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CVE-2026-74696
tcp: fix TFO max_qlen accounting across reuseport migration
Published 2026-08-22 (2 months ago) by the Linux kernel CNA. Subsystem net/ipv4. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-74696 in every release this site tracks, and what each one means in practice. Every status below is the one the Debian Security Team publishes for this CVE. source
| Release | Status | Version | What this means |
|---|---|---|---|
| Debian 12bookworm | fixedDLA-4777-1 · 2026-09-08 · text | 6.1.187-1 | Update the kernel and reboot. DLA-4777-1 shipped the fix on 2026-09-08, in 6.1.187-1. A running kernel keeps the old code until the machine restarts. |
| Debian 13trixie | fixedDSA-6466-1 · 2026-08-25 · text | 6.12.105-1 | Update the kernel and reboot. DSA-6466-1 shipped the fix on 2026-08-25, in 6.12.105-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 7.1.9-1 | Update the kernel and reboot. Fixed in 7.1.9-1. A running kernel keeps the old code until the machine restarts. |
| Debian 12bookworm · linux-6.12 | fixedDLA-4788-1 · 2026-09-19 · text | 6.12.107-1~deb12u1 | Update the kernel and reboot. DLA-4788-1 shipped the fix on 2026-09-19, in 6.12.107-1~deb12u1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
A listener's TCP_FASTOPEN max_qlen stops being accurate and lets through far more pending Fast Open requests than it was configured for. This only shows up with SO_REUSEPORT listener migration, where closing a listener hands its still-pending TFO children over to a surviving one. fastopenq.qlen is charged in tcp_fastopen_create_child() when the child is created and uncharged in reqsk_fastopen_remove() when the handshake completes. The uncharge follows rsk_listener of the request the child points at, and inet_reqsk_clone() has repointed the child at a new request owned by the new listener, so the ++ and the -- land on two different sockets. The new listener's qlen drifts negative and its limit no longer binds. Charge the new listener during migration, like reqsk_queue_migrated() already does for queue->young and queue->qlen.
Where it lives
The fix touches these files. If the subsystem is one you do not use, the practical exposure is lower, though the package is still the vulnerable one.
net/ipv4/inet_connection_sock.c
- What this is
- NETWORKING [TCP]MAINTAINERS section matched on net/ipv4/inet_connection_sock.c · netdev@vger.kernel.org · Maintained
Check it on the machine
grep -w CONFIG_INET /boot/config-$(uname -r)Reads the booted kernel's own configuration, on the machine and nowhere else. =y is built in, =m is a module, and an is not set line means the code is not there.
Show whether Debian builds this, per release
Debian builds this code. Every release below compiles the affected file into the kernels it ships.
- bookworm, trixie, forky, sid, bookworm · linux-6.12y
- y
- built into the kernel image on every flavour - only a patch removes it
- Build switch
- CONFIG_INET
From the configuration Debian publishes for every kernel flavour it builds. It covers only the architectures Debian ships a linux-config package for, and nothing about a kernel you built yourself. Debian's own status above stays authoritative about the source package.MAINTAINERS kbuild Makefiles Debian linux-config packages
Signals
- Published
- 2026-08-22 (2 months ago)source
- CVSS
- 7.5 high
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.49%, higher than 40.8% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/ipv4
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.14 | 5.15.216 | e98f0d80b9cc |
| 5.14 | 6.1.183 | b6247e0f96bd |
| 5.14 | 6.6.152 | 585fc5247d14 |
| 5.14 | 6.12.104 | 6e10ee56524a |
| 5.14 | 6.18.45 | a66e869cf0c9 |
| 5.14 | 7.1.9 | d974618b2097 |
| 5.14 | 7.2 | a0ab2ba83e35 |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DLA-4788-1 | 2026-09-19 | bookworm 6.12.107-1~deb12u1 | tracker |
| DLA-4777-1 | 2026-09-08 | bookworm 6.1.187-1 | tracker |
| DSA-6466-1 | 2026-08-25 | trixie 6.12.105-1 | tracker |
Why the CNA scored it this way
Quoted from the kernel CNA's own scoring file, word for word. A highlighted metric carries the most severe value CVSS v3.1 defines for it, which is what pushes the score up. source
- AV:NAttack vector, network
- The bug is in the inbound TCP SYN/TFO server path (tcp_v4_rcv/tcp_conn_request/tcp_try_fastopen), reachable by sending TCP Fast Open SYN packets to any internet-facing listener with server-side TFO enabled.
- AC:LAttack complexity, low
- An attacker fully controls the TFO SYN flood; once a SO_REUSEPORT worker closes while TFO children are pending (routine nginx/k8s reloads), migration deterministically skews qlen negative with no narrow race or special memory layout required.
- PR:NPrivileges required, none
- Exploitation needs only unauthenticated network reachability to the listening TCP port; no target credentials, capabilities, or local access are required because TFO SYN processing occurs before any application authentication.
- UI:NUser interaction, none
- No victim user action is needed beyond normal server operation; the remote attacker sends crafted TFO SYN traffic and benefits automatically when reuseport listener migration occurs during ordinary worker shutdown or reload.
- S:UScope, unchanged
- Impact is confined to the target host kernel TCP stack resource limits and availability; it does not cross VM, container, or IOMMU security boundaries to affect a different authority.
- C:NConfidentiality impact, none
- The flaw is a signed counter accounting mismatch on fastopenq.qlen; it does not corrupt memory, leak kernel pointers, or provide an out-of-bounds read or use-after-free disclosure primitive.
- I:NIntegrity impact, none
- No arbitrary memory write, control-flow hijack, or unauthorized persistent data modification occurs; bypassing max_qlen only relaxes a rate limit while TFO cookie validation and normal TCP semantics still govern accepted data.
- A:HAvailability impact, high
- With max_qlen disabled, a remote attacker can create far more pending TFO child sockets and SYN-carrying skbs than configured, exhausting kernel memory and socket tables and causing severe denial of service or OOM-induced host instability.