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TCP SACK Panics Linux Servers ![Clock Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-clock.svg) 5 min read * ![Profile Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-profile-grey.svg) By: * [Unit 42](https://unit42.paloaltonetworks.com/author/unit42/) * ![Published Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-calendar-grey.svg) Published:June 21, 2019 * ![Tags Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-category.svg) Categories: * [Cloud Cybersecurity Research](https://unit42.paloaltonetworks.com/category/cloud-cybersecurity-research/) * [Threat Research](https://unit42.paloaltonetworks.com/category/threat-research/) * [Vulnerabilities](https://unit42.paloaltonetworks.com/category/vulnerabilities/) * ![Tags Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-tags-grey.svg) Tags: * [AWS](https://unit42.paloaltonetworks.com/tag/aws/) * [Azure](https://unit42.paloaltonetworks.com/tag/azure/) * [CVE-2019-11477](https://unit42.paloaltonetworks.com/tag/cve-2019-11477/) * [CVE-2019-11478](https://unit42.paloaltonetworks.com/tag/cve-2019-11478/) * [CVE-2019-11479](https://unit42.paloaltonetworks.com/tag/cve-2019-11479/) * [GCP](https://unit42.paloaltonetworks.com/tag/gcp/) * [Linux](https://unit42.paloaltonetworks.com/tag/linux/) * [Public cloud](https://unit42.paloaltonetworks.com/tag/public-cloud/) * [SACK](https://unit42.paloaltonetworks.com/tag/sack/) * [![Download Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-download.svg)](https://unit42.paloaltonetworks.com/tcp-sack-panics-linux-servers/?pdf=download&lg=en&_wpnonce=49d7afdf0b "Click here to download") * [![Print Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-print.svg)](https://unit42.paloaltonetworks.com/tcp-sack-panics-linux-servers/?pdf=print&lg=en&_wpnonce=49d7afdf0b "Click here to print") Share![Down arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/down-arrow.svg) * ![Link Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-share-link.svg) * [![Link Email](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-sms.svg)](mailto:?subject=TCP%20SACK%20Panics%20Linux%20Servers&body=Check%20out%20this%20article%20https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F "Share in email") * [![Facebook Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-fb-share.svg)](https://www.facebook.com/sharer/sharer.php?u=https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F "Share in Facebook") * [![LinkedIn Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-linkedin-share.svg)](https://www.linkedin.com/sharing/share-offsite/?url=https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F&title=TCP%20SACK%20Panics%20Linux%20Servers "Share in LinkedIn") * [![Twitter Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-twitter-share.svg)](https://twitter.com/intent/tweet?url=https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F&text=TCP%20SACK%20Panics%20Linux%20Servers "Share in Twitter") * [![Reddit Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-reddit-share.svg)](https://www.paloaltonetworks.com//www.reddit.com/submit?url=https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F&ts=markdown "Share in Reddit") * [![Mastodon Icon](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-mastodon-share.svg)](https://mastodon.social/share?text=TCP%20SACK%20Panics%20Linux%20Servers%20https%3A%2F%2Funit42.paloaltonetworks.com%2Ftcp-sack-panics-linux-servers%2F "Share in Mastodon") #### Executive Summary The newly discovered [Linux vulnerabilities](https://access.redhat.com/security/vulnerabilities/tcpsack), [CVE-2019-11477](https://access.redhat.com/security/cve/cve-2019-11477), [CVE-2019-11478](https://access.redhat.com/security/cve/cve-2019-11478), and [CVE-2019-11479](https://access.redhat.com/security/cve/cve-2019-11479), affect all Linux operating systems newer than kernel 2.6.29 (released on March 2009) or above and can cause a kernel panic to systems with services listening on a TCP connection. This remote attack can put a server into a Denial of Service (DoS) state, but remote code execution is not of concern. The vulnerability roots on the flaws in the TCP Selective Acknowledgement (SACK) and Maximum Segment Size (MSS) implementation. The attack can be triggered by a remote user who sets the MSS to the lowest limit of 48 bytes and sends a sequence of specially crafted SACK packets to overflow the receiver's socket buffer. Most of the Linux distributions