CVE-2026-63984 ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

Q: Is CVE-2026-63984 being exploited?

There is no known exploitation of CVE-2026-63984 in the wild and no public exploit code.

Critical 9.8 Published 19 Jul 2026 Updated 19 Oct 2026

An integer overflow in the Linux kernel IPv6 RPL implementation allows memory corruption and potential denial of service via crafted packets.

Exploited in the wild: Not known
Login needed: Not needed
Attackable: Over the internet
Victim action needed: Not needed
Fix: Available

What is CVE-2026-63984?

CVE-2026-63984 is a critical vulnerability in Linux, published on 19 Jul 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.

Official description

In the Linux kernel, the following vulnerability has been resolved:

ipv6: rpl: fix hdrlen overflow in ipv6_rpl_srh_decompress()

ipv6_rpl_srh_decompress() computes:

outhdr->hdrlen = (((n + 1) * sizeof(struct in6_addr)) >> 3);

hdrlen is __u8. For n >= 127 the result exceeds 255 and silently truncates. With n=127 (cmpri=15, cmpre=15, pad=0, hdrlen=16):

(128 * 16) >> 3 = 256, truncated to 0 as __u8

The caller in ipv6_rpl_srh_rcv() then places the compressed header at buf + ((ohdr->hdrlen + 1) << 3). With hdrlen=0 this is buf + 8, but the decompressed region occupies buf[0..2055] (8-byte header plus 128 full addresses). The compressed header overlaps the decompressed data, and ipv6_rpl_srh_compress() writes into this overlap, corrupting the routing header of the forwarded packet.

The existing guard at exthdrs.c:546 checks (n + 1) > 255, which prevents n+1 from overflowing unsigned char (the segments_left field), but does not prevent the computed hdrlen from overflowing __u8. n=127 passes because 128 <= 255, yet hdrlen=256 does not fit.

Tighten the bound to (n + 1) > 127. This caps n at 126, giving hdrlen = (127 * 16) >> 3 = 254, which fits in __u8. The compressed header then lands at buf + ((254 + 1) << 3) = buf + 2040, exactly past the decompressed region (buf[0..2039]). No overlap. 127 segments is well beyond any realistic RPL deployment.

Weakness: CWE-190

Which versions are affected by CVE-2026-63984?

How do I fix CVE-2026-63984?

Apply the vendor's patch.

Is CVE-2026-63984 being exploited?