CVE-2026-40606: mitmproxy LDAP Auth Bypass Vulnerability

CVE-2026-40606 Overview

CVE-2026-40606 is an LDAP Injection vulnerability affecting mitmproxy, a popular interactive TLS-capable intercepting HTTP proxy used by penetration testers and software developers. The vulnerability exists in the builtin LDAP proxy authentication mechanism in mitmproxy 12.2.1 and below, where username input is not correctly sanitized when querying the LDAP server. This flaw allows a malicious client to craft specially formatted usernames that manipulate the LDAP query, ultimately bypassing authentication controls.

Critical Impact

Attackers can bypass LDAP-based proxy authentication to gain unauthorized access to mitmproxy instances, potentially intercepting or manipulating HTTP/HTTPS traffic passing through the proxy.

Affected Products

• mitmproxy versions 12.2.1 and below
• mitmweb (web-based interface for mitmproxy) versions 12.2.1 and below
• Only instances with proxyauth option configured for LDAP authentication

Discovery Timeline

• 2026-04-21 - CVE CVE-2026-40606 published to NVD
• 2026-04-22 - Last updated in NVD database

Technical Details for CVE-2026-40606

Vulnerability Analysis

This vulnerability is classified as CWE-90 (Improper Neutralization of Special Elements used in an LDAP Query, also known as LDAP Injection). The flaw resides in how mitmproxy handles user-supplied input during the LDAP authentication process. When the proxyauth option is configured to use LDAP backend authentication, user-provided credentials are used to construct LDAP queries without adequate sanitization.

The attack requires network access and exploits the lack of input validation in the username field. Although the attack complexity is considered high due to the specific configuration requirements (LDAP authentication must be explicitly enabled), successful exploitation allows attackers to bypass authentication entirely. This can result in unauthorized access to the proxy, potentially leading to confidentiality and integrity impacts as attackers could intercept or modify proxied traffic.

It's important to note that this vulnerability only affects mitmproxy instances where LDAP proxy authentication is explicitly configured using the proxyauth option. This option is not enabled by default, limiting the attack surface to specifically configured deployments.

Root Cause

The root cause is improper neutralization of special LDAP metacharacters in the username parameter before incorporating it into LDAP queries. When constructing the authentication query, the application fails to escape or sanitize characters that have special meaning in LDAP filter syntax, such as parentheses (), asterisks *, backslashes \, and other metacharacters. This allows an attacker to inject malicious LDAP filter components that alter the query logic.

Attack Vector

The attack is network-based and targets mitmproxy instances configured with LDAP authentication. An attacker would connect to the proxy and provide a maliciously crafted username containing LDAP injection payloads. These payloads manipulate the LDAP query structure, potentially modifying the filter to always evaluate as true or bypassing the password check entirely.

For example, an attacker might use LDAP filter injection techniques to close existing filter conditions and inject new ones that bypass authentication logic. The vulnerability requires no authentication or user interaction to exploit, though the specific configuration requirement (LDAP auth enabled) increases the overall attack complexity.

For technical details on the specific injection vectors, refer to the GitHub Security Advisory.

Detection Methods for CVE-2026-40606

Indicators of Compromise

• Unusual authentication attempts with usernames containing LDAP metacharacters such as *, (, ), \, |, &, or =
• Successful authentications from unexpected sources or IP addresses
• Log entries showing usernames with encoded or special characters in LDAP-related components
• Multiple rapid authentication attempts from the same source with varying username patterns

Detection Strategies

• Monitor proxy authentication logs for usernames containing LDAP special characters or injection patterns
• Implement application-layer logging to capture raw username inputs before LDAP query construction
• Deploy network-based intrusion detection signatures to identify LDAP injection payloads in proxy authentication traffic
• Review LDAP server logs for anomalous query patterns or unusual bind attempts

Monitoring Recommendations

• Enable verbose logging on mitmproxy instances to capture authentication events
• Configure LDAP server auditing to log all authentication queries and bind operations
• Set up alerts for authentication anomalies such as successful logins from unusual locations or times
• Implement network traffic analysis to detect suspicious patterns in proxy authentication flows

How to Mitigate CVE-2026-40606

Immediate Actions Required

• Upgrade mitmproxy to version 12.2.2 or above immediately
• Review current mitmproxy configurations to identify instances using LDAP proxy authentication
• Audit authentication logs for any signs of exploitation attempts prior to patching
• Consider temporarily disabling LDAP authentication or the affected proxy instances until patching is complete

Patch Information

The vulnerability has been fixed in mitmproxy version 12.2.2 and above. Organizations should upgrade all affected mitmproxy installations to the latest available version. The fix implements proper sanitization of username input before constructing LDAP queries, preventing injection attacks. For detailed information, see the GitHub Security Advisory.

Workarounds

• Disable LDAP-based proxy authentication by removing or modifying the proxyauth configuration option
• Switch to alternative authentication mechanisms such as local username/password authentication until the patch is applied
• Implement network-level access controls to restrict proxy access to trusted IP addresses or networks only
• Deploy a web application firewall (WAF) or reverse proxy in front of the mitmproxy instance to filter malicious authentication payloads

# Example: Disable LDAP authentication temporarily
# Remove or comment out the proxyauth LDAP configuration
# Original (vulnerable): --proxyauth ldap:ldap://ldap.example.com/dc=example,dc=com
# Temporary fix: Use local authentication or disable auth
mitmproxy --proxyauth username:password