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

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

- **Severity:** Critical 9.8 (`CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:H`)
- **Exploited in the wild:** Not known
- **Login needed:** Not needed
- **Attackable:** Over the internet
- **Victim action needed:** Not needed
- **Fix available:** Yes
- **Published:** 19 Jul 2026
- **Updated:** 19 Oct 2026
- **Also known as:** BELL-CVE-2026-63984, DEBIAN-CVE-2026-63984, GHSA-6hh9-w4p2-c5qm, ROOT-OS-UBUNTU-2204-CVE-2026-63984, UBUNTU-CVE-2026-63984

## 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 (Integer Overflow or Wraparound)

## 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?

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

## Fix prompt for AI coding agents

```text
Check this codebase for CVE-2026-63984 and fix it if it is affected.

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

Affected product: Linux. No package-level version data is available, so check the vendor advisory linked from the reference page.

Task:
1. Search the dependency manifests and lockfiles (for example package.json, package-lock.json, requirements.txt, poetry.lock, uv.lock, pom.xml, build.gradle, go.mod, Cargo.lock, Gemfile.lock) and any Dockerfiles for the affected packages, including transitive dependencies.
2. If an affected version is in use, upgrade it to the fixed version for its release line with the project's package manager, so the lockfile is regenerated. Prefer the smallest upgrade that fixes it.
3. If there is no fix, or the upgrade would break the build, do not guess: explain the risk and the safest workaround instead.
4. Run the tests and summarise what changed and why.
5. If nothing in the project is affected, say so and change nothing.

Reference: https://insights.sbomify.com/CVE-2026-63984/ (Markdown: https://insights.sbomify.com/CVE-2026-63984/index.md)
```

## Sources

- CVE record: https://www.cve.org/CVERecord?id=CVE-2026-63984
- NVD: https://nvd.nist.gov/vuln/detail/CVE-2026-63984
- OSV: https://osv.dev/vulnerability/CVE-2026-63984

---
From sbomify Insights: https://insights.sbomify.com/CVE-2026-63984/
Summaries are generated and may contain mistakes; the official description above is the record.