CVE-2025-20183: Cisco AsyncOS Auth Bypass Vulnerability

CVE-2025-20183 Overview

CVE-2025-20183 is an input validation flaw in the policy-based Cisco Application Visibility and Control (AVC) implementation of Cisco AsyncOS Software for Cisco Secure Web Appliance. The vulnerability lets an unauthenticated, remote attacker evade the antivirus scanner and deliver a malicious file to an endpoint behind the appliance. Cisco tracks this issue in advisory cisco-sa-swa-range-bypass-2BsEHYSu and classifies it under [CWE-20] Improper Input Validation.

Critical Impact
A crafted HTTP Range request header causes Cisco Secure Web Appliance to pass malware to an endpoint without triggering antivirus scanning, breaking a control that many enterprises rely on for web-borne threats.

Affected Products

Cisco AsyncOS Software for Secure Web Appliance (multiple 11.8, 12.0, 12.5, 14.0, 14.1, 14.5, 15.0, 15.1, and 15.2 builds)
Cisco Secure Web Appliance hardware models S196, S396, S696
Cisco Secure Web Appliance Virtual models S100v, S300v, S600v, S1000v

Discovery Timeline

2025-02-05 - CVE-2025-20183 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20183

Vulnerability Analysis

The vulnerability lives in how Cisco AsyncOS processes HTTP Range request headers within the AVC-driven scanning pipeline. AsyncOS relies on inspecting the response payload as a coherent object before releasing it to the requester. When a client submits a crafted Range header, the appliance mishandles the partial-content flow and forwards response bytes without applying antivirus policy to the reassembled object.

The result is a scanner evasion. Malicious executables, scripts, or archives fetched over HTTP can reach the endpoint even when the appliance is configured to block them. Because the flaw affects only the malware detection path, it does not disclose data or crash the device, which aligns with an integrity-only impact profile.

Root Cause

The root cause is improper input validation ([CWE-20]) on the Range request header. AsyncOS does not correctly normalize or reassemble range-based responses before invoking the AVC antivirus scanner, so the content that reaches the client differs from what the scanner evaluated.

Attack Vector

Exploitation is network-based and requires no authentication or user interaction beyond a victim client fetching attacker-controlled content through the proxy. An attacker hosts a malicious file on a web server and lures or coerces a downstream user to request it, or targets automated clients traversing the appliance. The client sends an HTTP request that includes a crafted Range header, and Cisco Secure Web Appliance forwards the malicious payload without antivirus verdicting. No public proof-of-concept, CISA KEV entry, or in-the-wild exploitation has been reported for CVE-2025-20183.

See the Cisco Security Advisory for vendor-confirmed technical detail.

Detection Methods for CVE-2025-20183

Indicators of Compromise

HTTP requests traversing Secure Web Appliance that contain unusual or fragmented Range: bytes= headers targeting executable, script, or archive content types.
Endpoint-side detections of malware where the corresponding proxy access log shows a clean AVC verdict for the same URL and hash.
Downloads of MZ/PE, ELF, or archive files from low-reputation hosts recorded by the appliance without antivirus scan metadata.

Detection Strategies

Correlate Cisco Secure Web Appliance access logs with endpoint detection alerts to surface files delivered without scanner engagement.
Alert on HTTP transactions where Range headers request non-contiguous byte spans of downloadable content, particularly for first-seen domains.
Hunt for post-download execution behavior on internal hosts when the proxy log lacks an AV_MALWARE_SCAN outcome for the fetched object.

Monitoring Recommendations

Forward AsyncOS accesslogs and w3c logs to a central analytics platform and retain full request headers for range analysis.
Track file hashes and reputation for objects downloaded via Range requests and compare against endpoint telemetry.
Baseline the volume of 206 Partial Content responses per user and site, then alert on statistical outliers tied to executable MIME types.

How to Mitigate CVE-2025-20183

Immediate Actions Required

Apply the fixed AsyncOS release identified in Cisco advisory cisco-sa-swa-range-bypass-2BsEHYSu to every affected physical and virtual Secure Web Appliance.
Inventory AsyncOS versions across the fleet and prioritize upgrades on appliances handling internet-facing user traffic first.
Increase endpoint protection posture on assets downstream of the appliance until patches are fully deployed.

Patch Information

Cisco has published fixed software addressing CVE-2025-20183 in the Cisco Security Advisory cisco-sa-swa-range-bypass-2BsEHYSu. Refer to the advisory's Fixed Software section for the specific AsyncOS build matching each appliance model and current train.

Workarounds

Cisco lists no workarounds for this vulnerability; upgrading AsyncOS is the required remediation path.
As a compensating control, tighten decryption and file-type policies so that risky downloads are blocked outright, reducing reliance on AVC signature matching.
Enforce strict egress filtering and reputation-based blocking on the appliance for uncategorized and newly registered domains.

bash

# Verify AsyncOS version on Cisco Secure Web Appliance
# From the appliance CLI:
version

# Review recent access log entries that include Range headers
grep -i "Range:" /data/log/accesslogs/aclog.current