CVE-2021-43963
📋 TL;DR
CVE-2021-43963 is a privilege escalation vulnerability in Couchbase Sync Gateway where bucket credentials are insecurely stored in sync documents. Users with read access can extract these credentials to gain unauthorized write access to Couchbase Server. This affects Sync Gateway deployments using shared bucket access without X.509 certificate authentication.
💻 Affected Systems
- Couchbase Sync Gateway
📦 What is this software?
Sync Gateway by Couchbase
⚠️ Risk & Real-World Impact
Worst Case
Attackers with read access can obtain write credentials, potentially leading to data manipulation, deletion, or complete compromise of the Couchbase Server bucket.
Likely Case
Internal users or compromised accounts with read permissions escalate to write access, enabling data tampering or unauthorized modifications.
If Mitigated
With X.509 certificate authentication or shared bucket access disabled, the vulnerability is not exploitable.
🎯 Exploit Status
Exploitation requires existing read access to sync documents. The vulnerability is straightforward to exploit once read access is obtained.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 2.8.3 and later
Vendor Advisory: https://www.couchbase.com/alerts
Restart Required: Yes
Instructions:
1. Download Sync Gateway version 2.8.3 or later from Couchbase downloads. 2. Stop the Sync Gateway service. 3. Install the updated version. 4. Restart the Sync Gateway service. 5. Verify the version is 2.8.3+.
🔧 Temporary Workarounds
Disable shared bucket access
allDisable shared bucket access feature in Sync Gateway configuration
Set "disable_shared_bucket_access": true in sync gateway configuration file
Enable X.509 certificate authentication
allConfigure Sync Gateway to use X.509 client certificates instead of bucket credentials
Configure "x509_cert_path" and "x509_key_path" in sync gateway configuration
🧯 If You Can't Patch
- Implement strict access controls to limit read access to sync documents
- Monitor for unusual read patterns on sync documents and audit credential usage
🔍 How to Verify
Check if Vulnerable:
Check Sync Gateway version and configuration. If version is between 2.7.0-2.8.2 and shared bucket access is enabled without X.509 authentication, the system is vulnerable.Check Version:
sync_gateway --version or check service logs for version informationVerify Fix Applied:
Verify Sync Gateway version is 2.8.3 or later and check that credentials are no longer stored in sync documents.📡 Detection & Monitoring
Log Indicators:
- Unusual read patterns on sync documents
- Multiple failed authentication attempts followed by successful writes
Network Indicators:
- Unexpected write operations from previously read-only users
- Credential extraction patterns in network traffic
SIEM Query:
source="sync_gateway" AND (event="document_read" AND user="read_only_user" AND document_type="sync")