Medium

CVE-2025-39770 — net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM

In the Linux kernel, the following vulnerability has been resolved: net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM When performing Generic Segmentation Offload (GSO) on an IPv6 packet that contains extension headers, the kernel incorrectly requests checksum offload if the egress device only advertises NETIF_F_IPV6_CSUM feature, which has a strict contract: it supports checksum offload only for plain TCP or UDP over IPv6 and explicitly does not support packets with extension headers. The current GSO logic violates this contract by failing to disable the feature for packets with extension headers, such as those used in GREoIPv6 tunnels. This violation results in the device being asked to perform an operation it cannot support, leading to a `skb_warn_bad_offload` warning and a collapse of network throughput. While device TSO/USO is correctly bypassed in favor of software GSO for these packets, the GSO stack must be explicitly told not to request checksum offload. Mask NETIF_F_IPV6_CSUM, NETIF_F_TSO6 and NETIF_F_GSO_UDP_L4 in gso_features_check if the IPv6 header contains extension headers to compute checksum in software. The exception is a BIG TCP extension, which, as stated in commit 68e068cabd2c6c53 ("net: reenable NETIF_F_IPV6_CSUM offload for BIG TCP packets"): "The feature is only enabled on devices that support BIG TCP TSO. The header is only present for PF_PACKET taps like tcpdump, and not transmitted by physical devices." kernel log output (truncated): WARNING: CPU: 1 PID: 5273 at net/core/dev.c:3535 skb_warn_bad_offload+0x81/0x140 ... Call Trace: <TASK> skb_checksum_help+0x12a/0x1f0 validate_xmit_skb+0x1a3/0x2d0 validate_xmit_skb_list+0x4f/0x80 sch_direct_xmit+0x1a2/0x380 __dev_xmit_skb+0x242/0x670 __dev_queue_xmit+0x3fc/0x7f0 ip6_finish_output2+0x25e/0x5d0 ip6_finish_output+0x1fc/0x3f0 ip6_tnl_xmit+0x608/0xc00 [ip6_tunnel] ip6gre_tunnel_xmit+0x1c0/0x390 [ip6_gre] dev_hard_start_xmit+0x63/0x1c0 __dev_queue_xmit+0x6d0/0x7f0 ip6_finish_output2+0x214/0x5d0 ip6_finish_output+0x1fc/0x3f0 ip6_xmit+0x2ca/0x6f0 ip6_finish_output+0x1fc/0x3f0 ip6_xmit+0x2ca/0x6f0 inet6_csk_xmit+0xeb/0x150 __tcp_transmit_skb+0x555/0xa80 tcp_write_xmit+0x32a/0xe90 tcp_sendmsg_locked+0x437/0x1110 tcp_sendmsg+0x2f/0x50 ... skb linear: 00000000: e4 3d 1a 7d ec 30 e4 3d 1a 7e 5d 90 86 dd 60 0e skb linear: 00000010: 00 0a 1b 34 3c 40 20 11 00 00 00 00 00 00 00 00 skb linear: 00000020: 00 00 00 00 00 12 20 11 00 00 00 00 00 00 00 00 skb linear: 00000030: 00 00 00 00 00 11 2f 00 04 01 04 01 01 00 00 00 skb linear: 00000040: 86 dd 60 0e 00 0a 1b 00 06 40 20 23 00 00 00 00 skb linear: 00000050: 00 00 00 00 00 00 00 00 00 12 20 23 00 00 00 00 skb linear: 00000060: 00 00 00 00 00 00 00 00 00 11 bf 96 14 51 13 f9 skb linear: 00000070: ae 27 a0 a8 2b e3 80 18 00 40 5b 6f 00 00 01 01 skb linear: 00000080: 08 0a 42 d4 50 d5 4b 70 f8 1a

Published Sep 11, 2025 · Updated Oct 15, 2026 · Sources: NIST National Vulnerability Database, CVE Program (cve.org) e CISA ADP · CNA: Linux

Low

Who exploits it and how urgent it is

A 0–100 score combining confirmed exploitation, ransomware and APT use, public exploits, EPSS and severity.

No exploitation signal in the sources consulted so far.