have released a patch, [RedHat](https://access.redhat.com/security/vulnerabilities/tcpsack), [Ubuntu](https://wiki.ubuntu.com/SecurityTeam/KnowledgeBase/SACKPanic), and [openSUSE](https://www.suse.com/support/kb/doc/?id=7023928). If immediately patching the system is not possible, the attack can also be blocked by firewall rules or traffic control policies. As the attack is fairly simple, a proof of concept (POC) exploit should be available soon and we expect to see large-scale DDoS attacks within a few days as attackers exploit unprotected systems. #### TCP SACK [RedHat](https://access.redhat.com/security/vulnerabilities/tcpsack) gives a detailed description of this attack; we've included a brief review of the attack. The TCP SACK attack is achieved by tipping the balance between the sender's TCP selective acknowledgment (SACK) mechanism and the receiver's maximum segment size (MSS) settings. SACK is a mechanism used to inform the sender system which TCP packets have been accepted successfully by the receiver and which packets need to be resent. MSS is a setting set by the receiver on how large of a packet can be received by its network. When a receiver's MSS is set to its lowest setting of only 48 bytes, the sender is required to send more segments to fulfill the request. When a receiver requests a large network transfer, i.e. media, or a large file transfer, the sender begins the monitoring the successful receipt of transmitted packets. However, the imbalance between the receiver's low MSS capability and the potential for a sender's limited volume to store a long SACK queue can cause an overflow in the sender's SACK queue. This overflow of unsent packets, when large enough, can trigger a kernel panic state causing the DoS. ##### Impact Any server with Linux kernel 2.6.29 or above and with open ports running TCP services are vulnerable to this attack. We perform a few queries on Shodan to understand the potential impact of this vulnerability, see Image 1. There are currently 1,944,975 Linux-based servers discoverable on the internet. 543,797 servers have http running on port 80, 152,416 servers have sshd running on port 22, and 126,143 servers have https running on port 443. If no special firewall rules or traffic control policy is enforced on these servers, they can all be potential targets to the attack. ## ![](https://unit42.paloaltonetworks.com/wp-content/uploads/2019/06/word-image-5.png) *Image 1. Publicly discoverable Linux systems via Shodan* ##### Resolution This attack targets Linux kernel systems. If you have a Linux system that is running a kernel version of 2.6.29 or higher, the system should be upgraded to the latest Linux release available. If the client is using Google Cloud Platform (GCP), Microsoft Azure, or another Infrastructure as a Service (IaaS), please follow their Linux upgrade procedures to upgrade all Linux systems to the latest release. Amazon has identified the following services which can fall victim to the identified SACK Panic attack: * AWS Elastic Beanstalk * Amazon Linux and Amazon Linux 2 * Amazon Elastic Compute Cloud (EC2) * Elastic Load Balancing (ELB) * Amazon WorkSpaces (Linux) * Amazon Elastic Container Service for Kubernetes (Amazon EKS) * Amazon ElastiCache * Amazon EMR Clients who use any or all of the identified Amazon services should update those services to their latest versions. Below is an abbreviated listing of actions to be taken for each of the listed Amazon Services. **AWS Elastic Beanstalk** * If clients are using Amazon's Managed Platform Updates, these systems will be updated during the selected maintenance window. If the client has not enabled Managed Platform Updates, the following link will assist them in setting up that process, [Managed Platform Updates Guide](https://docs.aws.amazon.com/elasticbeanstalk/latest/dg/environment-platform-update-managed.html). **Amazon Linux and Amazon Linux 2** * New Linux kernels have been uploaded and are available within the Amazon Linux repository. Clients should run the following command in each of their EC2 instances to receive the kernel update. sudo yum update kernel **Amazon Elastic Compute Cloud (EC2)** * Perform the following command within each EC2 instance sudo yum update kernel **Elastic Load Balancing (ELB)** * If your ELBs are configured to be classic load or application balancers, or if they are configured with TLS termination enabled, no action required.If you are running Linux-based EC2 instances within AWS