HIGH
netfilter OSF Fingerprint OOB
CVE-2026-23397
CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:N/A:H
KernelScan AI7.1HIGH
01Description
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.
02KernelScan AI Analysis
Risk summary
An attacker with administrative privileges can add malformed OS fingerprints that cause out-of-bounds memory reads during network packet processing. This could lead to kernel crashes, information disclosure, or potentially code execution. While adding fingerprints requires root access, the vulnerability can be triggered by regular network traffic once the malformed fingerprint is installed.
Vulnerability analysis
Root Cause: The nfnl_osf_add_callback() function validates the number of TCP options and string termination when adding OS fingerprints, but fails to validate individual option length fields. This allows malformed fingerprints with zero-length options or MSS options with length < 4 to be added to the kernel. During packet matching, nf_osf_match_one() trusts these length values and can perform out-of-bounds reads when accessing option data.
Attack Surface: Network-facing attack surface through netfilter packet processing. Requires CAP_NET_ADMIN privileges to add malformed fingerprints via netlink, but once added, the vulnerability can be triggered by any network packet that matches the fingerprint pattern. The vulnerability affects TCP packet processing in the netfilter framework.
Fix Mechanism: The patch adds validation in nfnl_osf_add_callback() to reject fingerprints at add-time if any option has zero length or if an MSS option (kind=2) has length < 4. It also validates that the total option length doesn't exceed MAX_IPOPTLEN. This prevents malformed fingerprints from being stored and later causing memory safety issues during packet matching.
03Fix Versions
| Branch | Fixed in | Patch commit |
|---|---|---|
| 5.10 | 5.10.253 | e9cf17b91e73 |
| 5.15 | 5.15.203 | 3c11b5c2436a |
| 6.1 | 6.1.167 | aa0574182c46 |
| 6.12 | 6.12.78 | ec8bf0571b14 |
| 6.18 | 6.18.20 | 3932620c04c2 |
| 6.19 | 6.19.10 | 4c6aa008b913 |
| 6.6 | 6.6.130 | 224f4678812e |
| mainline | 7.0 | dbdfaae96096 |