UBUNTU-CVE-2026-53349: Medium severity ubuntu/linux vulnerability
In the Linux kernel, the following vulnerability has been resolved: netfilter: nfconntrack: destroy stale expectfn expectations on unregister NAT helpers such as nfnath323 store a raw pointer to module text in exp->expectfn (e.g. ipnatq931expect). nfcthelperexpectfnunregister() only unlinks the callback descriptor and never walks the expectation table, so an expectation pending at module removal survives with a dangling exp->expectfn into freed module text. When the expected connection arrives, initconntrack() invokes exp->expectfn(), now a stale pointer into the unloaded module. Reproduced on a KASAN build by loading the H.323 helpers, creating a Q.931 expectation, unloading nfnath323, then connecting to the expected port: Oops: int3: 0000 [#1] SMP KASAN NOPTI RIP: 0010:0xffffffffa06102d1 initconntrack.isra.0 (net/netfilter/nfconntrackcore.c:1862) nfconntrackin (net/netfilter/nfconntrackcore.c:2049) ipv4conntracklocal (net/netfilter/nfconntrackproto.c:223) nfhookslow (net/netfilter/core.c:619) iplocalout (net/ipv4/ipoutput.c:120) tcptransmitskb (net/ipv4/tcpoutput.c:1715) tcpconnect (net/ipv4/tcpoutput.c:4374) tcpv4connect (net/ipv4/tcpipv4.c:345) sysconnect (net/socket.c:2167) Modules linked in: nfconntrackh323 [last unloaded: nfnath323] Reaching the dangling state requires CAPSYSMODULE in the initial user namespace to remove a NAT helper that still has live expectations, so this is a robustness fix; leaving an expectation pointing at freed text is wrong regardless. Add nfcthelperexpectfndestroy(), which walks the expectation table and drops every expectation whose ->expectfn matches the descriptor being torn down. Call it from each NAT helper's exit path after the existing RCU grace period, so no expectation outlives the code it points at and no extra synchronizercu() is introduced. With the fix, the same reproducer runs to completion without the Oops.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
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ubuntu/linux-riscvto a version that resolves this vulnerability.Fixed in 7.0.0-31.31.1 - Compensating control
Implement nf_ct_helper_expectfn_destroy() to walk the expectation table and drop every expectation whose exp->expectfn matches the descriptor being torn down; call it from each NAT helper's exit path after the existing RCU grace period, including helpers such as nf_nat_h323, so no expectation retains a pointer into unloaded module text.
Event History
Frequently Asked Questions
What conditions are required to trigger the failure?
An attacker needs local access with low privileges. The system must have a NAT helper that stores an expectation callback, an expectation must be created, and the helper module must then be unloaded before traffic arrives for the expected connection.
Which helper is demonstrated as affected?
The reported reproduction uses the H.323 NAT helpers: it creates a Q.931 expectation, unloads nf_nat_h323, and then connects to the expected port. The description also indicates that NAT helpers with raw expectation-function pointers can be affected.
What is the likely impact after triggering the stale callback?
When the expected connection arrives, conntrack invokes a function pointer into unloaded module text. The supplied reproduction results in a kernel Oops, making denial of service the stated practical impact.
How can an administrator determine whether the system is exposed to this scenario?
Check whether affected NAT helper modules, including the H.323 helper nf_nat_h323, are loaded and used to create expectations, and whether they can be unloaded while those expectations remain pending. The issue specifically depends on an expectation surviving helper-module removal.