CVE-2020-27635
📋 TL;DR
CVE-2020-27635 is a vulnerability in PicoTCP 1.7.0 where TCP Initial Sequence Numbers (ISNs) are generated with insufficient randomness, allowing attackers to predict sequence numbers and hijack TCP sessions. This affects systems using PicoTCP library for network communication, particularly in embedded and IoT devices. Attackers can exploit this to intercept, modify, or inject data into TCP connections.
💻 Affected Systems
- PicoTCP library
- Devices using PicoTCP for TCP/IP stack
📦 What is this software?
Picotcp by Capgemini
⚠️ Risk & Real-World Impact
Worst Case
Complete TCP session hijacking allowing man-in-the-middle attacks, data interception, injection of malicious traffic, and potential unauthorized access to sensitive systems.
Likely Case
TCP session prediction leading to connection hijacking, data manipulation, and potential denial of service in vulnerable embedded systems.
If Mitigated
Limited impact if network segmentation, encryption, and proper monitoring are in place, though the fundamental vulnerability remains.
🎯 Exploit Status
Exploitation requires network access and ability to sniff/analyze TCP traffic. Tools exist for TCP sequence prediction attacks.
🛠️ Fix & Mitigation
✅ Official Fix
Patch Version: 1.7.1 or later
Vendor Advisory: https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
Restart Required: Yes
Instructions:
1. Update PicoTCP library to version 1.7.1 or later. 2. Recompile applications using the library. 3. Restart affected devices/services. 4. Verify the fix by checking version and testing ISN randomness.
🔧 Temporary Workarounds
Network Segmentation
allIsolate vulnerable devices in separate network segments to limit attack surface
Encryption Enforcement
allRequire TLS/SSL encryption for all TCP communications to protect against session hijacking
🧯 If You Can't Patch
- Implement strict network access controls and firewall rules to limit connections to vulnerable devices
- Deploy network monitoring and intrusion detection systems to detect TCP sequence prediction attempts
🔍 How to Verify
Check if Vulnerable:
Check if system uses PicoTCP library version 1.7.0. Examine source code or binary for PicoTCP implementation.Check Version:
Check application documentation or source code for PicoTCP version referenceVerify Fix Applied:
Verify PicoTCP version is 1.7.1 or later. Test TCP ISN randomness using network analysis tools.📡 Detection & Monitoring
Log Indicators:
- Unexpected TCP connection resets
- Multiple failed connection attempts
- Suspicious TCP sequence patterns
Network Indicators:
- Predictable TCP sequence numbers in packet captures
- Unusual TCP handshake patterns
- Multiple SYN packets from same source
SIEM Query:
tcp.flags.syn==1 AND tcp.flags.ack==0 | stats count by src_ip, dst_ip, dst_port | where count > threshold🔗 References
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
- https://www.cisa.gov/news-events/ics-advisories/icsa-21-042-01
- https://www.forescout.com
- https://www.forescout.com/resources/numberjack-weak-isn-generation-in-embedded-tcpip-stacks/
