CVE-2021-3711: SM2 Decryption Buffer Overflow

In order to decrypt SM2 encrypted data an application is expected to call the API function EVPPKEYdecrypt(). Typically an application will call this function twice. The first time, on entry, the "out" parameter can be NULL and, on exit, the "outlen" parameter is populated with the buffer size required to hold the decrypted plaintext. The application can then allocate a sufficiently sized buffer and call EVPPKEYdecrypt() again, but this time passing a non-NULL value for the "out" parameter. A bug in the implementation of the SM2 decryption code means that the calculation of the buffer size required to hold the plaintext returned by the first call to EVPPKEYdecrypt() can be smaller than the actual size required by the second call. This can lead to a buffer overflow when EVPPKEYdecrypt() is called by the application a second time with a buffer that is too small. A malicious attacker who is able present SM2 content for decryption to an application could cause attacker chosen data to overflow the buffer by up to a maximum of 62 bytes altering the contents of other data held after the buffer, possibly changing application behaviour or causing the application to crash. The location of the buffer is application dependent but is typically heap allocated. Fixed in OpenSSL 1.1.1l (Affected 1.1.1-1.1.1k).

OpenSSL is vulnerable to a buffer overflow, caused by improper bounds checking by the EVPPKEYdecrypt() function within implementation of the SM2 decryption. By sending specially crafted SM2 content, a remote attacker could overflow a buffer and execute arbitrary code on the system or cause the application to crash.

— IBM

Severity: High

In order to decrypt SM2 encrypted data an application is expected to call the API function EVPPKEYdecrypt(). Typically an application will call this function twice. The first time, on entry, the "out" parameter can be NULL and, on exit, the "outlen" parameter is populated with the buffer size required to hold the decrypted plaintext. The application can then allocate a sufficiently sized buffer and call EVPPKEYdecrypt() again, but this time passing a non-NULL value for the "out" parameter.

A bug in the implementation of the SM2 decryption code means that the calculation of the buffer size required to hold the plaintext returned by the first call to EVPPKEYdecrypt() can be smaller than the actual size required by the second call. This can lead to a buffer overflow when EVPPKEYdecrypt() is called by the application a second time with a buffer that is too small.

A malicious attacker who is able present SM2 content for decryption to an application could cause attacker chosen data to overflow the buffer by up to a maximum of 62 bytes altering the contents of other data held after the buffer, possibly changing application behaviour or causing the application to crash. The location of the buffer is application dependent but is typically heap allocated.

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

OpenSSL 1.0.2 is not impacted by this issue.

OpenSSL 3.0 alpha/beta releases are also affected but this issue will be addressed before the final release.

This issue was reported to OpenSSL on 12th August 2021 by John Ouyang. The fix was developed by Matt Caswell.

— Red Hat