CVE-2020-11996
7.5 HIGH
📋 TL;DR
This CVE describes a denial-of-service vulnerability in Apache Tomcat's HTTP/2 implementation where specially crafted requests can cause high CPU usage, potentially making the server unresponsive. It affects Tomcat versions 10.0.0-M1 to 10.0.0-M5, 9.0.0.M1 to 9.0.35, and 8.5.0 to 8.5.55. Organizations running these Tomcat versions with HTTP/2 enabled are vulnerable.
💻 Affected Systems
Products:
- Apache Tomcat
Versions: 10.0.0-M1 to 10.0.0-M5, 9.0.0.M1 to 9.0.35, 8.5.0 to 8.5.55
Operating Systems: All operating systems running affected Tomcat versions
Default Config Vulnerable:
✅ No
Notes: Only vulnerable when HTTP/2 protocol is enabled. HTTP/1.1 connections are not affected.
📦 What is this software?
Leap by Opensuse
Leap by Opensuse
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Tomcat by Apache
Ubuntu Linux by Canonical
⚠️ Risk & Real-World Impact
Worst Case
Complete service unavailability due to resource exhaustion, leading to extended downtime for web applications hosted on affected Tomcat instances.
Likely Case
Temporary performance degradation or intermittent service interruptions during attack periods, affecting application availability.
If Mitigated
Minimal impact with proper monitoring and rate limiting in place, allowing quick detection and mitigation of attack attempts.
🌐 Internet-Facing: HIGH - Internet-facing Tomcat servers are directly exposed to potential DoS attacks from untrusted sources.
🏢 Internal Only: MEDIUM - Internal servers could still be targeted by compromised internal systems or malicious insiders.
🎯 Exploit Status
Public PoC:
⚠️ Yes
Weaponized:
LIKELY
Unauthenticated Exploit:
⚠️ Yes
Complexity:
LOW
Exploitation requires sending specially crafted HTTP/2 requests, which can be automated. No authentication is required to trigger the vulnerability.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: Tomcat 10.0.0-M6, 9.0.36, 8.5.56
Vendor Advisory: https://lists.apache.org/thread.html/r2529016c311ce9485e6f173446d469600fdfbb94dccadfcd9dfdac79%40%3Cusers.tomcat.apache.org%3E
Restart Required: Yes
Instructions:
1. Download the patched version from Apache Tomcat website. 2. Stop the Tomcat service. 3. Backup current installation. 4. Replace with patched version. 5. Restart Tomcat service.
🔧 Temporary Workarounds
Disable HTTP/2
allDisable HTTP/2 protocol support in Tomcat configuration to prevent exploitation
Edit server.xml and remove or comment out HTTP/2 connector configurations
Implement Rate Limiting
linuxConfigure web application firewall or load balancer to limit concurrent HTTP/2 connections
# Example using iptables for connection limiting
iptables -A INPUT -p tcp --dport 8443 -m connlimit --connlimit-above 100 -j REJECT
🧯 If You Can't Patch
- Implement network-level controls to limit HTTP/2 traffic to trusted sources only
- Deploy monitoring with alerting for abnormal CPU spikes on Tomcat servers
🔍 How to Verify
Check if Vulnerable:
Check Tomcat version and verify HTTP/2 is enabled in server.xml configurationCheck Version:
grep 'Server version' $CATALINA_HOME/logs/catalina.out | tail -1Verify Fix Applied:
Verify Tomcat version is 10.0.0-M6+, 9.0.36+, or 8.5.56+ and test HTTP/2 functionality📡 Detection & Monitoring
Log Indicators:
- Unusual patterns of HTTP/2 connection attempts
- High CPU usage alerts from monitoring systems
- Tomcat thread pool exhaustion warnings
Network Indicators:
- Spikes in HTTP/2 traffic from single or multiple sources
- Unusual HTTP/2 request patterns
SIEM Query:
source="tomcat.logs" AND ("HTTP/2" OR "h2") AND ("high cpu" OR "connection limit" OR "thread pool")🔗 References
- http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00064.html
- http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00072.html
- https://lists.apache.org/thread.html/r2529016c311ce9485e6f173446d469600fdfbb94dccadfcd9dfdac79%40%3Cusers.tomcat.apache.org%3E
- https://lists.apache.org/thread.html/r3ea96d8f36dd404acce83df8aeb22a9e807d6c13ca9c5dec72f872cd%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r5541ef6b6b68b49f76fc4c45695940116da2bcbe0312ef204a00a2e0%40%3Cannounce.tomcat.apache.org%3E
