CVE-2026-74434 in Linux
Summary
by MITRE • 08/15/2026
In the Linux kernel, the following vulnerability has been resolved:
rxrpc: Don't move a peeked OOB message onto the pending queue
rxrpc_recvmsg_oob() takes a received oob message off recvmsg_oobq and, if a response is needed, moves it onto the pending_oobq tree. However, only the unlink from recvmsg_oobq is guarded by MSG_PEEK; the move onto pending_oobq always runs.
As a result, reading a challenge with MSG_PEEK leaves the skb on recvmsg_oobq while also adding it to pending_oobq. Since struct sk_buff's rbnode shares storage with its next and prev pointers, rb_insert_color() overwrites the list linkage, and the skb, which holds a single reference, becomes reachable from both queues at once.
When the socket is closed both queues are drained in turn. While draining recvmsg_oobq, __skb_unlink() follows the next and prev pointers that rbnode has overwritten and writes to a bad address. Also, as the skb holds a single reference but is freed from each queue, both the skb and the connection reference it holds are released twice. This leads to memory corruption and to a use-after-free caused by the connection refcount underflow.
MSG_PEEK does not consume the message from the queue, so only unlink it from recvmsg_oobq and then move it onto pending_oobq or free it when the message is actually consumed.
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Analysis
by VulDB Data Team • 08/15/2026
The vulnerability described represents a critical memory corruption issue within the Linux kernel's rxrpc subsystem that stems from improper handling of out-of-band message queuing during peek operations. This flaw exists in the rxrpc_recvmsg_oob() function which manages the reception and processing of out-of-band messages. The core technical problem occurs when applications use the MSG_PEEK flag to inspect messages without consuming them from the queue, creating a race condition between queue management operations.
The vulnerability manifests through improper synchronization between two distinct queue operations within the rxrpc subsystem. When an out-of-band message is received and processed with MSG_PEEK, the system correctly removes it from the recvmsg_oobq queue but fails to apply the same peek semantics to the subsequent move operation to pending_oo
This inconsistency creates a scenario where the same socket buffer (skb) becomes simultaneously reachable through two different queue structures. The struct sk_buff data structure uses an rbnode field that shares memory storage with its next and prev pointers, enabling a dangerous overwrite condition during the red-black tree insertion process.According to CWE classification, this vulnerability maps to CWE-121: Stack-based Buffer Overflow and CWE-415: Double Free, as it involves both memory corruption through pointer overwrites and improper resource management leading to double reference counting. The ATT&CK framework categorizes this under T1059.008: Command and Scripting Interpreter: Python and T1566: Phishing, though more specifically relates to T1499.004: Endpoint Denial of Service - Network Denial of Service through kernel-level corruption.
The operational impact of this vulnerability extends beyond simple memory corruption to potentially enable arbitrary code execution or system crashes. When the socket is closed and both queues are drained in sequence, the __skb_unlink() function attempts to traverse list structures that have been overwritten by rb_insert_color() operations. This leads to writes to invalid memory addresses and ultimately causes a use-after-free condition when the connection reference count underflows due to double freeing of resources.
The fix requires implementing proper peek semantics throughout the entire message handling pipeline, ensuring that when MSG_PEEK is specified, the message is only unlinked from recvmsg_oobq without being moved to pending_oobq or freed until actual consumption occurs. This modification aligns with standard socket programming practices where peek operations should not alter queue state, maintaining consistency with POSIX socket semantics and preventing the dangerous concurrent access patterns that lead to memory corruption.
This vulnerability demonstrates the complexity of kernel-level resource management in network subsystems, where seemingly minor synchronization issues can cascade into critical security flaws. The issue highlights the importance of careful consideration of atomic operations in kernel code, particularly when dealing with shared data structures like red-black trees and linked lists that share underlying memory layouts. Proper implementation of peek semantics requires maintaining queue integrity while ensuring that reference counting mechanisms operate correctly across all access patterns. The vulnerability serves as a reminder of the critical need for thorough testing of edge cases in kernel networking code, especially around message queuing operations and resource management scenarios involving multiple concurrent access patterns.