Vulnerability Note VU#686224

OpenSSL does not securely handle invalid public key when configured to ignore errors

Original Release date: 30 Sep 2003 | Last revised: 01 Oct 2003

Overview

A vulnerability in the way OpenSSL handles invalid public keys in client certificate messages could allow a remote attacker to cause a denial of service. This vulnerability requires as a precondition that an application is configured to ignore public key decoding errors, which is typically only the case during debugging.

Description

OpenSSL implements the Secure Sockets Layer (SSL) and Transport Layer Security (TLS) protocols and includes a general-purpose cryptographic library. SSL and TLS are commonly used to provide authentication, encryption, integrity, and non-repudiation services to network applications such as HTTP, IMAP, POP3, LDAP, and others. Clients and servers exchange authentication information in X.509 certificates. While the SSL and TLS protocols are not directly based on ASN.1, they do rely on ASN.1 objects used in X.509 certificates.

OpenSSL certificate verification code contains a vulnerability that can be triggered by an invalid public key in a client certificate message. This vulnerability only occurs if the application is configured to ignore all errors or the specific error X509_V_ERR_UNABLE_TO_DECODE_ISSUER_PUBLIC_KEY. Typically, an application would only ignore public key decoding errors for debugging purposes. Production applications should not be configured this way, since ignoring these errors effectively disables certificate signature verification.

From the OpenSSL advisory:


3. A malformed public key in a certificate will crash the verify code if
it is set to ignore public key decoding errors. Public key decode errors
are not normally ignored, except for debugging purposes, so this is
unlikely to affect production code. Exploitation of an affected
application would result in a denial of service vulnerability.

The U.K. National Infrastructure Security Co-ordination Centre (NISCC) is tracking this vulnerability as NISCC/006489/openssl/2.

Impact

By sending a specially crafted client certificate message to a vulnerable system, a remote attacker could cause a denial of service.

Solution

Upgrade or Patch
Upgrade to OpenSSL 0.9.7c or 0.9.6k. Alternatively, upgrade or apply a patch as specified by your vendor. Further information is available in an advisory from OpenSSL. Note that it is necessary to recompile any applications that are statically linked to OpenSSL libraries.


Verify application configuration

A necessary precondition of this vulnerability is that an application is configured to ignore public key decoding errors. Verify that production applications are not configured this way.

Systems Affected (Learn More)

VendorStatusDate NotifiedDate Updated
OpenSSLAffected-30 Sep 2003
If you are a vendor and your product is affected, let us know.

CVSS Metrics (Learn More)

Group Score Vector
Base N/A N/A
Temporal N/A N/A
Environmental N/A N/A

References

Credit

This vulnerability was discovered and researched by NISCC. Thanks also to Stephen Henson of the OpenSSL project.

This document was written by Art Manion.

Other Information

  • CVE IDs: Unknown
  • CERT Advisory: CA-2003-26
  • Date Public: 30 Sep 2003
  • Date First Published: 30 Sep 2003
  • Date Last Updated: 01 Oct 2003
  • Severity Metric: 1.50
  • Document Revision: 34

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