which are not terminating TLS sessions, update the Linux kernels for those systems. Instructions can be found [here](https://docs.aws.amazon.com/AWSEC2/latest/UserGuide/install-updates.html). **Amazon WorkSpaces (Linux)** * All new Linux WorkSpaces will be updated automatically and do not require any updates by the client. **Amazon Elastic Container Service for Kubernetes (Amazon EKS)** * All currently-running EKS clusters are protected against this attack. Clients should replace all worker nodes to use the latest EKS version. **Amazon ElastiCache** * ElastiCache VPCs to not accept untrusted TCP connections by default and are not affected. * If a client has made changes to their default ElastiCache VPC they will need to ensure their ElastiCache security groups follow AWS best practices and configure them to block traffic from untrusted clients. Additional information can be found in Amazon's [VPC and ElastiCache Security](https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/VPCs.html) page. * If a client has an ElastiCache running outside of a VPC and has altered the ElastiCache from the default, should configure their instance using ElastiCache security groups. More information can be found in Amazon's [EC2-Classic Security Group](https://docs.aws.amazon.com/AmazonElastiCache/latest/red-ug/SecurityGroups.html) page. **Amazon EMR** * Amazon EMR launches EC2 instances running Linux into a client's VPC, and these do not accept untrusted TCP connections by default and are not affected. * Any client who has made changes to the EMR VPC should follow AWS best practices and configure them to block traffic from untrusted clients. Additional information can be found in Amazon's [Control Network Traffic with Security Groups](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-security-groups.html) page. * If clients choose not to configure EMR security groups according to AWS best practices can follow these guidelines: * New Clusters - use EMR bootstrap actions to update the Linux Kernel and reboot each instance. Assistance can be found in Amazon's [EMR Bootstrap Plan](https://docs.aws.amazon.com/emr/latest/ManagementGuide/emr-plan-bootstrap.html) page. * Existing Clusters - Update the Linux kernel on each instance within the cluster and reboot the instances one at a time. #### Conclusion Due to the low complexity and high severity of this vulnerability, it won't be surprising to see large scale DDoS attacks in a few days. It is critical to update the Linux kernel as soon as possible. For the servers running in the public cloud with TCP services open to the internet, it is even more critical to patch immediately or at least set up firewall rules to block the attack. The resolution guideline provided in this blog should help administrators secure their systems. Back to top ### Tags * [AWS](https://unit42.paloaltonetworks.com/tag/aws/ "AWS") * [Azure](https://unit42.paloaltonetworks.com/tag/azure/ "Azure") * [CVE-2019-11477](https://unit42.paloaltonetworks.com/tag/cve-2019-11477/ "CVE-2019-11477") * [CVE-2019-11478](https://unit42.paloaltonetworks.com/tag/cve-2019-11478/ "CVE-2019-11478") * [CVE-2019-11479](https://unit42.paloaltonetworks.com/tag/cve-2019-11479/ "CVE-2019-11479") * [GCP](https://unit42.paloaltonetworks.com/tag/gcp/ "GCP") * [Linux](https://unit42.paloaltonetworks.com/tag/linux/ "Linux") * [Public cloud](https://unit42.paloaltonetworks.com/tag/public-cloud/ "public cloud") * [SACK](https://unit42.paloaltonetworks.com/tag/sack/ "SACK") [Threat Research Center](https://unit42.paloaltonetworks.com "Threat Research") [Next: Unit 42 Discovers 10 New Microsoft Vulnerabilities](https://unit42.paloaltonetworks.com/unit-42-discovers-10-new-microsoft-vulnerabilities/ "Unit 42 Discovers 10 New Microsoft Vulnerabilities") ### Table of Contents * ### Related Articles * [From Exposure to Lockdown: How AWS Neutralizes Compromised IAM Credentials through Managed Policies](https://unit42.paloaltonetworks.com/detecting-exposed-aws-iam-credentials/ "article - table of contents") * [A Vault with a Heap-View: The Uncomfortable Space Between AgentCore Harness and Identity](https://unit42.paloaltonetworks.com/securing-aws-agentcore-harness-credentials/ "article - table of contents") * [The Machine With Many Faces: Post-Exploitation Identity Misuse in SPIFFE/SPIRE](https://unit42.paloaltonetworks.com/kubernetes-spiffe-spire-identity-spoofing/ "article - table of contents") ## Related Resources ![Pictorial representation of NetScaler zero days. 