CVE-2020-21699
📋 TL;DR
CVE-2020-21699 is an integer overflow vulnerability in Tengine's range filter module that allows attackers to leak potentially sensitive information from web servers. The vulnerability affects Tengine 2.2.2 and Nginx versions 0.5.6 through 1.13.2 when handling specially crafted HTTP range requests. This impacts organizations using these web server versions in production environments.
💻 Affected Systems
- Tengine
- Nginx
📦 What is this software?
Tengine by Alibaba
⚠️ Risk & Real-World Impact
Worst Case
Complete memory disclosure leading to exposure of sensitive data like session tokens, passwords, or application secrets stored in memory, potentially enabling further attacks.
Likely Case
Partial memory leakage revealing fragments of sensitive information that could be combined with other vulnerabilities for privilege escalation or data theft.
If Mitigated
Limited information disclosure with minimal impact if proper network segmentation and access controls prevent exploitation attempts.
🎯 Exploit Status
Exploitation requires sending specially crafted HTTP range requests. The vulnerability is well-documented and proof-of-concept code is available in security research repositories.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Nginx 1.13.3+, Tengine 2.2.3+
Vendor Advisory: https://nginx.org/en/security_advisories.html
Restart Required: Yes
Instructions:
1. Backup current configuration. 2. Download and compile patched version from official sources. 3. Replace existing binary with patched version. 4. Restart nginx/tengine service.
🔧 Temporary Workarounds
Disable Range Requests
allDisable HTTP range requests in nginx/tengine configuration to prevent exploitation
add 'max_ranges 0;' to http, server, or location blocks in nginx.conf
Request Limiting
allLimit request sizes and rates to reduce attack surface
client_max_body_size 1M;
limit_req_zone $binary_remote_addr zone=one:10m rate=1r/s;
🧯 If You Can't Patch
- Implement WAF rules to block malicious range requests
- Isolate vulnerable servers behind reverse proxies that filter suspicious requests
🔍 How to Verify
Check if Vulnerable:
Check nginx/tengine version with 'nginx -v' or 'tengine -v' and compare against affected versionsCheck Version:
nginx -v 2>&1 | head -1Verify Fix Applied:
Verify version is 1.13.3+ for nginx or 2.2.3+ for tengine, then test with known exploit payloads📡 Detection & Monitoring
Log Indicators:
- Unusual range request patterns
- Multiple requests with malformed range headers
- Requests resulting in 416 status codes
Network Indicators:
- HTTP requests with crafted Range headers containing large or negative values
- Unusual traffic patterns to web servers
SIEM Query:
source="nginx_access.log" AND (Range:"bytes=-*" OR Range:"bytes=*-*" OR status=416)