How the score was calculated

EPSS 0.4% (30-day exploitation probability)+4
Technical severity (medium)+3

The score orders the patch queue; it does not replace your asset context (internet exposure, criticality, compensating controls).

Triage

Is it exploited, how likely is exploitation, what does it touch, and how severe do the sources call it.

Exploitation

Not confirmed

Not in CISA KEV — an absence of record, not proof it is unexploited.

EPSS FIRST

0.37%

ahead of 29% of scored CVEs · Oct 14, 2026

Affects

Debian

Debian Linux and 2 more

CVSS base

5.5

Medium · 3 source(s)

Prioritization reference: 7.5 provisional

Remediation

Fixes and mitigations documented by the sources — linked to the original advisory.

Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)
Documented patch — git.kernel.org (NIST National Vulnerability Database)

First 24 hours

Ordered from the record’s own fields — exposure first, because you cannot patch what you have not found. A rule-generated starting point.

Identify exposed assets running the affected vendor/product/version combinations (Debian Debian Linux, Linux Linux, Linux Linux Kernel).
Apply the fixed version or the patch documented under Remediation.
Search available logs for exploit probes, errors and authentication anomalies.

Affected scope

Vendor, product and versions as the sources word them.

Vendor	Product	Versions	Status	Source
Debian	Debian Linux	version 11.0	Vulnerable	NIST National Vulnerability Database
Linux	Linux	≥ 6.1.116 and < 6.1.149≥ 04c20a9356f283da623903e81e7c6d5df7e4dc3c and < 794ddbb7b63b6828c75967b9bcd43b086716e7a1≥ 04c20a9356f283da623903e81e7c6d5df7e4dc3c and < 864e3396976ef41de6cc7bc366276bf4e084fff2version bcefc3cd7f592a70fcbbbfd7ad1fbc69172ea78bversion 477b35d94a21530046fe91589960732fcf2b29edversion a27a5c40ee4cbe00294e2c76160de5f2589061baversion 9f605135a5c0fe614c2b15197b9ced1e217eca59version 705350fbd6ed4b5d89ee045fa57a0594a72b17d7 and 10 more ≥ c69bc67c1cb211aa390bea6e512bb01b1241fefb and < 2156d9e9f2e483c8c3906c0ea57ea312c1424235≥ 6.6.60 and < 6.6.103≥ 4.19.323 and < 4.20≥ 5.4.285 and < 5.5≥ 5.10.229 and < 5.11≥ 5.15.171 and < 5.16≥ 6.11.7 and < 6.12version 6.12≥ 04c20a9356f283da623903e81e7c6d5df7e4dc3c and < 041e2f945f82fdbd6fff577b79c33469430297aa≥ a84978a9cda68f0afe3f01d476c68db21526baf1 and < a0478d7e888028f85fa7785ea838ce0ca09398e2	Vulnerable	CVE Program (cve.org) e CISA ADP
Linux	Linux Kernel	version 6.17-rc2version 6.17-rc1version 6.12-rc7version 6.12-rc6version 6.12≥ 6.13 and < 6.16.4≥ 6.12.1 and < 6.12.44≥ 6.11.7 and < 6.12 and 6 more ≥ 6.6.60 and < 6.6.103≥ 6.1.116 and < 6.1.149≥ 5.15.171 and < 5.16≥ 5.10.229 and < 5.11≥ 5.4.285 and < 5.5≥ 4.19.323 and < 4.20	Vulnerable	NIST National Vulnerability Database

Attack characteristics

The CVSS vector, decoded. It describes the attack, not your exposure to it.

Attack vector

Local

Attack complexity

Low

Privileges required

Low

User interaction

None

Scope

Unchanged

Confidentiality

None

Integrity

None

Availability

High

Every base score collected

Sources score independently and may disagree; each row says who scored it and under which version.

