CVE-2025-20255: Cisco Webex Meetings Cache Poisoning Flaw

CVE-2025-20255 Overview

CVE-2025-20255 is an HTTP cache poisoning vulnerability in the client join services of Cisco Webex Meetings. The flaw allows an unauthenticated, remote attacker to manipulate cached HTTP responses within the meeting join service. Exploitation requires user interaction and stems from improper handling of malicious HTTP requests. A successful attack causes the Webex Meetings service to return incorrect HTTP responses to legitimate clients. The vulnerability is tracked under [CWE-349: Acceptance of Extraneous Untrusted Data With Trusted Data].

Critical Impact
An attacker can poison the cache of the Webex Meetings client join service, causing it to serve manipulated HTTP responses to meeting participants and undermining response integrity.

Affected Products

Cisco Webex Meetings (cloud service)
Client join services component
Deployments prior to the Cisco-applied fix referenced in advisory cisco-sa-webex-cache-Q4xbkQBG

Discovery Timeline

2025-05-21 - CVE-2025-20255 published to NVD
2026-06-17 - Last updated in NVD database

Technical Details for CVE-2025-20255

Vulnerability Analysis

The vulnerability resides in the client join services that handle HTTP traffic when users connect to a Cisco Webex meeting. The service caches HTTP responses to reduce latency and backend load. It does not sufficiently validate attacker-controlled elements of incoming HTTP requests before storing the associated responses. As a result, an attacker can craft requests that cause the cache to store malicious or incorrect content keyed to legitimate resources.

Subsequent legitimate clients requesting the same cached resource receive the poisoned response. Because the attacker never authenticates and the exploit occurs over the network, the reach of a single successful poisoning event extends to any client that consumes the affected cache entry. The attack requires user interaction, typically a victim joining or interacting with a meeting through the compromised join flow. Impact is limited to integrity of returned responses. Confidentiality and availability are not directly affected according to the published vector.

Root Cause

The root cause is improper trust in extraneous data extracted from HTTP requests, matching the [CWE-349] weakness pattern. The join service treats untrusted request attributes such as headers or unkeyed parameters as safe inputs to cache logic. This mismatch between the cache key and the fields that influence the response body enables poisoning.

Attack Vector

An unauthenticated remote attacker sends crafted HTTP requests to the Cisco Webex Meetings client join endpoint. The malicious request influences the cached response for a shared resource. When a targeted user later joins a meeting and their client fetches the poisoned resource, they receive attacker-influenced content. No credentials, tokens, or prior access to the meeting are required.

No verified public proof-of-concept code is available. Refer to the Cisco Security Advisory for vendor-supplied technical detail.

Detection Methods for CVE-2025-20255

Indicators of Compromise

Unexpected or malformed HTTP responses returned by Cisco Webex Meetings client join endpoints during meeting join workflows.
Anomalous HTTP headers such as unusual X-Forwarded-Host, X-Forwarded-Scheme, or duplicated headers in requests sent to Webex join services.
Client-side errors, redirection anomalies, or altered content served during meeting join that does not match expected Cisco-hosted resources.

Detection Strategies

Monitor outbound traffic from endpoints to Webex Meetings domains for responses that deviate from known-good content hashes or sizes.
Inspect proxy and web gateway logs for HTTP requests to Webex join URLs containing suspicious header manipulation or cache-busting parameters.
Correlate meeting join failures or content anomalies across multiple users hitting the same regional Webex endpoint, which can indicate a shared poisoned cache entry.

Monitoring Recommendations

Alert on HTTP responses from Webex join endpoints that contain unexpected redirects, scripts, or content-type mismatches.
Ingest network and proxy telemetry into a centralized analytics platform to enable historical searching across all Webex-related sessions.
Track Cisco PSIRT advisories, including advisory cisco-sa-webex-cache-Q4xbkQBG, for updated fixed-service confirmations.

How to Mitigate CVE-2025-20255

Immediate Actions Required

Confirm your Cisco Webex Meetings tenants are running on Cisco-hosted infrastructure updated per the vendor advisory, since Cisco applies fixes to the cloud service.
Restrict user-side ability to join meetings through untrusted links and enforce use of official Cisco Webex clients and URLs.
Educate users to validate meeting invitations and avoid interacting with unexpected prompts during meeting join.

Patch Information

Cisco Webex Meetings is a cloud-delivered service. Cisco has addressed CVE-2025-20255 in the affected client join services on its infrastructure. Customers do not need to install patches manually. Full remediation details are documented in the Cisco Security Advisory cisco-sa-webex-cache-Q4xbkQBG. Administrators should verify service health and advisory status through the Cisco Webex Control Hub.

Workarounds

No standalone workarounds are published by Cisco. Rely on the vendor-applied service fix.
Reduce exposure by enforcing single sign-on and meeting authentication policies that limit anonymous join workflows where feasible.
Use enterprise web proxies to inspect and log traffic to Webex join endpoints, providing visibility if anomalies recur.

bash

# Example: verify Webex client version and force update via administrative policy
webex --version
# Windows administrators can push the latest Webex client through configuration management
# to ensure endpoints consume responses from the fixed service consistently