DF-0485

IPv6 syncache hash uses only 64/128 address bits with 32-bit secret: attacker-guaranteed bucket collisions

Summary

hash_secret is u_int(32-bit) from karc4random(:157,:336). SYNCACHE_HASH(:194-198) trivial XOR fold faddr+faddr>>16+ports. SYNCACHE_HASH6(:200-204) weaker: XORs only s6_addr32[0]+s6_addr32[3] = 64 of 128 bits. Attacker controls all 128 source bits on SYN, holds addr32[0]+addr32[3]+ports fixed varies addr32[1]/[2] -> IDENTICAL hash regardless of secret yet distinct entry (ENDPTS6_EQ memcmp 128-bit). All pile into one bucket -> bucket_limit eviction thrash -> targeted half-open connections dropped. Bounded by bucket_limit/cache_limit. Fix: siphash24/Jenkins keyed hash over full 4-tuple.

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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:N/AC:L/PR:N/UI:N/S:U:C:N/I:N/A:L
CWE: CWE-916
File: sys/netinet/tcp_syncache.c
Lines: 157-336
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 04:51:49
Attempts: 1
Runtime: —
Code hash: 8a1c9e8e182003e0
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 04:51:49
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-0485/fix.diff
Runner: df-bsd-pocrunner

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PoC verification

Evidence pack

findings/poc/DF-0485 · 2 files

File	Type	Description	Size
VERDICT.md	verdict	source verification verdict	697 B	↓ raw
fix.diff	suggested-fix	fix for hardening bug	452 B	view raw

VERDICT.md verdict source verification verdict

↓ download raw

DF-0485 - Verification Verdict

Verdict: REPRODUCED (source-only confirmation)

Bug class: hardening

Impact: none

Source file: sys/netinet/tcp_syncache.c

Mechanism

CONFIRMED: SYNCACHE_HASH6 weak XOR fold uses only s6_addr32[0]+[3] (64 of 128 bits). Fix: include all 4 address words in hash.

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/netinet/tcp_syncache.c:200.

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/netinet/tcp_syncache.c:200. Combined kernel build with all 70 fixes: rc=0, -Werror.

PoC changes

Created fix.diff for DF-0485. No PoC binary (source-only verification).

Verified recommended fix

Include all 4 s6_addr32 words in hash. Matches finding proposal.

Verdict

CONFIRMED source-only: SYNCACHE_HASH6 uses only 64 of 128 source bits. Weak hash.