CVE-2024-26752 in Linux
Summary
by MITRE • 04/03/2024
In the Linux kernel, the following vulnerability has been resolved:
l2tp: pass correct message length to ip6_append_data
l2tp_ip6_sendmsg needs to avoid accounting for the transport header twice when splicing more data into an already partially-occupied skbuff.
To manage this, we check whether the skbuff contains data using skb_queue_empty when deciding how much data to append using ip6_append_data.
However, the code which performed the calculation was incorrect:
ulen = len + skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0;
...due to C operator precedence, this ends up setting ulen to transhdrlen for messages with a non-zero length, which results in corrupted packets on the wire.
Add parentheses to correct the calculation in line with the original intent.
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Analysis
by VulDB Data Team • 08/03/2025
The vulnerability CVE-2024-26752 resides within the Linux kernel's Layer 2 Tunneling Protocol implementation for IPv6 environments, specifically affecting the l2tp_ip6_sendmsg function. This flaw demonstrates a critical issue in packet construction where the transport header is being accounted for twice during data splicing operations, leading to malformed packets on the network. The vulnerability impacts systems utilizing IPv6 L2TP tunnels and represents a significant concern for network infrastructure reliability and security. According to CWE-129, this constitutes an improper input validation issue where the kernel fails to properly validate the message length calculation, while the ATT&CK framework would categorize this under privilege escalation through kernel exploitation techniques that could lead to network traffic manipulation.
The technical root cause stems from an operator precedence error in the C programming language within the kernel's packet processing logic. The original code attempted to determine whether to include transport header length in the calculation by checking if the write queue was empty using skb_queue_empty. However, due to C operator precedence rules, the expression ulen = len + skb_queue_empty(&sk->sk_write_queue) ? transhdrlen : 0; was incorrectly evaluated, causing transhdrlen to be added to len even when the queue contained data. This resulted in packets being constructed with incorrect length fields, as the transport header was effectively counted twice during the ip6_append_data call. The flaw specifically affects the l2tp module's handling of partially occupied socket buffers during data splicing operations, where the kernel attempts to efficiently append additional data to existing network packets.
The operational impact of this vulnerability extends beyond simple packet corruption, potentially enabling attackers to exploit the malformed packet behavior for network disruption or injection attacks. Systems utilizing IPv6 L2TP tunnels become vulnerable to traffic manipulation where packets may be dropped by network devices due to incorrect length fields or could be interpreted inconsistently by receiving endpoints. The vulnerability affects all Linux kernel versions containing the problematic l2tp_ip6_sendmsg implementation, particularly those supporting IPv6 tunneling protocols. Network administrators should be aware that this issue could be exploited to create denial-of-service conditions or potentially enable more sophisticated attacks depending on the specific network environment and the nature of the affected L2TP implementations.
Mitigation strategies for CVE-2024-26752 require immediate kernel updates from vendors that include the corrected implementation with proper parentheses around the operator precedence issue. The fix involves adding explicit parentheses to ensure the conditional calculation evaluates correctly, thereby preventing the double accounting of transport headers. System administrators should prioritize patching affected systems, particularly those running IPv6 L2TP services, and monitor network traffic for anomalies that might indicate exploitation attempts. Organizations should also implement network monitoring solutions capable of detecting malformed L2TP packets that could indicate the presence of this vulnerability or related exploitation attempts. The corrected code ensures that ip6_append_data receives the proper message length by correctly accounting for transport headers only once during the data splicing process, restoring proper packet construction behavior in the Linux kernel's IPv6 L2TP implementation.