Non-constant-time SMB1 signature verification via bcmp — timing side channel for byte-by-byte signature forgery
Summary
smb_crypt.c:304 bcmp(mtod+14,digest,8) short-circuits on first differing byte. 8-byte HMAC-MD5 signature verification for SMB1 packet integrity. Active MITM on TCP/139/445 can measure timing delta per byte position = byte-by-byte signature-forgery oracle ~2048 trials to forge 8-byte signature allowing injection of attacker-controlled SMB commands into VC client believes integrity-protected. Network jitter makes practical exploitation hard hence likely not certain. Fix: constant-time comparison loop volatile diff |= a[i]^b[i].
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0737 · 2 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | git-apply-able fix for the cited path | 375 B | view raw |
| VERDICT.md | verdict | source-confirmation narrative | 997 B | ↓ raw |
DF-0737 source-confirmation
Verdict: REPRODUCED (source-confirmed)
Impact: leak Confidence: likely
Kernel ref: sys/netproto/smb/smb_crypt.c:304
Mechanism
bcmp short-circuits on first differing byte -> timing side channel on 8-byte SMB1 signature
Confirmation method
Source-trace confirmed the cited code path matches the finding (exact line/condition
verified against sys/). Runtime PoC not exercised for this source-only Low-severity
item; confirmation is by code inspection.
Recommended fix
See fix.diff in this folder (git-apply-able unified diff).
Phase 8 (combined build)
All 64 fixes were batched into one combined patch
(../_batch/combined_64.patch) and applied to in-guest /usr/src. A single
make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under
-Werror (../_batch/fix_build.log, 35374 lines). The GENERIC kernel + all modules
(drm, firewire, usb, netgraph, smbfs, fuse, crypto, vm, pmap) compiled clean.
Fix verification
not_testablenot_testable (source-only): no runtime PoC exercised; fix.diff applies cleanly and the combined single nativekernel build of all 64 fixes completed rc=0 with 0 errors under -Werror (findings/poc/_batch/fix_build.log). Bug source-confirmed in baseline tree. Compile-validation of the fix is the requested Phase-8 deliverable for this batch.
combined build: '=== NK_DONE rc=0 ===' and '>>> Kernel build for X86_64_GENERIC completed on Thu Jul 23 09:17:55 UTC 2026' (0 'error:' lines in 35374-line log).
Confirmed kernel references
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Detail
Exploit chain
none (non-corruption / source-only confirmation; no memory-corruption escalation chain developed for this Low-severity item)
Evidence (decisive lines)
source-only: bcmp short-circuits on first differing byte -> timing side channel on 8-byte SMB1 signature @ sys/netproto/smb/smb_crypt.c:304. Combined fix build: '=== NK_DONE rc=0 ===' / 'Kernel build for X86_64_GENERIC completed', 0 errors under -Werror (findings/poc/_batch/fix_build.log).
PoC changes
authored git-apply-able fix.diff targeting the cited line; source-trace verified the vulnerable path. Evidence pack (VERDICT.md, manifest.json, env.txt, fix.diff) in findings/poc/DF-0737/.
Verified recommended fix
bcmp->timingsafe_bcmp for the SMB1 signature check. matches finding proposal. Full diff in findings/poc/DF-0737/fix.diff.
Verdict
REPRODUCED (source-confirmed). Source-trace confirmed bcmp short-circuits on first differing byte -> timing side channel on 8-byte SMB1 signature at sys/netproto/smb/smb_crypt.c:304 against the audited sys/ tree; exact line/condition verified.
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