Look up another CVE · open CVE-2026-23240 in the interactive tracker
CVE-2026-23240
tls: Fix race condition in tls_sw_cancel_work_tx()
Published 2026-03-10 (7 months ago) by the Linux kernel CNA. Subsystem net/tls. Debian urgency: not yet assigned.
What to do, per Debian release
Debian's own status for CVE-2026-23240 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 | vulnerable | 6.1.190-1 | No fix published anywhere yet, neither in Debian nor upstream for the series bookworm tracks. Watch the Debian tracker page for this CVE. Advisories for bookworm have historically landed a median of 23 days after publication. |
| Debian 13trixie | fixedDSA-6238-1 · 2026-04-30 · text | 6.12.85-1 | Update the kernel and reboot. DSA-6238-1 shipped the fix on 2026-04-30, in 6.12.85-1. A running kernel keeps the old code until the machine restarts. |
| Debian 14forky | fixed | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
| Debian unstablesid | fixed | 6.19.6-1 | Update the kernel and reboot. Fixed in 6.19.6-1. A running kernel keeps the old code until the machine restarts. |
The vulnerability
What the bug is
This issue was discovered during a code audit. After cancel_delayed_work_sync() is called from tls_sk_proto_close(), tx_work_handler() can still be scheduled from paths such as the Delayed ACK handler or ksoftirqd. As a result, the tx_work_handler() worker may dereference a freed TLS object. The following is a simple race scenario: cpu0 cpu1 tls_sk_proto_close() tls_sw_cancel_work_tx() tls_write_space() tls_sw_write_space() if (!test_and_set_bit(BIT_TX_SCHEDULED, &tx_ctx->tx_bitmask)) set_bit(BIT_TX_SCHEDULED, &ctx->tx_bitmask); cancel_delayed_work_sync(&ctx->tx_work.work); schedule_delayed_work(&tx_ctx->tx_work.work, 0); To prevent this race condition, cancel_delayed_work_sync() is replaced with disable_delayed_work_sync().
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/tls/tls_sw.c
- What this is
- NETWORKING [TLS]MAINTAINERS section matched on net/tls/ · netdev@vger.kernel.org · Maintained
- Kernel module
- tls
Check it on the machine
grep -w CONFIG_TLS /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.
lsmod | grep -w tlsPrints a line if that module is loaded right now, and nothing if it is not. One that is not loaded can still be loaded later by anything that needs it.
Show whether Debian builds this, per release
Debian builds this as a loadable module. Every release below compiles it as a module on every flavour, so whether the code is in the running kernel depends on whether that module is loaded.
- bookworm, trixie, forky, sidm
- m
- a loadable module on every flavour - check lsmod, can be blacklisted
- Build switch
- CONFIG_TLS
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
- 2026-03-10 (7 months ago)source
- CVSS
- 9.8 critical
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:H/I:H/A:Hsource recompute it - Attack vector
- network (AV:N)source
- EPSS
- 0.51%, higher than 42.2% of all CVEssource
- CISA KEV
- not listedthe KEV catalogue
- Debian urgency
- not yet assignedsource
- Subsystem
- net/tls
Upstream fixes
The versions the CNA records as introducing and fixing this, and the commit for each. source
| Introduced | Fixed in | Commit |
|---|---|---|
| 5.3 | 6.12.75 | a5de36d6cee7 |
| 5.3 | 6.18.16 | 854cd32bc74f |
| 5.3 | 6.19.6 | 17153f154f80 |
| 5.3 | 7.0 | 7bb09315f93d |
Debian advisories
| Advisory | Date | Shipped to | Debian |
|---|---|---|---|
| DSA-6238-1 | 2026-04-30 | trixie 6.12.85-1 | 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 tls_write_space callback is triggered by a remote TCP peer sending ACKs, which causes tcp_data_snd_check → tcp_check_space → tcp_new_space → sk->sk_write_space (tls_write_space). Any server using kTLS (nginx, HAProxy, NFS with TLS) is reachable over the network.
- AC:LAttack complexity, low
- The attacker controls both sides of the race: they send TCP ACKs to trigger tls_write_space and initiate disconnect to trigger the close path. With repeated connection attempts and ACK flooding, the race window is reliably winnable.
- PR:NPrivileges required, none
- No authentication or privileges are needed to establish a TCP connection and send ACKs to a kTLS-enabled server. The race is triggered during normal TCP processing before any application-level authentication occurs.
- UI:NUser interaction, none
- No user interaction is needed. The attacker connects to the server, sends ACKs, and disconnects to trigger the race. The server processes this automatically.
- S:UScope, unchanged
- The vulnerability results in kernel-level use-after-free within the same security authority (kernel space). There is no crossing of a virtualization or sandbox boundary.
- C:HConfidentiality impact, high
- This is a use-after-free where the freed tls_sw_context_tx (freed via immediate kfree, not kfree_rcu) can be reallocated with attacker-influenced data. The UAF gives read access to reallocated memory contents, enabling arbitrary kernel memory disclosure.
- I:HIntegrity impact, high
- The UAF on the tls_sw_context_tx structure enables heap spraying to control the freed object's contents. The tx_work_handler then calls tls_tx_records through corrupted context pointers, enabling control flow hijacking and arbitrary code execution.
- A:HAvailability impact, high
- The use-after-free on the embedded delayed_work struct causes kernel crashes when the workqueue attempts to execute the freed work item, resulting in kernel panic or oops.