CRITICAL Introduced in 5.16
amt HeaderPull UAF
CVE-2026-68302
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H
KernelScan AI8.1HIGH
01Description
In the Linux kernel, the following vulnerability has been resolved: amt: re-read skb header pointers after every pull Several AMT receive and transmit paths cache a pointer into the skb head (ip_hdr(), ipv6_hdr(), eth_hdr() or the AMT message header) and then call a helper that can reallocate that head before the cached pointer is used again. pskb_may_pull(), ip_mc_may_pull(), ipv6_mc_may_pull(), iptunnel_pull_header(), ip_mc_check_igmp() and ipv6_mc_check_mld() can all free the old head and move the data, so a pointer taken before the call dangles afterwards and the later access is a use-after-free of the freed head. The affected sites are: amt_rcv() caches ip_hdr() before amt_parse_type() pulls, then reads iph->saddr. amt_dev_xmit() caches ip_hdr()/ipv6_hdr() before ip_mc_check_igmp()/ ipv6_mc_check_mld() and pskb_may_pull(), then reads the group address. amt_multicast_data_handler() caches eth_hdr() before pskb_may_pull(), then writes the L2 header. amt_membership_query_handler() caches the AMT header, the outer and inner eth_hdr() and ip_hdr() before iptunnel_pull_header() and several pulls, then reads and writes them. amt_igmpv3_report_handler() and amt_mldv2_report_handler() cache ip_hdr()/ipv6_hdr() and the current group record and read the record count from the report header inside the record loop, across the *_mc_may_pull() calls. amt_update_handler() caches ip_hdr() and the AMT membership-update header before pskb_may_pull(), iptunnel_pull_header(), ip_mc_check_igmp() and the report handler, then reads iph->daddr and amtmu->nonce / amtmu->response_mac. Fix each site by either snapshotting the scalar that is used after the pull before the first pull runs, or re-deriving the header pointer from the skb after the last pull that can move the head. Values that are stable across the pull (source and group address, the response MAC and nonce, the record count, the outer source MAC) are snapshotted; pointers that are written through or read repeatedly are re-derived.
02KernelScan AI Analysis
Risk summary
The AMT multicast tunneling driver caches pointers to packet headers and then calls buffer-pulling helpers that may reallocate the packet head, leaving the cached pointers dangling. An attacker who can send UDP packets to a configured AMT interface can trigger use-after-free reads and writes in the kernel, potentially leading to memory corruption, information disclosure, or a kernel crash. The transmit path is also reachable by any local user who can send traffic through an AMT interface.
Vulnerability analysis
The AMT (Automatic Multicast Tunneling) driver caches pointers into network packet headers—IP, IPv6, Ethernet, and AMT-protocol-specific headers—and then calls helper functions such as pull operations that may reallocate the underlying packet buffer to grow the linear data area. After such a reallocation the previously cached pointer refers to freed memory, and every subsequent read or write through it is a use-after-free. The fix addresses each affected code path by either snapshotting the scalar values needed after the pull (source address, group address, nonce, response MAC, record counts) before any pull runs, or by re-deriving header pointers from the packet after the last operation that could move the buffer. The receive path is triggered by incoming UDP packets sent to a configured AMT interface, so a remote attacker on the network can reach it without any privileges on the target host; the transmit path is reachable by any local user who can send multicast traffic through an AMT interface.
Lifecycle
03Fix Versions
| Branch | Introduced | Fixed in | Patch commit |
|---|---|---|---|
| 6.1 | 5.16 | 6.1.183 | 37ff890f9c18 |
| 6.12 | 5.16 | 6.12.101 | 7746d588d42a |
| 6.18 | 5.16 | 6.18.42 | ca0e8b661957 |
| mainline | 5.16 | 7.2 | 3656a79f94c4 |
| 7.1 | 5.16 | 7.1.6 | 7f48e3ddad8e |
| 6.6 | 5.16 | 6.6.148 | 9005b221cb1f |