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CVE-2026-63981— net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow

Quick assessment

Affected
Linux Linux
Exploitation
No confirmed in-the-wild exploitation; assess based on exposure
Recommended action
Check the vendor advisory and references for a fixed version. If immediate upgrade is impossible, restrict exposure and increase monitoring.

Linux kernel是美国Linux基金会开源的一款操作系统内核。 Linux kernel 6.19版本存在安全漏洞，该漏洞源于blockcast递归绕过导致栈溢出，未经身份验证的用户可通过创建共享TC出口块的设备并安装mirred blockcast规则，引发无限递归循环直至内核栈溢出。

AI Predicted 7.8 Difficulty: Easy EPSS 0.23% · P13

Possible ATT&CK Techniques 1 AI

T1134 · Access Token Manipulation

Affected Version Matrix 7

VendorProduct Version RangeStatus
Linux Linux 906736728cea480a85803c67fafb1b0e78491922< 25fc9352590f5ef21ebf290432bd768b336693bc affected
fe946a751d9b52b7c45ca34899723b314b79b249< 34457de389fb64a01fdcc71177dfebe65fd2d362 affected
fe946a751d9b52b7c45ca34899723b314b79b249< a005fa5d7502eefec7ee6e1c01adadc06de2f9ad affected
6.19 affected
< 6.19 unaffected
7.0.12≤ 7.0.* unaffected
7.1≤ * unaffected
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I. Basic Information for CVE-2026-63981

Vulnerability Information
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Vulnerability Title
net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow
Source: CVE Program / CVE List V5
Vulnerability Description
In the Linux kernel, the following vulnerability has been resolved: net/sched: act_mirred: Fix blockcast recursion bypass leading to stack overflow tcf_mirred_act() checks sched_mirred_nest against MIRRED_NEST_LIMIT (4) to prevent deep recursion. However, when the action uses blockcast (tcfm_blockid != 0), the function returns at the tcf_blockcast() call BEFORE reaching the counter increment. As a result, the recursion counter never advances and the limit check is entirely bypassed. When two devices share a TC egress block with a mirred blockcast rule, a packet egressing on device A is mirrored to device B via blockcast; device B's egress TC re-enters tcf_mirred_act() via blockcast and mirrors back to A, creating an unbounded recursion loop: tcf_mirred_act -> tcf_blockcast -> tcf_mirred_to_dev -> dev_queue_xmit -> sch_handle_egress -> tcf_classify -> tcf_mirred_act -> (repeat) This recursion continues until the kernel stack overflows. The bug is reachable from an unprivileged user via unshare(CLONE_NEWUSER | CLONE_NEWNET): user namespaces grant CAP_NET_ADMIN in the new network namespace, which is sufficient to create dummy devices, attach clsact qdiscs with shared blocks, and install mirred blockcast filters. BUG: TASK stack guard page was hit at ffffc90000b7fff8 Oops: stack guard page: 0000 [#1] SMP KASAN NOPTI CPU: 2 UID: 1000 PID: 169 Comm: poc Not tainted 7.0.0-rc7-next-20260410 RIP: 0010:xas_find+0x17/0x480 Call Trace: xa_find+0x17b/0x1d0 tcf_mirred_act+0x640/0x1060 tcf_action_exec+0x400/0x530 basic_classify+0x128/0x1d0 tcf_classify+0xd83/0x1150 tc_run+0x328/0x620 __dev_queue_xmit+0x797/0x3100 tcf_mirred_to_dev+0x7b1/0xf70 tcf_mirred_act+0x68a/0x1060 [repeating ~30+ times until stack overflow] Kernel panic - not syncing: Fatal exception in interrupt Fix this by incrementing sched_mirred_nest before calling tcf_blockcast() and decrementing it on return, mirroring the non-blockcast path. This ensures subsequent recursive entries see the updated counter and are correctly limited by MIRRED_NEST_LIMIT.
Source: CVE Program / CVE List V5
CVSS Information
N/A
Source: CVE Program / CVE List V5
Vulnerability Type
N/A
Source: CVE Program / CVE List V5
Vulnerability Title
Linux kernel 安全漏洞
Source: CNNVD (China National Vulnerability Database)
Vulnerability Description
Linux kernel是美国Linux基金会开源的一款操作系统内核。 Linux kernel 6.19版本存在安全漏洞，该漏洞源于blockcast递归绕过导致栈溢出，未经身份验证的用户可通过创建共享TC出口块的设备并安装mirred blockcast规则，引发无限递归循环直至内核栈溢出。
Source: CNNVD (China National Vulnerability Database)
CVSS Information
N/A
Source: CNNVD (China National Vulnerability Database)
Vulnerability Type
N/A
Source: CNNVD (China National Vulnerability Database)

