CVE-2026-99042

Published Oct 23, 2026

Description

In the Linux kernel, the following vulnerability has been resolved: io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6 io_connect_bpf_populate() fills connect.port and connect.v6_addr for AF_INET6 only when addr_len >= sizeof(struct sockaddr_in6), i.e. 28 bytes. The IPv6 connect paths accept SIN6_LEN_RFC2133 (24 bytes), which already holds sin6_port and sin6_addr. io_connect() passes addr_len to the socket layer unchanged. At addr_len 24 the filter sees family AF_INET6 with port and address zero, while the kernel connects to the real destination. A filter that denies an IPv6 port or address is bypassed. Denying connect.port == P rejects [::1]:P with -EACCES at addr_len 28 but connects at addr_len 24. Use SIN6_LEN_RFC2133 as the threshold. sin6_scope_id stays zero when absent. The threshold mismatch was found by an AI coding assistant (Claude Code) comparing this hook against the net/ipv6 length checks on my instructions; it also drafted the reproducer and this patch. I verified the bypass and the fix in a VM on 7.3-rc6 and that 7.2 has the same code.

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References

Frequently Asked Questions

What is CVE-2026-99042? +
In the Linux kernel, the following vulnerability has been resolved: io_uring/net: populate CONNECT filter fields for 24-byte sockaddr_in6 io_connect_bpf_populate() fills connect.port and connect.v6_addr for AF_INET6 only when addr_len >= sizeof(struct sockaddr_in6), i.e. 28 bytes. The IPv6 connect paths accept SIN6_LEN_RFC2133 (24 bytes), which already holds sin6_port and sin6_addr. io_connect() passes addr_len to the socket layer unchanged. At addr_len 24 the filter sees family AF_INET6 with port and address zero, while the kernel connects to the real destination. A filter that denies an IPv6 port or address is bypassed. Denying connect.port == P rejects [::1]:P with -EACCES at addr_len 28 but connects at addr_len 24. Use SIN6_LEN_RFC2133 as the threshold. sin6_scope_id stays zero when absent. The threshold mismatch was found by an AI coding assistant (Claude Code) comparing this hook against the net/ipv6 length checks on my instructions; it also drafted the reproducer and this patch. I verified the bypass and the fix in a VM on 7.3-rc6 and that 7.2 has the same code.
How do I check if I'm vulnerable to CVE-2026-99042? +
You can use Secably's free Website Scanner to check your website for known vulnerabilities. For infrastructure scanning, use the Port Scanner to identify exposed services that may be affected. Check the vendor advisories linked above for specific patch and version information.

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