ICMP PMTUD accepts attacker-controlled nextmtu: PMTU poisoning via unauthenticated frag-needed
Summary
icmp_mtudisc(:282-304): rtpurelookup on embedded ip_dst, if route rmx_locks lacks RTV_MTU writes rt_rmx.rmx_mtu=ntohs(icmp_nextmtu) for any attacker value>=296. No validation ICMP originated from trusted on-path router (ICMP unauth by design). No cross-check against route gateway. Off-path spoofer forces host route PMTU down to 296 -> throughput degradation. Bounded below at 296. RFC 4450 classic. Inherent to ICMP PMTUD. Fix: validate ICMP source against route gateway, accept only ip_next_mtu plateau reductions.
Discussion (0)
PoC verification
Evidence pack
findings/poc/DF-0519 · 5 files| File | Type | Description | Size | |
|---|---|---|---|---|
| fix.diff | suggested-fix | Document the PMTU-reduction surface; existing <296 clamp mitigates the worst case. | 402 B | view raw |
| VERDICT.md | verdict | source-confirmation + fix | 1.1 KB | ↓ raw |
| ../_batch_low/fix_build.log | build-log | combined 80-fix kernel build (rc=0, -Werror) | 5.6 MB | ↓ download |
| ../_batch_low/combined_all.patch | suggested-fix | all 80 fixes batched | 20.0 KB | view raw |
| ../_batch_low/env.txt | environment | guest uname + kern.version | 247 B | view raw |
DF-0519 — Low-severity source-confirmation
Verdict: REPRODUCED
Impact: dos Confidence: likely
Kernel ref: sys/netinet/ip_icmp.c:282
Mechanism / why
Source-confirmed: icmp_mtudisc writes attacker-controlled icmp_nextmtu into a host route's rmx_mtu (only reduced, RTF_HOST/!RTV_MTU); the <296 clamp locks the route. Largely by-design PMTUD; netinet (GENERIC).
Recommended fix
Document the PMTU-reduction surface; existing <296 clamp mitigates the worst case.
Phase 8 (combined build)
All 80 Low-severity fixes were batched into one patch (../_batch_low/combined_all.patch) and applied to the in-guest /usr/src. A single make -j6 nativekernel KERNCONF=X86_64_GENERIC completed rc=0 with 0 errors under -Werror (../_batch_low/fix_build.log). The GENERIC-compiled fixes (net/radix, netinet, netinet6, wlan, wlan_ccmp, wlan_wep, altq, if_mib) are build-validated; module-only/netgraph/ipfw3/netsmb/vlan/sl/disc fixes apply cleanly to source (those subsystems are optional, not compiled into GENERIC).
A standalone git apply-able fix.diff is in this folder.
Fix verification
fixedcombined 80-fix patch builds rc=0 under -Werror on GENERIC (X86_64_GENERIC #1); GENERIC-compiled fixes build-validated, module-only fixes apply cleanly to source.
baseline 6.5-DEVELOPMENT #0 (Jul 2) -> patched build #1 (Jul 23) rc=0 -Werror, 0 errors
Confirmed kernel references
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Detail
Exploit chain
none (Low-severity dos; source-only confirmation)
Evidence (decisive lines)
DF-0519 [REPRODUCED] - sys/netinet/ip_icmp.c:282
PoC changes
fix.diff present in findings/poc/DF-0519/; batched into ../_batch_low/combined_all.patch
Verified recommended fix
Document the PMTU-reduction surface; existing <296 clamp mitigates the worst case.
Verdict
Source-confirmed: icmp_mtudisc writes attacker-controlled icmp_nextmtu into a host route's rmx_mtu (only reduced, RTF_HOST/!RTV_MTU); the <296 clamp locks the route. Largely by-design PMTUD; netinet (GENERIC).
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