CVE-2025-3576: MIT Kerberos Auth Bypass Vulnerability

CVE-2025-3576 Overview

A vulnerability in the MIT Kerberos implementation allows GSSAPI-protected messages using RC4-HMAC-MD5 to be spoofed due to weaknesses in the MD5 checksum design. If RC4 is preferred over stronger encryption types, an attacker could exploit MD5 collisions to forge message integrity codes. This may lead to unauthorized message tampering.

Critical Impact

Attackers can forge message integrity codes by exploiting MD5 collisions, potentially enabling unauthorized message tampering in GSSAPI-protected communications when RC4-HMAC-MD5 encryption is used.

Affected Products

• MIT Kerberos implementations using RC4-HMAC-MD5 encryption
• Red Hat Enterprise Linux (multiple versions with krb5 packages)
• Debian Linux systems with affected Kerberos packages

Discovery Timeline

• April 15, 2025 - CVE-2025-3576 published to NVD
• September 2, 2025 - Last updated in NVD database

Technical Details for CVE-2025-3576

Vulnerability Analysis

This vulnerability affects the MIT Kerberos implementation when GSSAPI-protected messages utilize the legacy RC4-HMAC-MD5 encryption type. The weakness stems from the inherent cryptographic flaws in MD5, which has been known to be susceptible to collision attacks since the mid-2000s. When RC4-HMAC-MD5 is configured as a preferred encryption type, the GSSAPI layer relies on MD5-based checksums to ensure message integrity.

An attacker positioned on the network can exploit known MD5 collision techniques to craft malicious messages that produce identical checksum values to legitimate messages. This allows the attacker to forge message integrity codes (MICs) without possessing the actual session keys, effectively bypassing the integrity protection that GSSAPI is intended to provide.

The attack requires network access and the ability to intercept and modify Kerberos traffic. Additionally, the target environment must be configured to use or accept RC4-HMAC-MD5 as an encryption type, which is increasingly rare in modern deployments but may still exist in legacy environments or those requiring backward compatibility.

Root Cause

The root cause of CVE-2025-3576 is the use of a cryptographically weak hash algorithm (MD5) within the RC4-HMAC-MD5 encryption type (CWE-328: Use of Weak Hash). MD5 was designed in 1991 and has been progressively weakened by cryptographic research, with practical collision attacks demonstrated as early as 2004. The RC4-HMAC-MD5 encryption type in Kerberos relies on MD5 for computing message authentication codes, making it vulnerable to collision-based forgery attacks.

Modern Kerberos deployments should use stronger encryption types such as AES256-CTS-HMAC-SHA1-96 or AES128-CTS-HMAC-SHA1-96, which do not suffer from the same cryptographic weaknesses.

Attack Vector

The attack vector is network-based and requires the attacker to intercept GSSAPI-protected communications between a Kerberos client and server. The exploitation scenario involves:

1. Identifying a target environment where RC4-HMAC-MD5 is enabled and preferred
2. Capturing legitimate GSSAPI-protected messages
3. Computing MD5 collisions to create forged messages with valid integrity codes
4. Injecting the forged messages into the communication stream

The vulnerability specifically targets the message integrity protection mechanism rather than confidentiality, meaning attackers can tamper with message content but must work within the constraints of collision generation.

Detection Methods for CVE-2025-3576

Indicators of Compromise

• Unexpected modifications to GSSAPI-protected message payloads in application logs
• Anomalous Kerberos authentication patterns indicating message manipulation attempts
• Authentication failures following successful ticket acquisition when using RC4 encryption
• Network traffic analysis showing inconsistencies between expected and received message content

Detection Strategies

• Monitor Kerberos KDC logs for unusual encryption type negotiation patterns, specifically watching for RC4-HMAC-MD5 usage
• Implement network intrusion detection rules to identify potential message tampering attempts in GSSAPI traffic
• Audit Kerberos configuration files (krb5.conf, kdc.conf) for weak encryption type settings
• Deploy endpoint detection solutions to identify applications negotiating deprecated encryption types

Monitoring Recommendations

• Enable verbose logging on Kerberos KDC servers to track encryption type selection during ticket requests
• Configure SIEM rules to alert on RC4-HMAC-MD5 encryption type usage in enterprise environments
• Periodically audit Active Directory and MIT Kerberos environments for weak encryption type configurations
• Monitor for security advisories from Red Hat and MIT Kerberos for updates

How to Mitigate CVE-2025-3576

Immediate Actions Required

• Audit Kerberos configuration to identify if RC4-HMAC-MD5 encryption type is enabled
• Disable RC4-HMAC-MD5 encryption type in krb5.conf by modifying the permitted_enctypes and default_tgs_enctypes settings
• Update Kerberos packages to patched versions as referenced in vendor security advisories
• Test applications for compatibility with stronger encryption types before disabling RC4 in production

Patch Information

Multiple vendors have released security patches addressing this vulnerability. Red Hat has issued several security advisories including RHSA-2025:8411, RHSA-2025:9418, RHSA-2025:9430, and additional updates through RHSA-2025:15004. Debian has also released patches as documented in the Debian LTS Announcement. Administrators should apply the appropriate patches for their distribution and verify successful installation.

Workarounds

• Disable RC4-HMAC-MD5 encryption type in the krb5.conf configuration file by excluding it from permitted encryption types
• Configure Kerberos to prefer AES-based encryption types (AES256-CTS-HMAC-SHA1-96, AES128-CTS-HMAC-SHA1-96)
• If RC4 must remain enabled for legacy compatibility, implement network segmentation to limit exposure of affected systems
• Consider deploying additional application-layer integrity verification for sensitive communications

# Configuration example - krb5.conf encryption type hardening
[libdefaults]
    default_realm = EXAMPLE.COM
    # Disable RC4-HMAC-MD5 by specifying only strong encryption types
    permitted_enctypes = aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96
    default_tgs_enctypes = aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96
    default_tkt_enctypes = aes256-cts-hmac-sha1-96 aes128-cts-hmac-sha1-96