VulnerabilityPublished 14 Aug 2025

CVE-2025-20251: Cisco Adaptive Security Appliance (ASA) Software · Improper Validation of Specified Type of Input

Affects Cisco Adaptive Security Appliance (ASA) SoftwareSource CVE Database V5

Not known to be exploited · CVSS 8.5 High.

Data as of 26 Oct 2026, 14:05 UTC

CVSS 3.1 base score

8.5High

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

EPSS: 0.4% chance of exploitation in 30 days – higher than 40% of CVEsExploit Prediction Scoring System: the likelihood attackers will use this flaw soon.

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

Attack Vector: Network
Attack Complexity: Low
Privileges Required: Low
User Interaction: None
Scope: Changed
Confidentiality: None
Integrity: Low
Availability: High

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

Attack Vector: Network: The vulnerable component is bound to the network stack and the set of possible attackers extends beyond the other options listed, up to and including the entire Internet. Such a vulnerability is often termed 'remotely exploitable' and can be thought of as an attack being exploitable at the protocol level one or more network hops away (e.g., across one or more routers).
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: Changed: An exploited vulnerability can affect resources beyond the security scope managed by the security authority of the vulnerable component. In this case, the vulnerable component and the impacted component are different and managed by different security authorities.
Confidentiality: None: There is no loss of confidentiality within the impacted component.
Integrity: Low: Modification of data is possible, but the attacker does not have control over the consequence of a modification, or the amount of modification is limited. The data modification does not have a direct, serious impact on 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 14 Aug 2025
Detectable: no evidence on record.
No public nuclei template indexed
Weaponised: 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 26 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.
Weaponised: Public exploit code exists in Metasploit or Exploit-DB, 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.

What to do

Affected software

Product	Affected	Fixed in
Cisco Cisco Adaptive Security Appliance (ASA) Software	Affected 9.8.1 – 9.23.1 220 affected builds	Fixed in–

AI-suggested

Immediate patching: Organizations should prioritize applying Cisco's security updates or patches addressing CVE-2025-20251 as soon as they become available. Given the broad range of affected versions, verifying the exact version in use and upgrading to a fixed release is critical.
Restrict VPN user privileges: Limit VPN user permissions to the minimum necessary to reduce the risk of authenticated attackers exploiting this vulnerability.
Monitor VPN user activity: Implement enhanced logging and monitoring of VPN sessions to detect unusual file operations or service disruptions that could indicate exploitation attempts.
Network segmentation: Isolate management interfaces and restrict access to the ASA/FTD devices to trusted networks and users to reduce exposure.
Multi-factor authentication (MFA): Enforce MFA for VPN access to reduce the risk of credential compromise leading to exploitation.
Incident response readiness: Prepare for potential DoS incidents by establishing procedures for rapid detection, manual reboot, and recovery of affected devices.
Input validation hardening: While this is a vendor-side fix, organizations should engage with Cisco for timely updates and consider compensating controls such as web application firewalls (WAF) or intrusion prevention systems (IPS) that can detect and block malicious crafted HTTP requests targeting the VPN service.
Credential hygiene: Regularly rotate VPN user credentials and educate users on phishing risks to minimize the chance of credential theft.

Description

A vulnerability in the Remote Access SSL VPN service for Cisco Secure Firewall Adaptive Security Appliance (ASA) Software and Cisco Secure Firewall Threat Defense (FTD) Software could allow an authenticated, remote attacker to create or delete arbitrary files on the underlying operating system. If critical system files are manipulated, new Remote Access SSL VPN sessions could be denied and existing sessions could be dropped, causing a denial of service (DoS) condition. An exploited device requires a manual reboot to recover. This vulnerability is due to insufficient input validation when processing HTTP requests. An attacker could exploit this vulnerability by sending crafted HTTP requests to an affected device. A successful exploit could allow the attacker to create or delete files on the underlying operating system, which could cause the Remote Access SSL VPN service to become unresponsive. To exploit this vulnerability, the attacker must be authenticated as a VPN user of the affected device.

Impact

For European organizations, this vulnerability poses a significant risk to network security and business continuity. Cisco ASA and FTD devices are widely used across Europe in enterprises, government agencies, and critical infrastructure sectors to secure remote access via SSL VPNs. Exploitation could lead to denial of service on VPN services, disrupting remote workforce connectivity, secure access to internal resources, and potentially halting business operations. In sectors such as finance, healthcare, energy, and public administration, where secure remote access is vital, this could cause operational outages and impact service delivery. The requirement for attacker authentication limits exploitation to insiders or compromised VPN credentials, but insider threats or credential theft remain realistic risks. The need for manual reboot after exploitation increases downtime and operational impact. Additionally, the scope change in the CVSS vector indicates that the vulnerability affects components beyond the initial vulnerable module, potentially impacting other security functions. European organizations relying on Cisco ASA/FTD for perimeter security and VPN access must consider this vulnerability a critical operational risk.

References

sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-asaftd-http-file-hUyX2jL4 (opens in a new tab)

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Exploitation evidence

What to do

Affected software

Description

AI analysisMachine-generated, updated 14 Aug 2025, 17:04 UTC

Technical details

References

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