CVE-2026-23397 nfnetlink_osf: validate individual option lengths in fingerprints
A missing validation in the Linux kernel's netfilter OS fingerprinting module allows a remote attacker to cause a system crash via crafted TCP packets.
- Exploited in the wild
- Not known
- Login needed
- Any signed-in user
- Attackable
- Local or same network only
- Victim action needed
- Not needed
- Fix
- Available
What is CVE-2026-23397?
CVE-2026-23397 is a high-severity vulnerability in Linux kernel, published on 26 Mar 2026. It can only be exploited from the same machine or local network, with no action from the victim.
Official description
In the Linux kernel, the following vulnerability has been resolved:
nfnetlink_osf: validate individual option lengths in fingerprints
nfnl_osf_add_callback() validates opt_num bounds and string NUL-termination but does not check individual option length fields. A zero-length option causes nf_osf_match_one() to enter the option matching loop even when foptsize sums to zero, which matches packets with no TCP options where ctx->optp is NULL:
Oops: general protection fault KASAN: null-ptr-deref in range [0x0000000000000000-0x0000000000000007] RIP: 0010:nf_osf_match_one (net/netfilter/nfnetlink_osf.c:98) Call Trace: nf_osf_match (net/netfilter/nfnetlink_osf.c:227) xt_osf_match_packet (net/netfilter/xt_osf.c:32) ipt_do_table (net/ipv4/netfilter/ip_tables.c:293) nf_hook_slow (net/netfilter/core.c:623) ip_local_deliver (net/ipv4/ip_input.c:262) ip_rcv (net/ipv4/ip_input.c:573)
Additionally, an MSS option (kind=2) with length < 4 causes out-of-bounds reads when nf_osf_match_one() unconditionally accesses optp[2] and optp[3] for MSS value extraction. While RFC 9293 section 3.2 specifies that the MSS option is always exactly 4 bytes (Kind=2, Length=4), the check uses "< 4" rather than "!= 4" because lengths greater than 4 do not cause memory safety issues -- the buffer is guaranteed to be at least foptsize bytes by the ctx->optsize == foptsize check.
Reject fingerprints where any option has zero length, or where an MSS option has length less than 4, at add time rather than trusting these values in the packet matching hot path.
Weakness: CWE-125
Which versions are affected by CVE-2026-23397?
It affects 1 product.
Products include Linux kernel.
How do I fix CVE-2026-23397?
Apply the vendor's patch.
Is CVE-2026-23397 being exploited?
There is no known exploitation of CVE-2026-23397 in the wild and no public exploit code.
Other vulnerabilities in Linux kernel
Sources
Also known as AZL-80850, BELL-CVE-2026-23397, DEBIAN-CVE-2026-23397, GHSA-3rcm-5vqm-53w6, ROOT-OS-DEBIAN-11-CVE-2026-23397, ROOT-OS-DEBIAN-12-CVE-2026-23397 and 4 more.