CVE-2020-1893
📋 TL;DR
CVE-2020-1893 is an out-of-bounds read vulnerability in HHVM's JSON parsing that occurs due to insufficient boundary checks. When exploited, it can cause denial of service by reading beyond allocated memory boundaries. This affects HHVM installations running vulnerable versions when processing untrusted JSON input.
💻 Affected Systems
- HHVM
📦 What is this software?
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
Hhvm by Facebook
⚠️ Risk & Real-World Impact
Worst Case
Denial of service causing HHVM process crashes, potentially leading to application downtime and service disruption.
Likely Case
Application instability and intermittent crashes when processing malformed JSON payloads.
If Mitigated
Minimal impact with proper input validation and network segmentation limiting exposure.
🎯 Exploit Status
Exploitation requires sending specially crafted JSON to vulnerable HHVM endpoints. No authentication needed if endpoint is publicly accessible.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 4.8.7, 4.32.1, 4.38.1, 4.46.0 and later
Vendor Advisory: https://hhvm.com/blog/2020/02/20/security-update.html
Restart Required: Yes
Instructions:
1. Identify current HHVM version. 2. Upgrade to patched version: 4.8.7, 4.32.1, 4.38.1, or 4.46.0+. 3. Restart HHVM service. 4. Verify upgrade with version check.
🔧 Temporary Workarounds
Input Validation Filter
allImplement strict JSON input validation and size limits before HHVM processes the data
# Implement in application code: validate JSON structure, size, and content before passing to HHVM
WAF Rule
allDeploy web application firewall rules to block malformed JSON payloads
# Example ModSecurity rule: SecRule REQUEST_BODY "@rx malformed_json_pattern" "id:1001,phase:2,deny"
🧯 If You Can't Patch
- Implement reverse proxy with JSON validation to filter malicious payloads before reaching HHVM
- Isolate HHVM instances behind internal network boundaries with strict ingress filtering
🔍 How to Verify
Check if Vulnerable:
Check HHVM version against affected ranges: hhvm --versionCheck Version:
hhvm --versionVerify Fix Applied:
Confirm version is 4.8.7, 4.32.1, 4.38.1, or ≥4.46.0 and test JSON parsing functionality📡 Detection & Monitoring
Log Indicators:
- HHVM segmentation fault logs
- Unexpected process termination
- Error logs containing JSON parsing failures
Network Indicators:
- Unusually large JSON payloads to HHVM endpoints
- Repeated malformed JSON requests
SIEM Query:
source="hhvm.log" AND ("segmentation fault" OR "SIGSEGV" OR "json parse error")