Spoofing TCP RST Segments for Out-of-Band Security Devices
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Solution Overview
Problem
Newer TCP stacks, such as those implementing RFC 5961, make it difficult for out-of-band security devices to successfully spoof a TCP RST segment and terminate unwanted TCP connections due to stricter validation rules, requiring precise sequence number matching and challenge ACK handling.
Innovation Solution
A system and method that injects spoofed TCP RST segments with predicted sequence numbers into communication traffic between endpoints, using observed segment information to adjust sequence numbers and invoke resets, and continues to send additional segments in response to challenge ACKs until the connection is successfully terminated.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If stricter validation rules (RFC 5961) are implemented in TCP stacks, then security against spoofed RST segments is improved, but the ability of out-of-band security devices to terminate unwanted TCP connections deteriorates
Solution Approach 1:
The security device performs preliminary actions by sending multiple spoofed RST segments with different predicted sequence numbers before the target TCP stack can complete its validation. This preemptive approach increases the likelihood that at least one segment will match the expected sequence number and trigger connection termination, overcoming the stricter validation rules.
Solution Approach 2:
Instead of attempting a single RST segment injection, the system sends multiple RST segments with different sequence numbers (excessive action). This increases the probability of success by covering multiple possible sequence number predictions, effectively bypassing the stricter validation mechanism that would reject a single incorrect sequence number.
2Reliability
If multiple spoofed RST segments with different sequence numbers are sent, then the success rate of connection termination is improved, but the complexity of the security device operations increases
Solution Approach 1:
The device calculates and prepares multiple predicted sequence numbers based on observed TCP traffic patterns before injecting the RST segments. This preliminary calculation of sequence numbers (using formulas based on last seen sequence numbers and window sizes) automates the complexity reduction, allowing the system to handle multiple segments systematically rather than through complex real-time adaptation.
Data Source
AI summary
Systems and methods are provided to reset a TCP connection such as in response to a security policy violation. To address reset conditions under RFC 5961, for example, TCP RST segments may be spoofed and injected into communications between endpoints (e.g. client and server). In one example, three segments may be spoofed with two having respective predicted sequence numbers to invoke a reset and the third having a sequence number to invoke a challenge ACK. If a challenge ACK is received, one or more additional TCP RST segments may be injected using segment information from the challenge ACK to predict new sequence numbers, one to invoke a reset and the other, if used, to invoke a further challenge ACK. Further responsive pairs of additional TCP RST segments may be sent in response to any further challenge ACK observed until the reset is successful.