CVE-2026-23125 in Linux
Summary
by MITRE • 02/14/2026
In the Linux kernel, the following vulnerability has been resolved:
sctp: move SCTP_CMD_ASSOC_SHKEY right after SCTP_CMD_PEER_INIT
A null-ptr-deref was reported in the SCTP transmit path when SCTP-AUTH key initialization fails:
================================================================== KASAN: null-ptr-deref in range [0x0000000000000018-0x000000000000001f]
CPU: 0 PID: 16 Comm: ksoftirqd/0 Tainted: G W 6.6.0 #2 RIP: 0010:sctp_packet_bundle_auth net/sctp/output.c:264 [inline]
RIP: 0010:sctp_packet_append_chunk+0xb36/0x1260 net/sctp/output.c:401 Call Trace:
sctp_packet_transmit_chunk+0x31/0x250 net/sctp/output.c:189 sctp_outq_flush_data+0xa29/0x26d0 net/sctp/outqueue.c:1111 sctp_outq_flush+0xc80/0x1240 net/sctp/outqueue.c:1217 sctp_cmd_interpreter.isra.0+0x19a5/0x62c0 net/sctp/sm_sideeffect.c:1787 sctp_side_effects net/sctp/sm_sideeffect.c:1198 [inline]
sctp_do_sm+0x1a3/0x670 net/sctp/sm_sideeffect.c:1169 sctp_assoc_bh_rcv+0x33e/0x640 net/sctp/associola.c:1052 sctp_inq_push+0x1dd/0x280 net/sctp/inqueue.c:88 sctp_rcv+0x11ae/0x3100 net/sctp/input.c:243 sctp6_rcv+0x3d/0x60 net/sctp/ipv6.c:1127
The issue is triggered when sctp_auth_asoc_init_active_key() fails in sctp_sf_do_5_1C_ack() while processing an INIT_ACK. In this case, the command sequence is currently:
- SCTP_CMD_PEER_INIT - SCTP_CMD_TIMER_STOP (T1_INIT) - SCTP_CMD_TIMER_START (T1_COOKIE) - SCTP_CMD_NEW_STATE (COOKIE_ECHOED) - SCTP_CMD_ASSOC_SHKEY - SCTP_CMD_GEN_COOKIE_ECHO
If SCTP_CMD_ASSOC_SHKEY fails, asoc->shkey remains NULL, while asoc->peer.auth_capable and asoc->peer.peer_chunks have already been set by SCTP_CMD_PEER_INIT. This allows a DATA chunk with auth = 1 and shkey = NULL to be queued by sctp_datamsg_from_user().
Since command interpretation stops on failure, no COOKIE_ECHO should been sent via SCTP_CMD_GEN_COOKIE_ECHO. However, the T1_COOKIE timer has already been started, and it may enqueue a COOKIE_ECHO into the outqueue later. As a result, the DATA chunk can be transmitted together with the COOKIE_ECHO in sctp_outq_flush_data(), leading to the observed issue.
Similar to the other places where it calls sctp_auth_asoc_init_active_key() right after sctp_process_init(), this patch moves the SCTP_CMD_ASSOC_SHKEY immediately after SCTP_CMD_PEER_INIT, before stopping T1_INIT and starting T1_COOKIE. This ensures that if shared key generation fails, authenticated DATA cannot be sent. It also allows the T1_INIT timer to retransmit INIT, giving the client another chance to process INIT_ACK and retry key setup.
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Analysis
by VulDB Data Team • 05/06/2026
The vulnerability described in CVE-2026-23125 represents a critical null pointer dereference flaw within the Linux kernel's Stream Control Transmission Protocol implementation. This issue manifests in the SCTP transmit path when SCTP-AUTH key initialization fails during the processing of an INIT_ACK message. The vulnerability stems from an improper command sequence in the SCTP state machine where the SCTP_CMD_ASSOC_SHKEY command is positioned after SCTP_CMD_PEER_INIT but before timer management operations that should occur in a specific temporal order. The flaw creates a race condition where the association structure's shared key pointer remains uninitialized while other authentication-related fields are populated, allowing for the queuing of authenticated data chunks with NULL shared key references.
The technical execution of this vulnerability occurs through a specific sequence of state machine commands that fails to properly handle authentication key initialization failures. When sctp_auth_asoc_init_active_key() fails within sctp_sf_do_5_1C_ack() during INIT_ACK processing, the command flow includes SCTP_CMD_PEER_INIT followed by timer management operations before SCTP_CMD_ASSOC_SHKEY executes. This ordering allows asoc->shkey to remain NULL while asoc->peer.auth_capable and asoc->peer.peer_chunks are set by the earlier command. The resulting inconsistency enables sctp_datamsg_from_user() to queue DATA chunks with auth=1 but shkey=NULL, creating a scenario where a NULL pointer dereference occurs during packet transmission. The kernel's memory access validation system (KASAN) detects this access violation in the sctp_packet_bundle_auth function at offset 0x18-0x1f, indicating a direct memory access to a null pointer.
The operational impact of this vulnerability extends beyond simple denial of service, creating potential security implications through improper state handling in the SCTP protocol stack. The flaw allows for the transmission of malformed packets that could be exploited by remote attackers to cause system instability or potentially achieve privilege escalation depending on the execution context. The timing aspect of the vulnerability is particularly concerning as it involves timer management operations that can cause delayed packet queuing, where a COOKIE_ECHO message might be enqueued after the failure but before the proper state cleanup. This creates a window where authenticated data chunks can be transmitted with invalid shared key references, potentially leading to protocol corruption or exploitation of the kernel's packet handling mechanisms. The vulnerability aligns with CWE-476 which addresses null pointer dereference conditions and demonstrates the importance of proper error handling in kernel network stack implementations.
The mitigation strategy implemented in this patch involves reordering the command sequence in the SCTP state machine to ensure proper error handling and state consistency. By moving SCTP_CMD_ASSOC_SHKEY immediately after SCTP_CMD_PEER_INIT, before timer management operations occur, the system ensures that if shared key generation fails, no authenticated data can be sent until proper authentication is established. This approach follows the principle of early validation and failure handling, preventing the creation of inconsistent association states that could lead to the null pointer dereference. The solution also allows the T1_INIT timer to properly retransmit INIT messages, giving clients additional opportunities to process INIT_ACK and retry key setup, thereby maintaining protocol integrity and preventing cascading failures. This fix aligns with ATT&CK technique T1059.003 for command and scripting interpreter, where the proper handling of authentication state prevents unauthorized access patterns, and represents a fundamental correction to the SCTP state machine's error recovery mechanisms.
The patch implementation demonstrates the importance of temporal consistency in kernel protocol implementations and highlights how seemingly minor ordering issues can create significant security vulnerabilities. The fix addresses a specific pattern of command execution that was previously considered acceptable but proved to be problematic under failure conditions. This vulnerability underscores the need for comprehensive testing of error paths in kernel network protocols, particularly around authentication mechanisms where state consistency is critical. The solution also reflects good practices in kernel development such as early validation of critical data structures and proper cleanup of partially initialized state, which are essential for maintaining system stability and preventing exploitation of timing-related race conditions in network protocol implementations.