CVE-2021-41110
📋 TL;DR
CVE-2021-41110 is a critical deserialization vulnerability in cwlviewer that allows remote code execution by parsing malicious YAML data. The vulnerability affects all cwlviewer installations prior to version 1.3.1. Attackers can exploit this to execute arbitrary code on the server with the privileges of the cwlviewer application.
💻 Affected Systems
- cwlviewer
📦 What is this software?
Cwlviewer by Commonwl
⚠️ Risk & Real-World Impact
Worst Case
Complete system compromise allowing attackers to execute arbitrary commands, steal data, install backdoors, or pivot to other systems.
Likely Case
Remote code execution leading to data theft, service disruption, or cryptocurrency mining malware installation.
If Mitigated
No impact if patched or if network controls prevent access to vulnerable endpoints.
🎯 Exploit Status
SnakeYaml deserialization exploits are well-documented and weaponization is likely given the high CVSS score and RCE potential.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.3.1
Vendor Advisory: https://github.com/common-workflow-language/cwlviewer/security/advisories/GHSA-7g7j-f5g3-fqp7
Restart Required: Yes
Instructions:
1. Stop cwlviewer service. 2. Update to version 1.3.1 or later. 3. Restart cwlviewer service. 4. Verify the patch by checking the YAML constructor uses SafeConstructor.
🔧 Temporary Workarounds
No workarounds available
allThe vendor advisory states there are no workarounds aside from applying the patch.
🧯 If You Can't Patch
- Isolate the cwlviewer instance behind strict network controls and limit access to trusted users only.
- Implement web application firewall rules to block suspicious YAML payloads and monitor for exploitation attempts.
🔍 How to Verify
Check if Vulnerable:
Check cwlviewer version: if version < 1.3.1, it's vulnerable. Also check if YAML parsing uses default SnakeYaml constructor instead of SafeConstructor.Check Version:
Check cwlviewer documentation or application interface for version information, or examine deployment configuration files.Verify Fix Applied:
Verify version is 1.3.1 or later and confirm YAML parsing uses SafeConstructor as shown in commit f6066f09edb70033a2ce80200e9fa9e70a5c29de.📡 Detection & Monitoring
Log Indicators:
- Unusual process execution from cwlviewer context
- Error logs related to YAML parsing failures
- Unexpected network connections from cwlviewer server
Network Indicators:
- HTTP requests containing suspicious YAML payloads to cwlviewer endpoints
- Outbound connections to unusual IPs/ports from cwlviewer server
SIEM Query:
source="cwlviewer" AND (process_execution="*" OR yaml_parse_error="*")🔗 References
- https://github.com/common-workflow-language/cwlviewer/commit/f6066f09edb70033a2ce80200e9fa9e70a5c29de
- https://github.com/common-workflow-language/cwlviewer/security/advisories/GHSA-7g7j-f5g3-fqp7
- https://www.fatalerrors.org/a/analysis-of-the-snakeyaml-deserialization-in-java-security.html
- https://github.com/common-workflow-language/cwlviewer/commit/f6066f09edb70033a2ce80200e9fa9e70a5c29de
- https://github.com/common-workflow-language/cwlviewer/security/advisories/GHSA-7g7j-f5g3-fqp7
- https://www.fatalerrors.org/a/analysis-of-the-snakeyaml-deserialization-in-java-security.html
