CVE-2026-23247 Overview
A vulnerability has been resolved in the Linux kernel related to the TCP secure sequence number generation mechanism. The fix reverts a previous change (commit 28ee1b746f49) that had downgraded timestamp offsets to per-host level. This vulnerability allows off-path TCP source port leakage via a SYN cookie side-channel attack.
The original change was made when tcp_tw_recycle was removed in 2017, but it inadvertently created a security weakness. The resolution brings back TCP ports in timestamp (TS) offset randomization, which helps prevent the leakage of TCP source port information. As an optimization, the fix performs a single siphash() computation to provide both an ISN (Initial Sequence Number) and a TS offset.
Critical Impact
Off-path attackers can exploit this vulnerability to leak TCP source port information through SYN cookie timing side-channels, potentially enabling connection hijacking or reconnaissance attacks.
Affected Products
- Linux Kernel (various versions prior to the fix)
Discovery Timeline
- March 18, 2026 - CVE-2026-23247 published to NVD
- March 18, 2026 - Last updated in NVD database
Technical Details for CVE-2026-23247
Vulnerability Analysis
This vulnerability exists in the Linux kernel's TCP secure sequence number generation subsystem. The core issue stems from the previous decision to downgrade timestamp offsets from per-connection to per-host granularity. While this simplification was considered safe after the removal of tcp_tw_recycle in 2017, it inadvertently created a side-channel vulnerability.
The weakness allows remote attackers to infer TCP source port information by analyzing timing characteristics of SYN cookies. This type of side-channel attack can be performed off-path, meaning attackers don't need to be in the direct network path between the client and server.
Root Cause
The root cause is the removal of port-specific entropy from the TCP timestamp offset calculation. When timestamp offsets were downgraded to per-host values, it created predictable patterns that could be exploited through careful timing analysis of SYN cookie responses. The original per-connection randomization using ports provided necessary entropy to prevent such inference attacks.
Attack Vector
The attack leverages the predictable nature of per-host timestamp offsets. An off-path attacker can send crafted packets and analyze the timing characteristics of SYN cookie responses to deduce the TCP source ports being used by a target host. This information could be used for:
- Connection hijacking attempts
- Targeted denial of service attacks
- Network reconnaissance and fingerprinting
- Bypassing certain firewall or intrusion detection rules
The fix addresses this by reintroducing port values into the timestamp offset calculation and optimizing the implementation to use a single siphash() call for both the Initial Sequence Number (ISN) and timestamp offset generation.
Detection Methods for CVE-2026-23247
Indicators of Compromise
- Unusual patterns of SYN packets from multiple source IPs targeting the same destination
- Anomalous timing analysis activity in network traffic
- Increased volume of TCP connection attempts with timing correlation patterns
- Evidence of port scanning followed by targeted connection attempts
Detection Strategies
- Monitor network traffic for patterns consistent with SYN cookie timing side-channel attacks
- Implement deep packet inspection to identify unusual TCP timestamp analysis behavior
- Use intrusion detection systems configured to alert on suspicious TCP handshake patterns
- Deploy kernel-level monitoring to track TCP sequence number generation anomalies
Monitoring Recommendations
- Enable TCP connection logging to track unusual connection patterns
- Configure network monitoring tools to baseline normal SYN cookie behavior
- Implement alerting for statistical anomalies in TCP timestamp distributions
- Review kernel audit logs for signs of exploitation attempts
How to Mitigate CVE-2026-23247
Immediate Actions Required
- Update the Linux kernel to a patched version that includes the fix
- Review network security controls for exposure to off-path attacks
- Consider implementing additional network segmentation to limit attacker reconnaissance capabilities
- Monitor systems for signs of exploitation while awaiting patch deployment
Patch Information
The vulnerability has been fixed in the Linux kernel through multiple commits. The fix reverts the per-host timestamp offset approach and reintroduces port-specific randomization using an optimized single siphash() computation.
Relevant kernel commits:
Workarounds
- Deploy network-level defenses such as rate limiting SYN packets to reduce attack surface
- Consider enabling SYN proxy features on network security appliances
- Implement strict ingress filtering to limit off-path attack opportunities
- Use network segmentation to isolate critical systems from potential reconnaissance
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
