CVE-2026-74598: ipv6: fix Route Information option length validation
In the Linux kernel, the following vulnerability has been resolved:
ipv6: fix Route Information option length validation
rt6routercv() validates the Route Information option (RFC 4191) length against the prefix length, but both checks are off by one.
rinfo->length is the ND option length in units of 8 octets and it includes the 8-byte option header, so an option carrying N bytes of prefix has length == 1 + N/8. RFC 4191 section 2.3 requires length 3 when Prefix Length is greater than 64, and 2 or 3 when it is greater than 0. The code accepts length >= 2 and length >= 1 respectively.
ipv6addrprefix() then copies prefixlen/8 bytes out of rinfo->prefix, so a Router Advertisement with (prefixlen=128, length=2) or (prefixlen=64, length=1) makes the kernel read up to 8 bytes past the end of the option. Those bytes end up in the prefix of the route that gets installed, so they are visible to userspace:
# RA with a Route Information option (prefixlen=128, length=2) # followed by a source link-layer address option, 01 01 de ad be ef ca fe $ ip -6 route show 2001:db8:dead:beef:101:dead:beef:cafe via fe80::1234 dev veth0 proto ra ^^^^^^^^^^^^^^^^^^ the next option, read out of bounds
When the Route Information option is the last one in the packet, those eight bytes come from the skb tail room instead.
Reject the option lengths RFC 4191 does not allow.
Affected Software
Remediation
Recommended actions to resolve this vulnerability, in priority order.
- Upgrade
Upgrade to a fixed release to a version that resolves this vulnerability.
Fixed in 6.6.152.1-1
Event History
Frequently Asked Questions
Who can exploit this issue in practice?
An attacker must be able to send a crafted IPv6 Router Advertisement containing a malformed Route Information option to the affected host. The attacker-controlled advertisement can cause the kernel to read bytes from the option that follows it.
What information can be exposed?
Up to 8 bytes beyond the malformed Route Information option can be copied into the installed IPv6 route prefix. Those bytes may be visible to userspace through route-listing tools, as demonstrated when bytes from a subsequent source link-layer address option appeared in the route.
How can I tell whether a system has been affected?
Inspect IPv6 routes installed with the Router Advertisement protocol for unexpected prefix components that resemble data from a subsequent Router Advertisement option. The example route output shows leaked bytes incorporated into the route prefix.