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Digital illustration of Europe map highlighting network connections and nodes, depicted as glowing points and lines on a dark blue background, emphasizing major cities and connectivity across the continent.](https://unit42.paloaltonetworks.com/wp-content/uploads/2026/06/09_Cloud_cybersecurity_research_Overview_1920x900-786x368.jpg) [![category icon](https://unit42.paloaltonetworks.com/wp-content/uploads/2024/06/icon-threat-research.svg)Threat Research](https://unit42.paloaltonetworks.com/category/threat-research/) June 22, 2026 [#### The Global Namespace Risk: Universal Bucket Hijacking Technique for Cloud Data Exfiltration](https://unit42.paloaltonetworks.com/cloud-bucket-hijacking-risks/) * [AWS](https://unit42.paloaltonetworks.com/tag/aws/ "AWS") * [Bucket hijacking](https://unit42.paloaltonetworks.com/tag/bucket-hijacking/ "bucket hijacking") * [Cloud data exfiltration](https://unit42.paloaltonetworks.com/tag/cloud-data-exfiltration/ "cloud data exfiltration") [Read now ![Right arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-right-arrow-withtail.svg)](https://unit42.paloaltonetworks.com/cloud-bucket-hijacking-risks/ "The Global Namespace Risk: Universal Bucket Hijacking Technique for Cloud Data Exfiltration") ![Pictorial representation of Vertex AI model uploads. Close-up view of a digital wall displaying various glowing icons, representing a high-tech network interface.](https://unit42.paloaltonetworks.com/wp-content/uploads/2026/06/AdobeStock_1270203474-1-786x354.png) [![category icon](https://unit42.paloaltonetworks.com/wp-content/uploads/2024/06/icon-threat-research.svg)Threat Research](https://unit42.paloaltonetworks.com/category/threat-research/) June 16, 2026 [#### Pickle in the Middle -- Hijacking Vertex AI Model Uploads for Cross-Tenant RCE](https://unit42.paloaltonetworks.com/hijacking-vertex-ai-model/) * [Bucket squatting](https://unit42.paloaltonetworks.com/tag/bucket-squatting/ "bucket squatting") * [Google Cloud](https://unit42.paloaltonetworks.com/tag/google-cloud/ "Google Cloud") * [Joblib](https://unit42.paloaltonetworks.com/tag/joblib/ "joblib") [Read now ![Right arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-right-arrow-withtail.svg)](https://unit42.paloaltonetworks.com/hijacking-vertex-ai-model/ "Pickle in the Middle – Hijacking Vertex AI Model Uploads for Cross-Tenant RCE") ![Pictorial representation of Cloud Logging services for defense evasion. A vibrant digital illustration depicting a glowing, neon blue cloud symbol positioned over a circuit board landscape. The cloud symbolizes cloud computing technology, and the landscape features intricate electronic circuits with glowing lines and nodes, suggesting high-tech data transfer and connectivity.](https://unit42.paloaltonetworks.com/wp-content/uploads/2026/06/11_Cloud_cybersecurity_research_Overview_1920x900-786x368.jpg) [![category icon](https://unit42.paloaltonetworks.com/wp-content/uploads/2024/06/icon-threat-research.svg)Threat Research](https://unit42.paloaltonetworks.com/category/threat-research/) June 9, 2026 [#### Blinding the Watchmen: Abusing Cloud Logging Services for Defense Evasion and Visibility](https://unit42.paloaltonetworks.com/cloud-logging-defense-evasion/) * [AWS CloudTrail](https://unit42.paloaltonetworks.com/tag/aws-cloudtrail/ "AWS CloudTrail") * [Cloud logging](https://unit42.paloaltonetworks.com/tag/cloud-logging/ "cloud logging") * [Defense evasion](https://unit42.paloaltonetworks.com/tag/defense-evasion/ "defense evasion") [Read now ![Right arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/icon-right-arrow-withtail.svg)](https://unit42.paloaltonetworks.com/cloud-logging-defense-evasion/ "Blinding the Watchmen: Abusing Cloud Logging Services for Defense Evasion and Visibility") * ![Slider arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/slider-arrow-left.svg) * ![Slider arrow](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/slider-arrow-left.svg) ![Close button](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/icons/close-modal.svg) ![Enlarged Image]() ![Newsletter](https://unit42.paloaltonetworks.com/wp-content/uploads/2026/03/unit42-footer-subscribe-desktop.png) ![UNIT 42 Small Logo](https://unit42.paloaltonetworks.com/wp-content/themes/unit42-v6/dist/images/palo-alto-logo-small.svg) Get updates from Unit 42 ## Peace of mind comes from staying ahead of threats. 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