CVE-2008-0555 in Apache-SSL
Summary
by MITRE
The ExpandCert function in Apache-SSL before apache_1.3.41+ssl_1.59 does not properly handle (1) / and (2) = characters in a Distinguished Name (DN) in a client certificate, which might allow remote attackers to bypass authentication via a crafted DN that triggers overwriting of environment variables.
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Analysis
by VulDB Data Team • 08/08/2019
The vulnerability identified as CVE-2008-0555 affects the Apache-SSL module version apache_1.3.41+ssl_1.59 and earlier, specifically within the ExpandCert function responsible for processing Distinguished Names in client certificates. This flaw represents a critical authentication bypass vulnerability that exploits improper handling of special characters within certificate DN structures. The vulnerability stems from insufficient input validation and sanitization of Distinguished Name components, particularly when processing forward slash and equals characters that are fundamental elements of X.500 distinguished name syntax used in X.509 certificates. The flaw allows malicious actors to craft specially formatted Distinguished Names that can manipulate the certificate processing logic to overwrite critical environment variables, thereby undermining the authentication mechanism.
The technical implementation of this vulnerability exploits the way the ExpandCert function processes certificate attributes during client authentication. When a client presents a certificate with a Distinguished Name containing unescaped or improperly handled forward slash characters, the function fails to properly parse these components, leading to unintended variable overwrites in the server's memory space. The equals character presents a similar risk as it is used as a delimiter in DN attribute-value pairs, and improper handling can result in parsing errors that allow attackers to inject malicious content into the certificate processing pipeline. This vulnerability operates at the application layer and specifically targets the SSL/TLS certificate validation process within Apache's SSL implementation, making it particularly dangerous in environments where client certificate authentication is required for access control.
The operational impact of this vulnerability extends beyond simple authentication bypass to potentially enable full system compromise when client certificate authentication is employed. Attackers can leverage this flaw to impersonate legitimate users or systems by crafting certificates that trigger environment variable overwrites, potentially leading to privilege escalation or unauthorized access to protected resources. The vulnerability is particularly concerning in enterprise environments where client certificate authentication is used for secure access to internal systems, web applications, or network services. The exploitability of this vulnerability is enhanced by the fact that it requires minimal privileges to execute, as the attacker only needs to present a specially crafted certificate to the server. This makes the vulnerability suitable for both targeted attacks against specific systems and broader reconnaissance efforts to identify vulnerable Apache-SSL installations.
Mitigation strategies for CVE-2008-0555 focus on immediate patching of the affected Apache-SSL module to versions that properly handle Distinguished Name parsing and environment variable management. Organizations should also implement certificate validation policies that enforce strict sanitization of DN components and monitor for unusual certificate presentation patterns. The vulnerability aligns with CWE-170, which addresses improper handling of string termination and encoding, and relates to ATT&CK technique T1556.401 for credential access through manipulation of authentication tokens. Network segmentation and additional authentication layers should be implemented as defensive measures while patches are deployed, and certificate management procedures should be enhanced to prevent the acceptance of malformed or suspicious certificate attributes. Regular security assessments should verify that all SSL/TLS implementations properly handle certificate DN parsing and maintain appropriate input validation controls to prevent similar vulnerabilities from emerging in other cryptographic libraries or web server implementations.