CVE-2026-98531 in Linux
Summary
by MITRE • 10/18/2026
In the Linux kernel, the following vulnerability has been resolved:
ipv6: sr: enforce exact attribute length for SEG6_ATTR_DST
In seg6_genl_policy, SEG6_ATTR_DST is defined with .type = NLA_BINARY and .len = sizeof(struct in6_addr). For NLA_BINARY, .len only enforces the maximum payload length and permits shorter payloads (e.g., 0 bytes). When seg6_genl_set_tunsrc() copies sizeof(struct in6_addr) bytes via kmemdup(val, sizeof(*val), GFP_KERNEL), a short SEG6_ATTR_DST attribute triggers a 16-byte out-of-bounds read past skb->tail into uninitialized skb->head memory, which is stored in sdata->tun_src and leaked back to userspace via SEG6_CMD_GET_TUNSRC.
Switch SEG6_ATTR_DST in seg6_genl_policy to NLA_POLICY_EXACT_LEN(sizeof(struct in6_addr)) so that generic netlink validation rejects any attribute whose length is not exactly sizeof(struct in6_addr) with -ERANGE.
Tested in QEMU against Linux 7.3.0-rc3 by sending a SEG6_CMD_SET_TUNSRC Generic Netlink message with a 0-byte SEG6_ATTR_DST attribute followed by SEG6_CMD_GET_TUNSRC. On the unfixed kernel, SEG6_CMD_SET_TUNSRC succeeds (err = 0) and SEG6_CMD_GET_TUNSRC leaks 16 bytes of uninitialized kernel heap memory (tun_src = 836a61ecc4d25a1042a8d60411cfb378); with this patch applied, SEG6_CMD_SET_TUNSRC is rejected by netlink policy validation with -ERANGE (-34) and tun_src remains zeroed.
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Analysis
by VulDB Data Team • 10/18/2026
The Linux kernel's IPv6 Segment Routing implementation contains a critical input validation flaw within the generic netlink interface used to configure tunnel source addresses. Specifically, the SEG6_ATTR_DST attribute in seg6_genl_policy is defined with NLA_BINARY type and a length constraint that only enforces a maximum payload size rather than an exact match. This configuration allows user-space applications to send attributes with payloads shorter than the expected sizeof(struct in6_addr), including zero-length payloads. When such a short attribute is processed, the kernel attempts to copy sixteen bytes into memory using kmemdup without verifying if the source buffer contains sufficient data.
This insufficient validation leads directly to an out-of-bounds read vulnerability where the kernel reads uninitialized memory from beyond the end of the socket buffer tail and potentially into adjacent heap regions. The copied data is then stored in sdata->tun_src, a structure member intended for valid IPv6 addresses. Because the source memory was not initialized prior to this copy operation, it contains arbitrary residual data that may include sensitive kernel information such as pointers, cryptographic keys, or other process-specific details. This constitutes an Information Leak vulnerability where local users can extract internal kernel state through subsequent queries.
The operational impact of this flaw is realized when a user-space application issues a SEG6_CMD_GET_TUNSRC command after having previously sent a malformed SEG6_CMD_SET_TUNSRC message with an undersized attribute. In the vulnerable configuration, the set operation succeeds silently despite the invalid input, and the get operation subsequently returns the leaked uninitialized memory contents to the caller. This allows unprivileged local users to read kernel heap memory, potentially facilitating further exploitation or reconnaissance against system security boundaries. The vulnerability aligns with CWE-20 Improper Input Validation as the root cause and CWE-200 Exposure of Sensitive Information to an Unauthorized Actor regarding the impact.
To mitigate this risk, the generic netlink policy for SEG6_ATTR_DST must be updated to use NLA_POLICY_EXACT_LEN instead of the permissive binary length constraint. This change ensures that any attribute not exactly matching the size of struct in6_addr is rejected by the kernel's netlink validation layer with an ERANGE error code before any memory copy operations occur. By enforcing strict length requirements at the input boundary, the kernel prevents the out-of-bounds read from being triggered and ensures that tun_src remains zeroed or properly initialized when valid inputs are provided. This fix aligns with ATT&CK technique T1083 File and Directory Discovery in contexts where attackers probe for information leakage vectors within operating system kernels.