CVE-2010-4020 in Kerberos
Summary
by MITRE
MIT Kerberos 5 (aka krb5) 1.8.x through 1.8.3 does not reject RC4 key-derivation checksums, which might allow remote authenticated users to forge a (1) AD-SIGNEDPATH or (2) AD-KDC-ISSUED signature, and possibly gain privileges, by leveraging the small key space that results from certain one-byte stream-cipher operations.
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Analysis
by VulDB Data Team • 10/06/2021
The vulnerability identified as CVE-2010-4020 affects MIT Kerberos 5 versions 1.8.x through 1.8.3, specifically targeting the RC4 key-derivation checksum mechanism. This flaw represents a critical weakness in the authentication system's cryptographic implementation that could enable authenticated attackers to forge critical signatures within the Kerberos infrastructure. The vulnerability stems from the improper handling of checksums during the key derivation process, creating a pathway for attackers to manipulate authentication tokens and potentially escalate privileges within the domain.
The technical flaw manifests in the RC4 encryption algorithm's key derivation function where certain one-byte stream-cipher operations produce a small key space that can be exploited through checksum manipulation. This weakness allows attackers who have authenticated to the Kerberos system to manipulate the signature verification process, specifically targeting AD-SIGNEDPATH and AD-KDC-ISSUED signature types. The vulnerability exploits the mathematical properties of RC4's key scheduling algorithm where specific byte operations result in predictable patterns that can be reverse-engineered to forge valid signatures without possessing the legitimate private keys.
From an operational impact perspective, this vulnerability undermines the fundamental security guarantees provided by Kerberos authentication, potentially allowing attackers to bypass authentication mechanisms and gain unauthorized access to network resources. The ability to forge AD-SIGNEDPATH signatures compromises the integrity of path validation within Active Directory environments, while AD-KDC-ISSUED signature forgery can enable privilege escalation attacks. Attackers could leverage this vulnerability to impersonate legitimate users or services, potentially gaining administrative access to critical systems within the Kerberos domain.
The vulnerability aligns with CWE-327, which addresses the use of weak cryptographic algorithms, and relates to ATT&CK technique T1550.003 for use of Kerberos. Organizations using affected Kerberos versions face significant risk as the flaw requires only authenticated access to exploit, making it particularly dangerous in environments where users have legitimate access to the system. The small key space resulting from the one-byte stream-cipher operations reduces the computational complexity required to perform successful forgery attacks, making this vulnerability practical for exploitation in real-world scenarios.
Mitigation strategies should focus on immediate patching of affected Kerberos versions to 1.8.4 or later, which addressed the RC4 key-derivation checksum issue. Organizations should also consider disabling RC4 encryption entirely from their Kerberos configurations and implementing stronger cryptographic algorithms such as AES. Network segmentation and monitoring for unusual authentication patterns can help detect potential exploitation attempts. Additionally, implementing proper key management practices and regular cryptographic audits will help prevent similar vulnerabilities from emerging in other components of the authentication infrastructure. Security teams should also review their Kerberos configuration to ensure that checksum validation is properly enforced and that appropriate access controls are in place to limit the impact of potential exploitation attempts.