A security vulnerability has been identified in the OpenSSL library that could potentially be exploited by an attacker to cause a denial of service (DoS) attack. This vulnerability, designated as CVE-2022-4450, affects the function PEM_read_bio_ex() and other related functions. In this post, we will discuss the details of this vulnerability, including the affected code, exploit conditions, and possible mitigation options.

Vulnerability Details

The function PEM_read_bio_ex() reads a PEM file from a BIO (Basic Input/Output) and is responsible for parsing and decoding the name (e.g., "CERTIFICATE"), any header data, and the payload data. If the function succeeds, the name_out, header, and data arguments are populated with pointers to buffers containing the relevant decoded data. The caller is then responsible for freeing those buffers.

It is possible to construct a malicious PEM file that results in  bytes of payload data. In this scenario, PEM_read_bio_ex() will return a failure code but will populate the header argument with a pointer to a buffer that has already been freed. If the caller also frees this buffer, a double-free vulnerability will occur, likely leading to a crash.

This vulnerability could be exploited by an attacker who has the ability to supply malicious PEM files for parsing to achieve a denial of service attack. The functions PEM_read_bio() and PEM_read() are simple wrappers around PEM_read_bio_ex() and are therefore also directly affected. These functions are also called indirectly by various other OpenSSL functions, including PEM_X509_INFO_read_bio_ex() and SSL_CTX_use_serverinfo_file(), which are also vulnerable.

Here is an example code snippet of the vulnerable PEM_read_bio_ex() function

int PEM_read_bio_ex(BIO *bp, char name, char header, unsigned char **data,
                    long *len, unsigned int flags)
{
    // vulnerable code logic
}

Some OpenSSL internal uses of these functions are not vulnerable because the caller does not free the header argument if PEM_read_bio_ex() returns a failure code. These locations include the PEM_read_bio_TYPE() functions and the decoders introduced in OpenSSL 3..

The OpenSSL asn1parse command-line application is also impacted by this issue.

OpenSSL Security Advisory: [CVE-2022-4450](

https://www.openssl.org/news/secadv/20220113.txt)
- CVE Details: CVE-2022-4450

Exploit Details

To exploit this vulnerability, an attacker must first be able to supply a malicious PEM file to an application using the vulnerable OpenSSL functions for parsing. The attacker can then craft a PEM file with specific characteristics to trigger the double-free situation, ultimately causing a crash and potential denial of service attack.

Mitigations

It is highly recommended to apply the latest available OpenSSL patches to all software that relies on OpenSSL for PEM file processing. Affected users should upgrade to OpenSSL versions 3..1, 1.1.1n, or later, which contain the necessary fixes for this vulnerability.

In addition to updating OpenSSL, it is a good security practice to validate user input and sanitize any data received from untrusted sources. This can help mitigate the risk of denial of service attacks from malicious input files.

Conclusion

CVE-2022-4450 is a double-free vulnerability in the OpenSSL library affecting the PEM_read_bio_ex() function and related functions. An attacker can potentially exploit this issue to cause a denial of service attack by supplying a malicious PEM file for parsing. It is crucial to apply the latest OpenSSL patches and follow secure programming practices to mitigate the risk and protect your systems from this vulnerability.

Timeline

Published on: 02/08/2023 20:15:00 UTC
Last modified on: 02/24/2023 15:15:00 UTC