CyberSecBulletin
CVE-2025-21855NVD status: Modified

Linux: Use After Free

Published Updated By the CyberSecBulletin desk

CVE-2025-21855 is a high-severity use after free 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 501ac6a7e21b82e05207c6b4449812d82820f306, 093b0e5c90592773863f300b908b741622eef597, 25dddd01dcc8ef3acff964dbb32eeb0d89f098e9 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 0.44% chance of exploitation within 30 days, which is low compared with other CVEs.

Next patch cycle

High severity with no evidence of exploitation so far.

What to do

  1. Update Linux to 501ac6a7e21b82e05207c6b4449812d82820f306 or 093b0e5c90592773863f300b908b741622eef597 or 25dddd01dcc8ef3acff964dbb32eeb0d89f098e9 or abaff2717470e4b5b7c0c3a90e128b211a23da09, whichever matches your release line, or a later version.
  2. Update Linux Kernel to 6.1.130 or 6.6.80 or 6.12.17 or 6.13.5, whichever matches your release line, or a later version.

Affected products and versions

VendorProductAffected versionsFixed in
LinuxLinux032c5e82847a2214c3196a90f0aeba0ce252de58 up to, but not including, 501ac6a7e21b82e05207c6b4449812d82820f306; 032c5e82847a2214c3196a90f0aeba0ce252de58 up to, but not including, 093b0e5c90592773863f300b908b741622eef597; 032c5e82847a2214c3196a90f0aeba0ce252de58 up to, but not including, 25dddd01dcc8ef3acff964dbb32eeb0d89f098e9; 032c5e82847a2214c3196a90f0aeba0ce252de58 up to, but not including, abaff2717470e4b5b7c0c3a90e128b211a23da09; 032c5e82847a2214c3196a90f0aeba0ce252de58 up to, but not including, bdf5d13aa05ec314d4385b31ac974d6c7e0997c9; 4.5501ac6a7e21b82e05207c6b4449812d82820f306, 093b0e5c90592773863f300b908b741622eef597, 25dddd01dcc8ef3acff964dbb32eeb0d89f098e9, abaff2717470e4b5b7c0c3a90e128b211a23da09, bdf5d13aa05ec314d4385b31ac974d6c7e0997c9
LinuxLinux Kernel4.5 up to, but not including, 6.1.130; 6.2 up to, but not including, 6.6.80; 6.7 up to, but not including, 6.12.17; 6.13 up to, but not including, 6.13.5; 6.146.1.130, 6.6.80, 6.12.17, 6.13.5

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 NVD.

Scoring systemScoreSeverityScored by
CVSS 3.18.6HighCNA
CVSS 3.17.8HighNVD
CVSS 3.17.8HighCISA (ADP)

Official description

In the Linux kernel, the following vulnerability has been resolved: ibmvnic: Don't reference skb after sending to VIOS Previously, after successfully flushing the xmit buffer to VIOS, the tx_bytes stat was incremented by the length of the skb. It is invalid to access the skb memory after sending the buffer to the VIOS because, at any point after sending, the VIOS can trigger an interrupt to free this memory. A race between reading skb->len and freeing the skb is possible (especially during LPM) and will result in use-after-free: ================================================================== BUG: KASAN: slab-use-after-free in ibmvnic_xmit+0x75c/0x1808 [ibmvnic] Read of size 4 at addr c00000024eb48a70 by task hxecom/14495 <...> Call Trace: [c000000118f66cf0] [c0000000018cba6c] dump_stack_lvl+0x84/0xe8 (unreliable) [c000000118f66d20] [c0000000006f0080] print_report+0x1a8/0x7f0 [c000000118f66df0] [c0000000006f08f0] kasan_report+0x128/0x1f8 [c000000118f66f00] [c0000000006f2868] __asan_load4+0xac/0xe0 [c000000118f66f20] [c0080000046eac84] ibmvnic_xmit+0x75c/0x1808 [ibmvnic] [c000000118f67340] [c0000000014be168] dev_hard_start_xmit+0x150/0x358 <...> Freed by task 0: kasan_save_stack+0x34/0x68 kasan_save_track+0x2c/0x50 kasan_save_free_info+0x64/0x108 __kasan_mempool_poison_object+0x148/0x2d4 napi_skb_cache_put+0x5c/0x194 net_tx_action+0x154/0x5b8 handle_softirqs+0x20c/0x60c do_softirq_own_stack+0x6c/0x88 <...> The buggy address belongs to the object at c00000024eb48a00 which belongs to the cache skbuff_head_cache of size 224 ==================================================================

Source: the CVE record as published by its numbering authority, via NVD and CVE.org.

The underlying weakness: Use After Free

The product reuses or references memory after it has been freed. At some point afterward, the memory may be allocated again and saved in another pointer, while the original pointer references a location somewhere within the new allocation. Any operations using the original pointer are no longer valid because the memory belongs to the code that operates on the new pointer.

How developers prevent this class of flaw

  • Choose a language that provides automatic memory management.
  • When freeing pointers, be sure to set them to NULL once they are freed. However, the utilization of multiple or complex data structures may lower the usefulness of this strategy.

More Use After Free vulnerabilities (CWE-416)

Timeline

  1. CVE record published.
  2. CISA published its SSVC assessment (exploitation: none, automatable: no, technical impact: total).
  3. Record last changed materially (score, affected products, status or references).

References

Recent Use After Free vulnerabilities

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