CVE-2026-99191
NoneCVSS
—
EPSS
0.2%
Published
Oct 28, 2026
CISA due date
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Cloudanix analysis
A Linux kernel networking bug can make large IPv6 packets with security labels get handled incorrectly, breaking firewalls, connection tracking, and traffic shaping.
- Exploitation
- Remote Unauthenticated
- Attacker needs
- None
- Complexity
- Moderate
- Impact
- Denial Of Service
- Component
- OS kernel
- Class
- Improper Input Validation
- Remediation
- Patch
- Effort
- Medium
- Patch risk
- Low
What it actually does
calipso_skbuff_setattr() incorrectly mutates the IPv6 payload_len field for BIG TCP packets (payload_len==0) and packets crossing IPV6_MAXPLEN, breaking ipv6_payload_len() fallback to skb->len and causing arithmetic wrap for nf_tables, conntrack and sch_cake.
Why it matters to the business
Network devices or servers running affected kernels may silently drop or mishandle large IPv6 flows when CALIPSO labeling is active, leading to connectivity loss, policy bypass, or degraded QoS on production traffic.
How an attack unfolds
An attacker forwards or injects large IPv6 GSO packets toward a destination mapped to a CALIPSO DOI; the kernel path through netlbl_skbuff_setattr invokes the buggy function, corrupting payload_len and causing downstream network stack components to operate on incorrect length values.
How to fix it
Apply the upstream fix that replaces the manual payload arithmetic with ipv6_set_payload_len(skb) in calipso_skbuff_setattr; rebuild and deploy the corrected kernel on all affected systems.
How to confirm the fix worked
Reproduce the provided test cases with 70000-byte GSO packets and confirm payload_len is correctly restored to 0 (above MAXPLEN) or the true length (below MAXPLEN) after the label is applied.
Typical attack path
- Send or forward IPv6 packets exceeding IPV6_MAXPLEN via GSO
- Ensure destination is subject to CALIPSO labeling via netlabel
- Trigger calipso_skbuff_setattr on the forward or output path
- Observe malformed payload_len affecting conntrack, nftables or qdisc decisions
How to detect it
Inspect kernel logs for IPv6 truncation or malformed packet warnings on CALIPSO-enabled interfaces; correlate with nf_tables or conntrack drops on large flows.
What your security team should do
- Prioritize kernel upgrade on all IPv6 routers and hosts that enable CALIPSO or NetLabel
- Monitor EPSS and KEV feeds for any subsequent exploit publication
- Temporarily disable CALIPSO mappings on high-risk segments until patched
The record
In the Linux kernel, the following vulnerability has been resolved: calipso: keep BIG TCP payload_len in calipso_skbuff_setattr() calipso_skbuff_setattr() adjusts payload_len by the length of the option it inserts or resizes: payload = ntohs(ip6_hdr->payload_len); ip6_hdr->payload_len = htons(payload + len_delta); GRO can build BIG TCP packets larger than IPV6_MAXPLEN, and stores payload_len == 0 for them; ipv6_payload_len() then falls back to skb->len. For such a packet forwarded to a destination mapped to CALIPSO, the code above writes len_delta (or 65536 + len_delta when shrinking) into payload_len, and ipv6_payload_len() no longer falls back to skb->len. nf_tables, conntrack and sch_cake rely on it. The same arithmetic also wraps when a packet grows past IPV6_MAXPLEN, and a BIG TCP packet that shrinks below IPV6_MAXPLEN should get its real length back. Set payload_len from the skb length with ipv6_set_payload_len(), which stores 0 above IPV6_MAXPLEN and the real length otherwise, as ipv6_gro_complete() does. The network header was just reset to skb->data, so skb->len - sizeof(*ip6_hdr) is the payload length. Tested on one kernel (arm64, QEMU) with SELinux permissive, a minimal mdp-generated policy (network_peer_controls=1), "netlabelctl calipso add pass doi:16" and a NetLabel mapping of fd00:2::/64 to CALIPSO DOI 16. calipso_skbuff_setattr() was called through netlbl_skbuff_setattr() from a test-only debugfs hook (not part of this patch) adding a level-only label; payload_len after the call: case before fix after fix real length 1000, unlabeled 1016 1016 1016 70000 GSO, payload_len 0 16 0 70016 65530 GSO, grows past MAXPLEN 10 0 65546 65540 GSO with a 24 byte hop header, shrinks below MAXPLEN 65528 65532 65532 1000 with a 24 byte hop header 992 992 992 (0 means ipv6_payload_len() returns the real length.) On the forward path, with a client injecting unlabeled UDP through a router to a server, the router adds the CALIPSO option and the server delivers all datagrams (Udp6InDatagrams 5, Ip6InTruncatedPkts 0) with this patch.
Affected packages
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