CVE-2025-20335
📋 TL;DR
An authentication bypass vulnerability in Cisco desk and IP phones allows unauthenticated remote attackers to write arbitrary files to specific directories when web access is enabled. This affects Cisco Desk Phone 9800 Series, IP Phone 7800/8800 Series, and Video Phone 8875. Web access is disabled by default, limiting exposure.
💻 Affected Systems
- Cisco Desk Phone 9800 Series
- Cisco IP Phone 7800 Series
- Cisco IP Phone 8800 Series
- Cisco Video Phone 8875
📦 What is this software?
⚠️ Risk & Real-World Impact
Worst Case
Complete device compromise allowing persistence, credential theft, or lateral movement within the network
Likely Case
Service disruption, configuration modification, or installation of malicious scripts
If Mitigated
No impact if web access remains disabled or proper network segmentation is in place
🎯 Exploit Status
Requires crafted HTTP request to vulnerable endpoint when web access enabled
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Check Cisco advisory for specific firmware versions
Vendor Advisory: https://sec.cloudapps.cisco.com/security/center/content/CiscoSecurityAdvisory/cisco-sa-phone-write-g3kcC5Df
Restart Required: No
Instructions:
1. Access Cisco advisory 2. Download appropriate firmware for your phone model 3. Upload firmware via phone administration interface 4. Apply update without restart required
🔧 Temporary Workarounds
Disable Web Access
allDisable the web access feature on all affected phones
Navigate to Phone Settings > Security > Web Access > Disable
🧯 If You Can't Patch
- Disable web access on all affected phones
- Implement network segmentation to isolate phones from untrusted networks
🔍 How to Verify
Check if Vulnerable:
Check if web access is enabled on phone and firmware version is unpatchedCheck Version:
Check phone information via Settings > Status > Firmware VersionVerify Fix Applied:
Verify firmware version matches patched version in Cisco advisory and web access remains disabled📡 Detection & Monitoring
Log Indicators:
- Unusual file write operations in phone logs
- Multiple failed authentication attempts followed by successful file writes
Network Indicators:
- HTTP requests to phone web interface from unexpected sources
- Unusual outbound connections from phones
SIEM Query:
source="cisco-phone" AND (event="file_write" OR event="web_access")