Affected Products

Vendor Product Affected Versions CPE Subscribe
Linux Linux 906736728cea480a85803c67fafb1b0e78491922 ~ 25fc9352590f5ef21ebf290432bd768b336693bc -
Linux Linux 6.19 -

II. Public POCs for CVE-2026-63981

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III. Intelligence Information for CVE-2026-63981

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Patches & Fixes for CVE-2026-63981 (3)

Same Patch Batch · Linux · 2026-07-19 · 429 CVEs total

CVE-2026-63795 10.0 CRITICAL 9p: avoid putting oldfid in p9_client_walk() error path
CVE-2026-63808 9.8 CRITICAL exfat: fix potential use-after-free in exfat_find_dir_entry()
CVE-2026-64122 9.8 CRITICAL net/mlx5e: Fix use-after-free in mlx5e_tx_reporter_timeout_recover
CVE-2026-64055 9.8 CRITICAL net: ethernet: cortina: Carry over frag counter
CVE-2026-64046 9.8 CRITICAL net: tls: prevent chain-after-chain in plain text SG
CVE-2026-63994 9.8 CRITICAL tunnels: load network headers after skb_cow() in iptunnel_pmtud_build_icmp[v6]()
CVE-2026-64142 9.8 CRITICAL ksmbd: close durable scavenger races against m_fp_list lookups
CVE-2026-64132 9.8 CRITICAL ipv6: ioam: refresh hdr pointer before ioam6_event()
CVE-2026-53399 9.8 CRITICAL nfsd: release layout stid on setlease failure
CVE-2026-53398 9.8 CRITICAL NFSD: Fix SECINFO_NO_NAME decode error cleanup
CVE-2026-63993 9.8 CRITICAL vxlan: do not reuse cached ip_hdr() value after skb_tunnel_check_pmtu()
CVE-2026-64089 9.8 CRITICAL batman-adv: tt: fix negative last_changeset_len
CVE-2026-64025 9.8 CRITICAL bpf, skmsg: fix verdict sk_data_ready racing with ktls rx
CVE-2026-64113 9.8 CRITICAL ixgbevf: fix use-after-free in VEPA multicast source pruning
CVE-2026-64136 9.8 CRITICAL smb: client: protect tc_count increment in smb2_find_smb_sess_tcon_unlocked()
CVE-2026-63888 9.8 CRITICAL scsi: target: iscsi: Fix CRC overread and double-free in iscsit_handle_text_cmd()
CVE-2026-63984 9.8 CRITICAL ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()
CVE-2026-64037 9.8 CRITICAL wifi: iwlwifi: mld: fix TSO segmentation explosion when AMSDU is disabled
CVE-2026-63800 9.8 CRITICAL pNFS: Fix use-after-free in pnfs_update_layout()
CVE-2026-64091 9.8 CRITICAL batman-adv: tt: fix TOCTOU race for reported vlans

Showing top 20 of 429 CVEs. View all on vendor page &rarr; →

IV. Related Vulnerabilities

Same product: Linux
Same vendor: Linux

V. Comments for CVE-2026-63981

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