Executive Summary

This vulnerability is a signed integer overflow in the function ASN1_mbstring_ncopy() used in OpenSSL. When calculating the size of the destination buffer for Unicode output, the signed integer used for sizing can overflow if the input is very large (around 2^30 characters). This overflow causes the buffer size to wrap around to a small value, leading to a heap buffer overflow when copying characters.

Specifically, the overflow happens during size computation for BMPSTRING (UTF-16) and UNIVERSALSTRING (UTF-32) by left shifting the input character count, or by summing byte counts for UTF8STRING. In the worst case, this results in allocating a very small buffer but writing several gigabytes of data past it.

This bug is not triggered by normal X.509 certificate processing in OpenSSL, but requires an application to call ASN1_mbstring_copy() or ASN1_mbstring_ncopy() directly with attacker-controlled input of very large size, or to register a custom string type that can be exploited similarly.

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Impact Analysis

The heap buffer overflow caused by this vulnerability can lead to a crash of the affected application or potentially allow an attacker to execute arbitrary code or cause other undefined behavior.

However, exploitation requires very large attacker-controlled input (on the order of half a gigabyte or more) and direct use of vulnerable functions, which limits the practical risk in typical OpenSSL usage.

Because of these factors, this issue is assigned a Low severity rating despite the high CVSS base score.

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Detection Guidance

This vulnerability occurs in specific OpenSSL functions (ASN1_mbstring_copy() or ASN1_mbstring_ncopy()) when processing very large input sizes (around 2^30 characters). Detection would require monitoring for applications that directly call these functions with attacker-controlled input of half a gigabyte or more.

Since no network protocol or certificate-handling path in OpenSSL exercises this overflow, and the issue requires direct application calls with large inputs, typical network scanning or system commands are unlikely to detect exploitation attempts.

No specific commands or detection methods are provided in the available information.

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Mitigation Strategies

Immediate mitigation involves ensuring that applications do not call ASN1_mbstring_copy() or ASN1_mbstring_ncopy() directly with untrusted or attacker-controlled large inputs.

Since the vulnerability requires very large input sizes and direct function calls, restricting or validating input sizes before passing them to these functions can prevent exploitation.

Additionally, using OpenSSL FIPS modules (versions 4.0, 3.6, 3.5, 3.4, and 3.0) can mitigate risk as they are not affected by this issue.

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