CVE-2004-0644 in Kerberos
Summary
by MITRE
The asn1buf_skiptail function in the ASN.1 decoder library for MIT Kerberos 5 (krb5) 1.2.2 through 1.3.4 allows remote attackers to cause a denial of service (infinite loop) via a certain BER encoding.
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Analysis
by VulDB Data Team • 12/16/2024
The vulnerability described in CVE-2004-0644 represents a critical denial of service flaw within the ASN.1 decoder component of MIT Kerberos 5 versions 1.2.2 through 1.3.4. This issue specifically targets the asn1buf_skiptail function which processes BER (Basic Encoding Rules) encoded data during Kerberos authentication operations. The flaw occurs when the decoder encounters certain malformed ASN.1 structures that trigger an infinite loop condition, effectively causing the Kerberos service to become unresponsive and preventing legitimate authentication requests from being processed. This vulnerability directly impacts the availability aspect of the Kerberos authentication system, which forms the backbone of many enterprise security infrastructures relying on centralized authentication services.
The technical implementation of this vulnerability stems from improper bounds checking and loop termination conditions within the asn1buf_skiptail function. When processing BER encoded data, the function fails to properly validate the structure of the encoded elements, particularly when encountering certain combinations of constructed and primitive encodings. The infinite loop manifests when the decoder attempts to skip trailing bytes in an ASN.1 structure but encounters malformed encoding that causes the loop counter to never reach its termination condition. This type of flaw falls under CWE-835, which specifically addresses infinite loops in software implementations where loop termination conditions are not properly handled. The vulnerability demonstrates a classic example of how improper input validation can lead to resource exhaustion and service disruption.
The operational impact of this vulnerability extends beyond simple service interruption, as it affects the fundamental reliability of Kerberos-based authentication systems. When exploited, the infinite loop causes the Kerberos server process to consume excessive CPU resources and become unresponsive to legitimate authentication requests, effectively creating a denial of service condition that can impact entire organizational networks relying on Kerberos for authentication. Attackers can leverage this vulnerability by crafting specially crafted ASN.1 encoded packets that trigger the problematic code path, making it particularly dangerous in environments where Kerberos is used for critical authentication services such as single sign-on systems, file server access, or network service authentication. This vulnerability aligns with ATT&CK technique T1499.004, which covers network denial of service attacks targeting authentication services.
Mitigation strategies for this vulnerability involve immediate patching of affected Kerberos installations to versions that contain the corrected ASN.1 decoder implementation. Organizations should also implement network monitoring to detect unusual patterns of authentication service degradation that might indicate exploitation attempts. Additional protective measures include implementing rate limiting on authentication requests, deploying intrusion detection systems that can identify malformed ASN.1 traffic patterns, and ensuring proper input validation at network boundaries. The fix typically involves adding proper bounds checking and loop termination validation to prevent the infinite loop condition from occurring when processing malformed BER encoded data. Security teams should also consider implementing redundant authentication mechanisms and monitoring systems to maintain service availability even when individual components are under attack. This vulnerability underscores the importance of thorough input validation in cryptographic libraries and the critical need for proper bounds checking in protocol implementation code.