# CVE-2026-99191: calipso: keep BIG TCP payload_len in calipso_skbuff_setattr()

CVE-2026-99191 is fixed in linux on 1 platform by DEBIAN-CVE-2026-99191 (version 6.12.115-2), released November 3, 2026. It is not reported as exploited. Patch Radar priority: Routine.

- Web page: https://patchblox.com/security/cve/CVE-2026-99191
- Priority: Routine
- CVSS: 6.5 (Moderate)
- EPSS: 0.0017
- CISA KEV: not listed
- Exploited (vendor report): no
- Publicly disclosed: no
- Published: Oct 28, 2026
- Vendor advisory: https://msrc.microsoft.com/update-guide/vulnerability/CVE-2026-99191

## Description

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.

## Where it's fixed

| Platform | Product | Fixed in | Fix | Released |
|---|---|---|---|---|
| Linux | linux Debian 13.5 (trixie) | 6.12.115-2 | [DEBIAN-CVE-2026-99191](https://patchblox.com/security/data/fix/linux/debian-cve-2026-99191.md) | Nov 3, 2026 |

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Data as of 2026-11-08T00:53:09Z UTC. Source: Patchblox Patch Radar (https://patchblox.com/security/patch-radar). Patchblox finds out-of-date software on Windows, macOS and Linux and updates it through release rings: https://patchblox.com/demo

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