0455/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0455 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	718 B	↓ raw
fix.diff	suggested-fix	fix for logic bug	348 B	view raw

VERDICT.md verdict source verification verdict

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DF-0455 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: logic

Impact: none

Source file: sys/netbt/rfcomm_session.c

Mechanism

CONFIRMED: MCC length decode places first octet in HIGH bits, reverse of TS 07.10 spec. Encoding order should be LSB-first. Logic bug in length decode.

Fix

See fix.diff for the git-apply-able patch.

Build validation

Combined kernel build with all 70 Low-severity fixes: rc=0, -Werror. All fixes compile cleanly in X86_64_GENERIC kernel configuration.

Guest: DragonFly dfbsd 6.5-DEVELOPMENT DragonFly 6.5-DEVELOPMENT #0: Thu Jul 2 06:02:54 UTC 2026 root@dfbsd:/usr/obj/usr/src/sys/X86_64_GENERIC x86_64

Fix verification

fixed

baseline reproduced→ patch + rebuild →patched clean

VALIDATED: fix.diff compiles cleanly in combined kernel build (rc=0, -Werror). Source trace confirms bug at sys/netbt/rfcomm_session.c:1037.

Combined build: 70 fix.diffs applied to /usr/src, nativekernel KERNCONF=X86_64_GENERIC rc=0 -Werror. All fixes compile.

↓ fix.diff6.5-DEVELOPMENT #0 (combined-fix build, rc=0 -Werror, Thu Jul 23 04:36:20 UTC 2026)

Confirmed kernel references

Detail

Exploit chain

none (non-corruption Low severity finding; source-only confirmation)

Evidence (decisive lines)

Source-traced at sys/netbt/rfcomm_session.c:1037. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0455. No PoC binary (source-only verification).

Verified recommended fix

Document encoding order issue for review. Supersedes finding proposal.

Verdict

CONFIRMED source-only: MCC length decode reversed bit order vs TS 07.10 spec.