Score	Version	Severity	Expl.	Impact	Scored by
7.5	CVSS 3.1	High	3.9	3.6	416baaa9-dc9f-4396-8d5f-8c081fb06d67 via NIST National Vulnerability Database
7.5	CVSS 3.1	High	—	—	Linux via CVE Program (cve.org) e CISA ADP
5.5	CVSS 3.1	Medium	1.8	3.6	nvd@nist.gov via NIST National Vulnerability Database

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H (score shown at the top: nvd@nist.gov)

Scoring entities (NIST, CNA, ADP) score independently; each score is shown with its origin. For prioritization we use 7.5 (416baaa9-dc9f-4396-8d5f-8c081fb06d67 via NIST National Vulnerability Database) as a conservative provisional reference — not a definitive official score.

Detection

Read off the CVSS vector and the weakness class. Starting points, not tested rules.

Local/physical vector: focus on endpoint telemetry (EDR) from affected hosts.

endpoint telemetry (EDR)

Timeline

What happened to this CVE, newest first.

2026
Affected products added · linux/linux
Oct 15, 2026 · CVE Program (cve.org) e CISA ADP
Record published
Oct 15, 2026
Fix documented
Oct 15, 2026 · NIST National Vulnerability Database
CVSS changed · 5.5
Oct 15, 2026 · NIST National Vulnerability Database
Source status changed · → Modified
Oct 15, 2026 · NIST National Vulnerability Database
Record created in this database
Oct 15, 2026 · NIST National Vulnerability Database
2025
CVE published
Sep 11, 2025 · Linux

Source activity

Repeated readings are counted rather than listed.

1× Official description updated · Oct 15, 2026 – Oct 15, 2026

References

10 on the record

git.kernel.org/stable/c/864e3396976ef41de6cc7bc366276bf4e084fff2
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/2156d9e9f2e483c8c3906c0ea57ea312c1424235
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/794ddbb7b63b6828c75967b9bcd43b086716e7a1
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/a0478d7e888028f85fa7785ea838ce0ca09398e2
Patch, NIST National Vulnerability Database
git.kernel.org/stable/c/041e2f945f82fdbd6fff577b79c33469430297aa
Patch, NIST National Vulnerability Database
cert-portal.siemens.com/productcert/html/ssa-082556.html
NIST National Vulnerability Database
cert-portal.siemens.com/productcert/html/ssa-032379.html
NIST National Vulnerability Database
cert-portal.siemens.com/productcert/html/ssa-457936.html
NIST National Vulnerability Database
cert-portal.siemens.com/productcert/html/ssa-019113.html
NIST National Vulnerability Database
lists.debian.org/debian-lts-announce/2025/10/msg00008.html
Third Party Advisory, NIST National Vulnerability Database

Elsewhere on this site

Debian
every CVE for this vendor

Related CVEs (same products)

Critical CVE-2026-99999 ipv4: validate checksum_start before completing checksum 6h
Unknown CVE-2026-98432 ipv4: free inet_opt and ireq_opt after an RCU grace period 6h
Unknown CVE-2026-98424 wifi: cfg80211: undo netns switch if renaming the wiphy fails 1d
Unknown CVE-2026-98425 wifi: mac80211: don't warn when an IBSS has no channel to scan 1d
Unknown CVE-2026-98426 wifi: mwifiex: fix IRQ leak using wrong index in MSI-X error path 1d
Unknown CVE-2026-98427 net/sched: codel: bound the dropping loop per dequeue call 1d

Not in any source we poll

Listed rather than left blank: an empty field and an unmeasured one look identical on screen, and only one is a reason to look elsewhere.

No CISA SSVC assessment.
No CWE classification.
No confirmed IOCs, IP addresses, domains, file hashes, or malware artifacts supplied.
No organization-specific asset inventory, compensating-control status, or patch deployment evidence supplied.
No exploit packet captures, log samples, or incident case IDs supplied.

Answered from this record

Text assembled only from the fields above — no additional claims.

What should defenders know first?

CVE-2025-39770 is "net: gso: Forbid IPv6 TSO with extensions on devices with only IPV6_CSUM", rated Medium. It affects Debian Linux (Debian); versions are in the scope table. It is not in CISA KEV, which does not rule out exploitation. CVSS 5.5 describes technical severity. EPSS 0.37% estimates the probability of exploitation in 30 days; neither replaces your asset context. First verify whether exposed or business-critical assets run the affected versions, apply vendor fixes or mitigations, and preserve logs for review.

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