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CVE-2026-52998 is a vulnerability in the Linux kernel’s netfilter framework, specifically within the `nfnetlink_osf` module responsible for passive operating system fingerprinting via Netlink. The flaw resides in the `nf_osf_ttl()` function, which performs Time-To-Live (TTL) checks during packet inspection.
The root cause is twofold. First, `nf_osf_ttl()` accesses skb->dev—the network device pointer associated with a packet—without first validating that the pointer is non-NULL. In certain packet paths, `skb->dev` can be NULL, leading to a NULL pointer dereference when the kernel attempts to read device properties for a local interface address lookup. This immediately triggers a kernel panic or system crash, resulting in a denial-of-service condition.
Second, the function uses an `in_dev_for_each_ifa_rcu` loop to iterate over local interface addresses, matching the packet’s source address against them. The logic assumes that if the source address appears to belong to the same subnet as a local interface, the TTL should not be decremented. However, this assumption is invalid in modern virtualized and containerized environments, where network namespaces, bridges, and overlay networks can cause packets to appear as if they originate from the same subnet when they actually do not. This can lead to incorrect TTL evaluation and potentially bypass fingerprinting logic.
The vulnerability was introduced in kernel version 2.6.31 with commit `11eeef41d5f6` and has persisted across numerous stable releases. The fix removes the unsafe device dereference and the interface address loop entirely, replacing the logic with a clean `switch` statement that evaluates the TTL based on the `ttl_check` parameter directly. This eliminates both the NULL dereference path and the flawed subnet assumption.
The CVE was published on June 24, 2026, and affects a wide range of Linux distributions, including Debian, Ubuntu, and Red Hat Enterprise Linux. Patches have been backported to all supported stable kernel branches.
DailyCVE Form
Platform: ……. Linux Kernel
Version: …….. 2.6.31 – 7.0.9
Vulnerability :.. NULL Pointer Dereference (CWE-476)
Severity: ……. Critical (CVSS: 7.5)
date: ………. June 24, 2026
Prediction: ….. July 2026 (patches already released)
What Undercode Say: Analytics & Bash Commands
The vulnerability is triggered in the `nf_osf_ttl()` function within net/netfilter/nfnetlink_osf.c. To determine if a system is vulnerable, check the kernel version:
uname -r
Affected versions (introduced in 2.6.31, fixed in the following commits):
| Kernel Branch | Fixed Commit |
||–|
| 5.10 | f4de0777e455 |
| 5.15 | c996a90f3071 |
| 6.1 | edc806f91229 |
| 6.6 | 5d05de2f0928 |
| 6.12 | 95be653a7679 |
| 6.18 | 79b90a96688e |
| 7.0 | 83fc5dd63455 |
To check if the fix is applied, verify the presence of the patched commit in your kernel source:
git log --oneline | grep -E "f4de0777e455|c996a90f3071|edc806f91229|5d05de2f0928|95be653a7679|79b90a96688e|83fc5dd63455"
For Debian-based systems, check the package status:
apt-cache policy linux-image-$(uname -r)
For RHEL/CentOS:
rpm -q kernel
The vulnerable code pattern (pre-patch) appears as:
static int nf_osf_ttl(const struct sk_buff skb, const struct nf_osf_ttl ttl_check)
{
struct in_device in_dev = __in_dev_get_rcu(skb->dev); // skb->dev not validated
// ...
in_dev_for_each_ifa_rcu(ifa, in_dev) { // loop over interfaces
if (ipv4_is_loopback(ifa->ifa_address) || ...)
continue;
if (skb->sk && inet_sk(skb->sk)->inet_saddr == ifa->ifa_address)
return 0;
}
// ...
}
The patched version replaces the above with a direct `switch` statement on ttl_check->ttl:
switch (ttl_check->check) {
case NF_OSF_TTL_TRUE: return 0;
case NF_OSF_TTL_FALSE: return 1;
// ...
}
Exploit
This vulnerability can be exploited by a remote attacker sending a specially crafted packet to a vulnerable system. The packet must be designed such that:
1. The `skb->dev` pointer is NULL when processed by nf_osf_ttl().
2. The packet reaches the netfilter OS fingerprinting path (e.g., via passive fingerprinting rules enabled with nf_osf).
When these conditions are met, the kernel dereferences the NULL pointer, causing an immediate kernel panic and system crash. This results in a denial-of-service condition.
Exploitation requirements:
- The target system must have the `nfnetlink_osf` module loaded or compiled into the kernel.
- Passive OS fingerprinting must be active (e.g., via `iptables` rules using
-m osf). - The attacker can be remote, as the packet only needs to reach the netfilter hook.
No public exploit is currently available, but the technical details are known, making exploitation feasible for skilled attackers. The attack complexity is considered low, and the required privileges are none (remote unauthenticated).
Protection
Immediate Mitigation:
- Upgrade the kernel to a patched version. The following minimum versions contain the fix:
– 5.10.258
– 5.15.209
– 6.1.175
– 6.6.141
– 6.12.91
– 6.18.33
– 7.0.10
2. For Debian/Ubuntu:
sudo apt update sudo apt install linux-image-$(uname -r) sudo reboot
3. For RHEL/CentOS:
sudo yum update kernel sudo reboot
4. Temporary workaround (if patching is not immediately possible):
– Unload the `nfnetlink_osf` module if not required:
sudo modprobe -r nfnetlink_osf
– Block the module from loading on boot:
echo "blacklist nfnetlink_osf" | sudo tee /etc/modprobe.d/blacklist-nfnetlink_osf.conf
– Disable OS fingerprinting rules in iptables/nftables.
5. Verify the fix:
dmesg | grep -i "nf_osf_ttl"
No NULL dereference panics should appear after patching.
Impact
- Denial of Service (DoS): A remote attacker can crash the system by sending a single crafted packet, causing a kernel panic and forcing a reboot.
- System Availability: Critical for production environments where netfilter OS fingerprinting is used (e.g., intrusion detection, traffic classification, network monitoring).
- Wide Affected Range: The vulnerability exists in all Linux kernels from 2.6.31 (2009) up to 7.0.9, affecting numerous enterprise distributions including Debian, Ubuntu, RHEL, SUSE, and Gentoo.
- Low Complexity: The attack requires no authentication and can be performed remotely, making it highly accessible to attackers.
- No Data Breach: The vulnerability does not allow data exfiltration or privilege escalation; its primary impact is availability.
- Patch Availability: Fixes have been backported to all supported LTS kernels, and distributors have released updates.
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Sources:
Reported By: nvd.nist.gov
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