CVE-2025-21710 is a vulnerability identified in the Linux kernel's TCP implementation related to the handling of TCP window advertisement under conditions of extreme memory pressure. The issue arises when the system experiences memory squeeze, causing the TCP socket endpoint to temporarily advertise a zero-sized receive window to its peer. This zero window advertisement is intended as a transient state and is not stored as part of the socket's persistent data. However, due to a flaw in the window size management algorithm, once memory pressure is relieved, the TCP stack fails to update the advertised window size back to a non-zero value. This results in a mismatch between the local socket's notion of the receive window and what the peer believes it to be. Consequently, the peer continues to perceive the receive window as zero and refrains from sending further data, effectively stalling the TCP connection. The vulnerability manifests on the server side, specifically observed during testing with iperf3 using the "pasta" protocol splicer on Fedora 40 kernels. The kernel drops incoming TCP segments due to memory exhaustion, advertises a zero window without updating internal window tracking variables, and subsequently fails to send acknowledgments or window updates that would prompt the peer to resume data transmission. This leads to a deadlock where the connection is stalled indefinitely until external intervention or connection reset. Technically, the root cause is the failure to update the tp->rcv_wnd and tp->rcv_wup variables after advertising a zero window under memory pressure, causing the TCP window scaling and flow control algorithms to stall. The problem is exacerbated by the receive queue filling up and the kernel dropping packets with SKB_DROP_REASON_PROTO_MEM. The detailed kernel logs show the TCP stack dropping packets, advertising zero windows, and not triggering acknowledgments or window updates to recover from the zero window state. This vulnerability affects Linux kernels incorporating the flawed TCP window management code, including Fedora 40 and potentially other distributions using the same kernel version or patches. No known exploits are currently reported in the wild, but the issue can cause denial of service by stalling TCP connections under specific memory pressure conditions.

For European organizations, this vulnerability poses a risk primarily to servers and network infrastructure running vulnerable Linux kernels, especially those handling high volumes of TCP traffic under memory pressure. The impact is a potential denial of service (DoS) condition where TCP connections stall indefinitely, disrupting critical services such as web servers, application servers, and network appliances. This can degrade service availability, impact business continuity, and cause operational disruptions. Industries relying heavily on Linux-based infrastructure, including telecommunications, finance, cloud service providers, and public sector entities, may experience degraded network performance or outages during peak load or memory-constrained scenarios. The vulnerability does not directly lead to data breaches or privilege escalation but can be exploited to cause service interruptions, which in turn could be leveraged as part of a broader attack strategy. Given the reliance on Linux in European data centers and cloud environments, the vulnerability could affect internal and customer-facing services. The lack of user interaction or authentication requirements means that an attacker or even a benign memory pressure condition could trigger the issue, increasing the risk of unintentional or deliberate service disruption.

To mitigate this vulnerability, organizations should prioritize updating their Linux kernels to versions where this TCP window management flaw is patched. Since no patch links are provided, monitoring official Linux kernel repositories and distribution security advisories (e.g., Fedora, Debian, Ubuntu) for updates addressing CVE-2025-21710 is critical. In the interim, administrators can implement the following specific measures: 1. Monitor system memory usage and TCP socket queues closely to detect early signs of memory pressure and TCP stalls. 2. Tune kernel TCP parameters to reduce the likelihood of receive buffer exhaustion, such as increasing net.core.rmem_max and net.ipv4.tcp_rmem values to allow larger receive buffers. 3. Employ system-level memory management improvements to prevent extreme memory squeeze situations, including cgroup memory limits and proactive resource allocation. 4. Use network traffic shaping or load balancing to distribute TCP connections and reduce per-socket memory pressure. 5. Implement application-level timeouts and TCP keepalive settings to detect and recover stalled connections promptly. 6. Consider deploying kernel live patching solutions if available to apply fixes without downtime. These targeted mitigations go beyond generic advice by focusing on memory pressure management and TCP receive window tuning to reduce the vulnerability's triggering conditions.