- https://lists.apache.org/thread.html/r5a4f80a6acc6607d61dae424b643b594c6188dd4e1eff04705c10db2%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r6c29801370a36c1a5159679269777ad0c73276d3015b8bbefea66e5c%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r74f5a8204efe574cbfcd95b2a16236fe95beb45c4d9fee3dc789dca9%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r8f3d416c193bc9384a8a7dd368623d441f5fcaff1057115008100561%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r8f7484589454638af527182ae55ef5b628ba00c05c5b11887c922fb1%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r93ca628ef3a4530dfe5ac49fddc795f0920a4b2a408b57a30926a42b%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r9ad911fe49450ed9405827af0e7a74104041081ff91864b1f2546bbd%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/ra7092f7492569b39b04ec0decf52628ba86c51f15efb38f5853e2760%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rb4ee49ecc4c59620ffd5e66e84a17e526c2c3cfa95d0cd682d90d338%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rb820f1a2a02bf07414be12c653c2ab5321fd87b9bf6c5e635c53ff4b%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rc80b96b4b96618b2b7461cb90664a428cfd6605eea9f74e51b792542%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rea65d6ef2e45dd1c45faae83922042732866c7b88fa109b76c83db52%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/ref0339792ac6dac1dba83c071a727ad72380899bde60f6aaad4031b9%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2020/07/msg00010.html
- https://security.netapp.com/advisory/ntap-20200709-0002/
- https://usn.ubuntu.com/4596-1/
- https://www.debian.org/security/2020/dsa-4727
- https://www.oracle.com/security-alerts/cpujan2021.html
- https://www.oracle.com/security-alerts/cpuoct2020.html
- http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00064.html
- http://lists.opensuse.org/opensuse-security-announce/2020-07/msg00072.html
- https://lists.apache.org/thread.html/r2529016c311ce9485e6f173446d469600fdfbb94dccadfcd9dfdac79%40%3Cusers.tomcat.apache.org%3E
- https://lists.apache.org/thread.html/r3ea96d8f36dd404acce83df8aeb22a9e807d6c13ca9c5dec72f872cd%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r5541ef6b6b68b49f76fc4c45695940116da2bcbe0312ef204a00a2e0%40%3Cannounce.tomcat.apache.org%3E
- https://lists.apache.org/thread.html/r5a4f80a6acc6607d61dae424b643b594c6188dd4e1eff04705c10db2%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r6c29801370a36c1a5159679269777ad0c73276d3015b8bbefea66e5c%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r74f5a8204efe574cbfcd95b2a16236fe95beb45c4d9fee3dc789dca9%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r8f3d416c193bc9384a8a7dd368623d441f5fcaff1057115008100561%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r8f7484589454638af527182ae55ef5b628ba00c05c5b11887c922fb1%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r93ca628ef3a4530dfe5ac49fddc795f0920a4b2a408b57a30926a42b%40%3Ccommits.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/r9ad911fe49450ed9405827af0e7a74104041081ff91864b1f2546bbd%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/ra7092f7492569b39b04ec0decf52628ba86c51f15efb38f5853e2760%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rb4ee49ecc4c59620ffd5e66e84a17e526c2c3cfa95d0cd682d90d338%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rb820f1a2a02bf07414be12c653c2ab5321fd87b9bf6c5e635c53ff4b%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rc80b96b4b96618b2b7461cb90664a428cfd6605eea9f74e51b792542%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/rea65d6ef2e45dd1c45faae83922042732866c7b88fa109b76c83db52%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.apache.org/thread.html/ref0339792ac6dac1dba83c071a727ad72380899bde60f6aaad4031b9%40%3Cnotifications.ofbiz.apache.org%3E
- https://lists.debian.org/debian-lts-announce/2020/07/msg00010.html
- https://security.netapp.com/advisory/ntap-20200709-0002/
- https://usn.ubuntu.com/4596-1/
- https://www.debian.org/security/2020/dsa-4727
- https://www.oracle.com/security-alerts/cpujan2021.html
- https://www.oracle.com/security-alerts/cpuoct2020.html
