REDHAT-BUG-2481879: Buffer Overflow

Severity: Low

Issue summary: A signed integer overflow when sizing the destination buffer for Unicode output in ASN1mbstringncopy() can lead to a heap buffer overflow.

Impact summary: A heap buffer overflow may lead to a crash or possibly attacker controlled code execution or other undefined behaviour.

In ASN1mbstringcopy() and ASN1mbstringncopy() the destination size for Unicode output is computed in a signed int: by left shift of the input character count for BMPSTRING (UTF-16) and UNIVERSALSTRING (UTF-32), and by summing per-character byte counts for UTF8STRING. The calculation overflows when the input reaches around 2^30 characters. In the worst case (UNIVERSALSTRING at 2^30 characters) the size wraps to zero, OPENSSLmalloc(1) is called, and the subsequent character copy writes several gigabytes past the one-byte allocation.

X.509 certificate processing routes through ASN1STRINGsetbyNID(), whose DIRSTRINGTYPE mask excludes UNIVERSALSTRING and whose per-NID size limits cap the input length; no network protocol or certificate-handling path in OpenSSL exercises the overflow. Triggering the bug requires an application that calls ASN1mbstringcopy() or ASN1mbstringncopy() directly, or registers a custom string type via ASN1STRINGTABLEadd(), with attacker-controlled input on the order of half a gigabyte or more. For these reasons this issue was assigned Low severity.

The FIPS modules in 4.0, 3.6, 3.5, 3.4 and 3.0 are not affected by this issue, as the affected code is outside the OpenSSL FIPS module boundary.

OpenSSL 4.0, 3.6, 3.5, 3.4, 3.0, 1.1.1 and 1.0.2 are vulnerable to this issue.

OpenSSL 4.0 users should upgrade to OpenSSL 4.0.1. OpenSSL 3.6 users should upgrade to OpenSSL 3.6.3. OpenSSL 3.5 users should upgrade to OpenSSL 3.5.7. OpenSSL 3.4 users should upgrade to OpenSSL 3.4.6. OpenSSL 3.0 users should upgrade to OpenSSL 3.0.21. OpenSSL 1.1.1 users should upgrade to OpenSSL 1.1.1zh. (premium support customers only). OpenSSL 1.0.2 users should upgrade to OpenSSL 1.0.2zq. (premium support customers only).

This issue was reported on 27th February 2026 by Zehua Qiao and Jinwen He. The fix was developed by Viktor Dukhovni.