CVE-2025-20393
Cisco Multiple Products Improper Input Validation Vulnerability
Description
CVE-2025-20393 is a critical remote code execution vulnerability in Cisco AsyncOS Software affecting both Cisco Secure Email Gateway and Cisco Secure Email and Web Manager. The flaw exists in the Spam Quarantine feature and allows an unauthenticated, remote attacker to execute arbitrary system commands with root privileges on affected devices. Rated with a maximum CVSS v3.1 base score of 10.0, this vulnerability has a changed scope and impacts confidentiality, integrity, and availability at the highest level. It is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of December 24, 2025, and carries an EPSS score of 0.0496 (89.5th percentile), indicating a significantly elevated probability of exploitation compared to most CVEs.
KEV Information
CVSS Score
CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:HOpen in CalculatorAffected Products
| Vendor | Product | Version |
|---|---|---|
| cisco | asyncos | < 15.0.5-016; >= 15.5, < 15.5.4-012; >= 16.0, < 16.0.4-016; < 15.0.2-007; >= 15.5, < 15.5.4-007; >= 16.0, < 16.0.4-010 |
References
Weakness Type
CWE-20: Improper Input Validation
CVE-2025-20393 is classified under CWE-20, which describes situations where an application fails to properly validate input before processing it. In this case, the Spam Quarantine feature of Cisco AsyncOS Software does not sufficiently validate HTTP requests, enabling an attacker to inject crafted input that is interpreted as system commands. Improper input validation is one of the most prevalent weakness types in software, particularly dangerous when the application processes input in contexts that can lead to command execution. When validation is absent or incomplete on a network-facing service running with elevated privileges, as is the case here, the impact can be catastrophic. Learn more about CWE-20
Impact Analysis
Confidentiality, Integrity, and Availability are all maximally impacted by CVE-2025-20393. The CVSS vector specifies C:H/I:H/A:H with a changed scope (S:C), meaning the attacker can compromise not only the vulnerable Spam Quarantine component but also other resources on the affected system and potentially within the broader network. Successful exploitation grants root-level command execution, giving the attacker complete control over the email gateway or web manager appliance.
Business Impact is severe for organizations relying on Cisco Secure Email Gateway for email security. An attacker with root access to the email gateway can intercept, modify, or block all email traffic passing through the appliance. This enables corporate espionage through email interception, disruption of business communications, and potential pivoting into the internal network from the compromised gateway. The changed scope in the CVSS vector reflects that the attacker can affect resources beyond the vulnerable component itself.
Exploitation Probability is elevated. The EPSS score of 0.0496 places this vulnerability in the 89.5th percentile, meaning it is more likely to be exploited than roughly 89% of all known vulnerabilities. The fact that no authentication or user interaction is required (PR:N, UI:N) and the attack complexity is low (AC:L) makes this vulnerability highly attractive for automated exploitation. Its presence in the CISA KEV catalog confirms that exploitation has been observed in the wild.
Exploit Maturity
CVE-2025-20393 is confirmed to be actively exploited in the wild, as demonstrated by its inclusion in the CISA Known Exploited Vulnerabilities (KEV) catalog with a remediation deadline of December 24, 2025. The EPSS score of 0.0496 (89.5th percentile) indicates that this vulnerability is exploited at a materially higher rate than the majority of known vulnerabilities, though it has not yet reached the extreme exploitation levels seen with some long-standing CVEs.
The attack requires no authentication and no user interaction, with low complexity, making it straightforward for attackers to weaponize. The vulnerability is accessible over the network to any unauthenticated attacker who can reach the Spam Quarantine HTTP interface. CISA's advisory and Cisco's own security advisory provide detailed information about the affected components. The ransomware association for this CVE is currently listed as "Unknown" by CISA, but the root-level command execution capability makes it a highly viable vector for ransomware deployment on compromised appliances.
