high CVSS 7.8

CVE-2025-21703·Kernel vulnerability

In the Linux kernel, the following vulnerability has been resolved: netem: Update sch->q.qlen before qdisc_tree_reduce_backlog() qdisc_tree_reduce_backlog() notifies parent qdisc only if child qdisc becomes empty, therefore we need to reduce the backlog of the child qdisc before calling it. Otherwise it would miss the opportunity to call cops->qlen_notify(), in the case of DRR, it resulted in UAF since DRR uses ->qlen_notify() to maintain its active list.

Severity
high
Software
Kernel
Fixed in
6.13.3
Published
2025-02-18

Affected versions

From: 6.13

Until: 6.13.3

Fixed in: 6.13.3

How to fix this CVE

CVE-2025-21703 is a use-after-free (UAF) vulnerability in the Linux kernel's netem (network emulation) qdisc that occurs when the backlog queue length is not updated before notifying parent qdiscs. This can lead to memory corruption and system crashes. Update your kernel to version 6.13.3 or later to resolve this issue.

sudo dnf update kernel

Defensia detects this vulnerability

How to check if you are affected

  1. Step 1: Check your current kernel version by running: uname -r
  2. Step 2: Verify if netem qdisc is active on your system by running: tc qdisc show | grep netem
  3. Step 3: Search system logs for UAF or memory corruption errors: sudo dmesg | grep -i 'use.after.free\|uaf\|page.fault'
  4. Step 4: After patching, confirm the new kernel version with: uname -r and verify it is 6.13.3 or later

FAQ

What is CVE-2025-21703?

CVE-2025-21703 is a use-after-free vulnerability in the Linux kernel's netem qdisc implementation that occurs when the child qdisc queue length is not reduced before invoking qdisc_tree_reduce_backlog(). This causes the DRR (Deficit Round Robin) qdisc to lose track of active queues, leading to memory corruption and potential kernel crashes.

Is CVE-2025-21703 being actively exploited?

No, CVE-2025-21703 is not currently listed on CISA's Known Exploited Vulnerabilities catalog and no public exploits are available.

What versions of Kernel are affected by CVE-2025-21703?

Linux kernel versions 6.13 through 6.13.2 are affected. Version 6.13.3 and later contain the fix.

How do I check if my server is vulnerable to CVE-2025-21703?

Run 'uname -r' to check your kernel version. If it shows 6.13.0, 6.13.1, or 6.13.2, your system is vulnerable. Then verify if netem qdisc is in use with 'tc qdisc show | grep netem'.

Does Defensia detect CVE-2025-21703?

Yes — Defensia's CVE advisory scanner compares installed package versions against the NVD database. If the Linux kernel is installed on a monitored server, CVE-2025-21703 will appear in your dashboard with remediation steps.

Related Kernel CVEs

CVE-2024-42256CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: cifs: Fix server re-repick on subrequest retry When a subrequest is marked for needing retry, netfs will call cifs_prepare_write() which will make cifs repick the server for the op before renegotiating credits; it then calls cifs_issue_write() which invokes smb2_async_writev() - which re-repicks the server. If a different server is then selected, this causes the increment of server->in_flight to happen against one record and the decrement to happen against another, leading to misaccounting. Fix this by just removing the repick code in smb2_async_writev(). As this is only called from netfslib-driven code, cifs_prepare_write() should always have been called first, and so server should never be NULL and the preparatory step is repeated in the event that we do a retry. The problem manifests as a warning looking something like: WARNING: CPU: 4 PID: 72896 at fs/smb/client/smb2ops.c:97 smb2_add_credits+0x3f0/0x9e0 [cifs] ... RIP: 0010:smb2_add_credits+0x3f0/0x9e0 [cifs] ... smb2_writev_callback+0x334/0x560 [cifs] cifs_demultiplex_thread+0x77a/0x11b0 [cifs] kthread+0x187/0x1d0 ret_from_fork+0x34/0x60 ret_from_fork_asm+0x1a/0x30 Which may be triggered by a number of different xfstests running against an Azure server in multichannel mode. generic/249 seems the most repeatable, but generic/215, generic/249 and generic/308 may also show it.
CVE-2024-36031CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: keys: Fix overwrite of key expiration on instantiation The expiry time of a key is unconditionally overwritten during instantiation, defaulting to turn it permanent. This causes a problem for DNS resolution as the expiration set by user-space is overwritten to TIME64_MAX, disabling further DNS updates. Fix this by restoring the condition that key_set_expiry is only called when the pre-parser sets a specific expiry.
CVE-2024-38612CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ipv6: sr: fix invalid unregister error path The error path of seg6_init() is wrong in case CONFIG_IPV6_SEG6_LWTUNNEL is not defined. In that case if seg6_hmac_init() fails, the genl_unregister_family() isn't called. This issue exist since commit 46738b1317e1 ("ipv6: sr: add option to control lwtunnel support"), and commit 5559cea2d5aa ("ipv6: sr: fix possible use-after-free and null-ptr-deref") replaced unregister_pernet_subsys() with genl_unregister_family() in this error path.
CVE-2024-38623CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: fs/ntfs3: Use variable length array instead of fixed size Should fix smatch warning: ntfs_set_label() error: __builtin_memcpy() 'uni->name' too small (20 vs 256)
CVE-2021-47548CVSS 9.8In the Linux kernel, the following vulnerability has been resolved: ethernet: hisilicon: hns: hns_dsaf_misc: fix a possible array overflow in hns_dsaf_ge_srst_by_port() The if statement: if (port >= DSAF_GE_NUM) return; limits the value of port less than DSAF_GE_NUM (i.e., 8). However, if the value of port is 6 or 7, an array overflow could occur: port_rst_off = dsaf_dev->mac_cb[port]->port_rst_off; because the length of dsaf_dev->mac_cb is DSAF_MAX_PORT_NUM (i.e., 6). To fix this possible array overflow, we first check port and if it is greater than or equal to DSAF_MAX_PORT_NUM, the function returns.

References

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