CVE-2026-39418: MaxKB Authentication Bypass Vulnerability

CVE-2026-39418 Overview

CVE-2026-39418 is a Server-Side Request Forgery (SSRF) vulnerability affecting MaxKB, an open-source AI assistant platform for enterprise environments. The vulnerability allows authenticated users with tool-editing permissions to bypass sandbox network protection mechanisms, potentially enabling access to internal services that should be blocked by the sandbox's banned hosts configuration.

Critical Impact

Authenticated attackers can bypass network sandbox restrictions to reach internal services, potentially accessing sensitive internal infrastructure and data that was explicitly meant to be protected.

Affected Products

• MaxKB versions 2.7.1 and below

Discovery Timeline

• 2026-04-14 - CVE CVE-2026-39418 published to NVD
• 2026-04-14 - Last updated in NVD database

Technical Details for CVE-2026-39418

Vulnerability Analysis

This vulnerability exists in MaxKB's sandbox implementation, which relies on LD_PRELOAD to hook the connect() function and block connections to banned IP addresses. However, Linux provides an alternative path for establishing TCP connections through the sendto() system call when combined with the MSG_FASTOPEN flag. This TCP Fast Open mechanism allows the kernel to establish connections directly without invoking the hooked connect() function, completely circumventing the IP validation logic.

The sandbox's security model is fundamentally flawed because it attempts to enforce network restrictions at the glibc level rather than at the kernel level. When sendto() is called with MSG_FASTOPEN, glibc invokes the kernel syscall directly rather than routing through any hooked syscall() function, making the sendto entry in the syscall wrapper ineffective.

Root Cause

The root cause stems from incomplete syscall coverage in the sandbox's network interception mechanism. The sandbox implementation hooked only the connect() function via LD_PRELOAD, failing to account for TCP Fast Open's ability to establish connections through sendto() with MSG_FASTOPEN. This architectural oversight in CWE-918 (Server-Side Request Forgery) allows attackers to bypass the intended network isolation.

Attack Vector

An authenticated user with tool-editing permissions can exploit this vulnerability by crafting network requests that use socket.sendto() with the MSG_FASTOPEN flag instead of traditional connect() calls. This allows the attacker to establish connections to internal services on the banned hosts list, potentially accessing internal APIs, databases, or other protected infrastructure.

     free(list);
     return blocked;
 }
-
-// ------------------ 网络拦截 ------------------
-int connect(int sockfd, const struct sockaddr *addr, socklen_t addrlen) {
-    RESOLVE_REAL(connect);
-    ensure_config_loaded();
-    if (is_sandbox_user() && addr->sa_family == AF_UNIX) {
+static int match_banned_addr(const struct sockaddr *addr) {
+    if (addr->sa_family == AF_UNIX) {
         struct sockaddr_un *un = (struct sockaddr_un *)addr;
         throw_permission_denied_err(false, "access unix socket: %s", un->sun_path[0] ? un->sun_path : "(abstract)");
-        return -1;
+        return 1;
     }
     char ip[INET6_ADDRSTRLEN] = {0};
     if (addr->sa_family == AF_INET) {

Source: GitHub Commit Changes

Detection Methods for CVE-2026-39418

Indicators of Compromise

• Unusual outbound connections from MaxKB sandbox processes to internal IP addresses
• Network traffic using TCP Fast Open (MSG_FASTOPEN) from the MaxKB application
• Connections to banned hosts originating from sandbox-restricted user contexts
• Audit logs showing tool-editing activities followed by unexpected internal service access

Detection Strategies

• Monitor network connections from MaxKB processes for attempts to reach internal services or banned IP ranges
• Implement network-level monitoring to detect TCP Fast Open connections originating from sandbox environments
• Review user activity logs for tool-editing permissions being used in conjunction with suspicious network behavior
• Deploy intrusion detection rules to flag sendto() syscalls with MSG_FASTOPEN from sandboxed processes

Monitoring Recommendations

• Enable detailed network logging for all MaxKB sandbox processes
• Implement egress filtering at the network level to enforce banned host restrictions independently of application-level controls
• Set up alerts for any connections from sandbox users to internal infrastructure
• Regularly audit users with tool-editing permissions and their activities

How to Mitigate CVE-2026-39418

Immediate Actions Required

• Upgrade MaxKB to version 2.8.0 or later immediately
• Review and restrict tool-editing permissions to only trusted users
• Implement network-level controls (firewall rules, network segmentation) to enforce banned host restrictions
• Audit logs for any exploitation attempts prior to patching

Patch Information

The vulnerability has been addressed in MaxKB version 2.8.0. The fix refactors the network interception code to use a unified match_banned_addr() function that can validate addresses across multiple syscall pathways, including sendto() and sendmsg(). Users should upgrade to version 2.8.0 or later. For detailed patch information, see the GitHub Security Advisory GHSA-w9g4-q3gm-6q6w and the v2.8.0 release.

Workarounds

• Implement network-level firewall rules to block sandbox processes from accessing internal services, independent of application controls
• Restrict tool-editing permissions to only essential users until patching is complete
• Disable TCP Fast Open at the kernel level using sysctl -w net.ipv4.tcp_fastopen=0 as a temporary measure
• Use container networking or network namespaces to provide isolation at the operating system level

# Disable TCP Fast Open as temporary mitigation
sysctl -w net.ipv4.tcp_fastopen=0

# Persist the setting across reboots
echo "net.ipv4.tcp_fastopen = 0" >> /etc/sysctl.conf