Remediation
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Apply Cisco AsyncOS Software Updates — Upgrade Cisco Secure Email Gateway to version 15.0.5-016, 15.5.4-012, or 16.0.4-016 or later. For Cisco Secure Email and Web Manager, upgrade to version 15.0.2-007, 15.5.4-007, or 16.0.4-010 or later. Consult Cisco's security advisory for the exact fixed release corresponding to your deployment.
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Restrict Network Access to Spam Quarantine — Until patching is complete, limit network access to the Spam Quarantine HTTP interface using firewall rules or access control lists. Only allow connections from trusted management networks and authenticated administrators.
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Monitor for Indicators of Compromise — Review system logs on Cisco Secure Email Gateway and Web Manager appliances for unusual command execution, unexpected process creation, or anomalous HTTP requests targeting the Spam Quarantine feature. Look for signs of unauthorized root-level activity.
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Implement Network Segmentation — Ensure that email gateway appliances are deployed in a segmented network zone with restricted outbound access. This limits an attacker's ability to pivot from a compromised appliance to other internal systems.
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Establish Continuous Vulnerability Monitoring — Subscribe to Cisco security advisories and monitor the CISA KEV catalog to ensure timely awareness of future vulnerabilities affecting your Cisco email security infrastructure.
Technical Details
CVE-2025-20393 targets the Spam Quarantine feature within Cisco AsyncOS Software, which powers both Cisco Secure Email Gateway and Cisco Secure Email and Web Manager. The vulnerability exists because HTTP requests processed by the Spam Quarantine feature are not sufficiently validated, allowing an attacker to inject commands that are executed at the operating system level with root privileges.
CVSS Vector Breakdown: The CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:C/C:H/I:H/A:H vector represents the most severe possible rating. Attack Vector (AV:N) means the vulnerability is exploitable remotely over the network. Attack Complexity (AC:L) indicates no special conditions or race conditions are needed. Privileges Required (PR:N) confirms no authentication is necessary. User Interaction (UI:N) means no action from a legitimate user is required. Scope (S:C) indicates the vulnerability can impact resources beyond the vulnerable component. Confidentiality, Integrity, and Availability (C:H/I:H/A:H) are all rated high, reflecting complete system compromise.
Attack Mechanism: The attacker sends a specially crafted HTTP request to the Spam Quarantine endpoint on the affected appliance. Due to insufficient input validation, the crafted payload is processed in a context where it can be interpreted as system commands. Because the Spam Quarantine service runs with root privileges on the underlying AsyncOS operating system, the injected commands execute with full administrative control over the appliance. This allows the attacker to read or modify any data on the system, install persistent backdoors, intercept email traffic, or use the compromised appliance as a launching point for further attacks within the network.
Frequently Asked Questions
How severe is CVE-2025-20393?
CVE-2025-20393 carries the maximum possible CVSS v3.1 base score of 10.0 (Critical). It allows unauthenticated remote command execution with root privileges and has a changed scope, meaning it can impact systems beyond the vulnerable component itself. It is one of the most severe types of vulnerabilities possible.
Which Cisco products are affected by CVE-2025-20393?
Cisco Secure Email Gateway and Cisco Secure Email and Web Manager running vulnerable versions of Cisco AsyncOS Software are affected. Specifically, versions prior to 15.0.5-016, 15.5.4-012, and 16.0.4-016 of the Email Gateway, and versions prior to 15.0.2-007, 15.5.4-007, and 16.0.4-010 of the Email and Web Manager are vulnerable.
Is CVE-2025-20393 being actively exploited?
Yes. This vulnerability is listed in CISA's Known Exploited Vulnerabilities (KEV) catalog, confirming active exploitation in the wild. The EPSS score of 0.0496 (89.5th percentile) further indicates elevated exploitation probability.
Can CVE-2025-20393 be exploited without authentication?
Yes. The vulnerability requires no authentication (PR:N) and no user interaction (UI:N). Any attacker who can reach the Spam Quarantine HTTP interface over the network can exploit this vulnerability with low attack complexity.
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