CVE-2021-3712: Read buffer overruns processing ASN.1 strings

ASN.1 strings are represented internally within OpenSSL as an ASN1STRING structure which contains a buffer holding the string data and a field holding the buffer length. This contrasts with normal C strings which are repesented as a buffer for the string data which is terminated with a NUL (0) byte. Although not a strict requirement, ASN.1 strings that are parsed using OpenSSL's own "d2i" functions (and other similar parsing functions) as well as any string whose value has been set with the ASN1STRINGset() function will additionally NUL terminate the byte array in the ASN1STRING structure. However, it is possible for applications to directly construct valid ASN1STRING structures which do not NUL terminate the byte array by directly setting the "data" and "length" fields in the ASN1STRING array. This can also happen by using the ASN1STRINGset0() function. Numerous OpenSSL functions that print ASN.1 data have been found to assume that the ASN1STRING byte array will be NUL terminated, even though this is not guaranteed for strings that have been directly constructed. Where an application requests an ASN.1 structure to be printed, and where that ASN.1 structure contains ASN1STRINGs that have been directly constructed by the application without NUL terminating the "data" field, then a read buffer overrun can occur. The same thing can also occur during name constraints processing of certificates (for example if a certificate has been directly constructed by the application instead of loading it via the OpenSSL parsing functions, and the certificate contains non NUL terminated ASN1STRING structures). It can also occur in the X509get1email(), X509REQget1email() and X509get1ocsp() functions. If a malicious actor can cause an application to directly construct an ASN1STRING and then process it through one of the affected OpenSSL functions then this issue could be hit. This might result in a crash (causing a Denial of Service attack). It could also result in the disclosure of private memory contents (such as private keys, or sensitive plaintext). Fixed in OpenSSL 1.1.1l (Affected 1.1.1-1.1.1k). Fixed in OpenSSL 1.0.2za (Affected 1.0.2-1.0.2y).

It was found that openssl assumed ASN.1 strings to be NUL terminated. A malicious actor may be able to force an application into calling openssl function with a specially crafted, non-NUL terminated string to deliberately hit this bug, which may result in a crash of the application, causing a Denial of Service attack, or possibly, memory disclosure. The highest threat from this vulnerability is to data confidentiality and system availability.

OpenSSL could allow a remote attacker to obtain sensitive information, caused by an out-of-bounds read when processing ASN.1 strings. By sending specially crafted data, an attacker could exploit this vulnerability to read contents of memory on the system or perform a denial of service attack.

— IBM

Severity: Moderate

ASN.1 strings are represented internally within OpenSSL as an ASN1STRING structure which contains a buffer holding the string data and a field holding the buffer length. This contrasts with normal C strings which are repesented as a buffer for the string data which is terminated with a NUL (0) byte.

Although not a strict requirement, ASN.1 strings that are parsed using OpenSSL's own "d2i" functions (and other similar parsing functions) as well as any string whose value has been set with the ASN1STRINGset() function will additionally NUL terminate the byte array in the ASN1STRING structure.

However, it is possible for applications to directly construct valid ASN1STRING structures which do not NUL terminate the byte array by directly setting the "data" and "length" fields in the ASN1STRING array. This can also happen by using the ASN1STRINGset0() function.

Numerous OpenSSL functions that print ASN.1 data have been found to assume that the ASN1STRING byte array will be NUL terminated, even though this is not guaranteed for strings that have been directly constructed. Where an application requests an ASN.1 structure to be printed, and where that ASN.1 structure contains ASN1STRINGs that have been directly constructed by the application without NUL terminating the "data" field, then a read buffer overrun can occur.

The same thing can also occur during name constraints processing of certificates (for example if a certificate has been directly constructed by the application instead of loading it via the OpenSSL parsing functions, and the certificate contains non NUL terminated ASN1STRING structures). It can also occur in the X509get1email(), X509REQget1email() and X509get1ocsp() functions.

If a malicious actor can cause an application to directly construct an ASN1STRING and then process it through one of the affected OpenSSL functions then this issue could be hit. This might result in a crash (causing a Denial of Service attack). It could also result in the disclosure of private memory contents (such as private keys, or sensitive plaintext).

OpenSSL versions 1.1.1k and below are affected by this issue. Users of these versions should upgrade to OpenSSL 1.1.1l.

OpenSSL versions 1.0.2y and below are affected by this issue. However OpenSSL 1.0.2 is out of support and no longer receiving public updates. Premium support customers of OpenSSL 1.0.2 should upgrade to 1.0.2z. Other users should upgrade to 1.1.1l.

An initial instance of this issue in the X509auxprint() function was reported to OpenSSL on 18th July 2021 by Ingo Schwarze. The bugfix was developed by Ingo Schwarze and first publicly released in OpenBSD-current on 10th July 2021 and subsequently in OpenSSL on 20th July 2021 (commit d9d838d). Subsequent analysis by David Benjamin on 17th August 2021 identified more instances of the same bug. Additional analysis was performed by Matt Caswell. Fixes for the additional instances of this issue were developed by Matt Caswell.

— Red Hat