VulnerabilityPublished 1 Apr 2025Updated 30 Oct 2025

CVE-2025-21977: Linux kernel · fbdev: hyperv_fb: Fix hang in kdump kernel when on Hyper-V Gen 2 VMs

Source GCVE Database

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Not known to be exploited · CVSS 5.5 Medium.

Data as of 29 Oct 2026, 23:17 UTC

CVSS 3.1 base score

5.5Medium

How severe the flaw is on the 0–10 CVSS scale.

CVSS:3.1/AV:L/AC:L/PR:L/UI:N/S:U/C:N/I:N/A:H

Attack Vector: Local
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Unchanged
Confidentiality: None
Integrity: None
Availability: High

Bold values are the worst case: easiest to exploit or largest impact.

Attack Vector: Local: The vulnerable component is not bound to the network stack and the attacker’s path is via read/write/execute capabilities. Either: the attacker exploits the vulnerability by accessing the target system locally (e.g., keyboard, console), or remotely (e.g., SSH); or the attacker relies on User Interaction by another person to perform actions required to exploit the vulnerability (e.g., tricking a legitimate user into opening a malicious document).
Attack Complexity: Low: Specialized access conditions or extenuating circumstances do not exist. An attacker can expect repeatable success against the vulnerable component.
Privileges Required: Low: The attacker is authorized with (i.e., requires) privileges that provide basic user capabilities that could normally affect only settings and files owned by a user. Alternatively, an attacker with Low privileges may have the ability to cause an impact only to non-sensitive resources.
User Interaction: None: The vulnerable system can be exploited without any interaction from any user.
Scope: Unchanged: An exploited vulnerability can only affect resources managed by the same security authority. In this case, the vulnerable component and the impacted component are either the same, or both are managed by the same security authority.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: None: There is no loss of integrity within the impacted component.
Availability: High: There is total loss of availability, resulting in the attacker being able to fully deny access to resources in the impacted component; this loss is either sustained (while the attacker continues to deliver the attack) or persistent (the condition persists even after the attack has completed). Alternatively, the attacker has the ability to deny some availability, but the loss of availability presents a direct, serious consequence to the impacted component (e.g., the attacker cannot disrupt existing connections, but can prevent new connections; the attacker can repeatedly exploit a vulnerability that, in each instance of a successful attack, leaks a only small amount of memory, but after repeated exploitation causes a service to become completely unavailable).

Definitions: FIRST CVSS v3.1 specification.

Exploitation evidence

Disclosed: evidence found.
CVE published 1 Apr 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponized: no evidence on record.
No public exploit code indexed in Metasploit or Exploit-DB
Exploited in the wildExploited: no evidence on record.
Not in CISA KEV and no source reports exploitation
Observed by OffSeq MirageSeen by Mirage: no evidence on record.
No attack traffic recorded by OffSeq Mirage sensors

A missing step is not evidence of safety. Sources: cve.org (opens in a new tab), the Metasploit, Exploit-DB and nuclei-templates index as of 29 Oct 2026.

Disclosed: The CVE record is public on cve.org. Its publication is t = 0.
Detectable: A public nuclei template can find exposed instances, for defenders and attackers alike.
Weaponized: A Metasploit exploit module or Exploit-DB entry exists (Metasploit scanner modules do not count), or CISA SSVC or the CVSS exploit maturity reports a public proof of concept.
Exploited in the wild: CISA KEV lists it, CISA SSVC rates exploitation active, or a source reports attacks.
Observed by OffSeq Mirage: OffSeq Mirage honeypot sensors recorded attack traffic for this CVE, or found exposed hosts without attack traffic.

Moments are spaced evenly, not to scale; a dashed line marks a gap of six months or more. Deltas count from t = 0. Sources that give only a date (CISA KEV, Exploit-DB, or a time of exactly 00:00Z) read “same day” rather than hours. Metasploit modules carry the flaw’s disclosure date, not their release date, and nuclei templates carry no date, so both appear above but not on the timeline.

ghsa-fcxj-8496-8247

What to do

AI-suggested

A fix has been implemented in the Linux kernel that reorders framebuffer allocation steps in the hyperv_fb driver to prevent framebuffer address conflicts. Users should update to a Linux kernel version that includes this fix. No other mitigation is required as the issue is resolved by this official fix.

Description

CVE-2025-21977 addresses a hang issue in the Linux kernel's kdump kernel when running on Hyper-V Generation 2 virtual machines. The problem arises because the hyperv_fb driver moves the framebuffer to a different memory address after conflicting with other framebuffer drivers, but the kdump kernel does not update to this new address. This causes the efifb driver to access an invalid framebuffer address, leading the Hyper-V host to throttle the VM, resulting in a hang or severe slowdown. The fix reorders framebuffer allocation steps to avoid address conflicts and ensures the kdump kernel uses the correct framebuffer address.

Impact

The vulnerability causes the kdump kernel to hang or run very slowly on Hyper-V Gen 2 VMs due to framebuffer address conflicts leading to invalid memory accesses. This results in denial of service conditions during kernel crash dumps, impacting system reliability and recovery operations. There is no impact on confidentiality or integrity, only availability.

Summary

This vulnerability involves the Linux kernel's framebuffer driver for Hyper-V (hyperv_fb) on Generation 2 VMs that boot via EFI. The hyperv_fb driver moves the framebuffer to a new MMIO address when conflicts with efifb or simplefb drivers occur, informing Hyper-V of the change. However, when the kdump kernel is loaded via kexec_file_load(), it uses the original framebuffer address, unaware of the move. The efifb driver then accesses an invalid framebuffer address, causing the Hyper-V host to throttle the VM due to perceived malicious behavior, resulting in a hang or severe slowdown. The issue was previously fixed in 2020 by preventing the kdump kernel from loading efifb, but a 2024 commit reverted that fix. The current resolution reorders the hyperv_fb driver to remove conflicting framebuffers before allocating the MMIO address, ensuring the framebuffer address remains consistent and preventing the hang.

References

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