CVE-2026-7383
HIGHIssue summary: A signed integer overflow when sizing the destination buffer for Unicode output in ASN1_mbstring_ncopy() can lead to a heap buffer overflow. Impact summary: A heap buffer…
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
Description
Issue summary: A signed integer overflow when sizing the destination buffer for Unicode output in ASN1_mbstring_ncopy() 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 ASN1_mbstring_copy() and ASN1_mbstring_ncopy() 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, OPENSSL_malloc(1) is called, and the subsequent character copy writes several gigabytes past the one-byte allocation.
X.509 certificate processing routes through ASN1_STRING_set_by_NID(), whose DIRSTRING_TYPE 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 ASN1_mbstring_copy() or ASN1_mbstring_ncopy() directly, or registers a custom string type via ASN1_STRING_TABLE_add(), 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.
Affected Products
opensslopensslDetection & mitigation playbook
Vendor / applianceDetect
Inventory every openssl openssl deployment and check each version against the affected-products list above. Because the exploit targets the running system rather than your application code, also watch for remote code execution at the network and runtime layer — O3 flags the exploit behaviour from runtime telemetry and egress traffic even before a vulnerable build is confirmed.
Fix
Apply the openssl openssl security patch or hotfix for CVE-2026-7383 on the affected version, following the vendor advisory for your exact build.
Workarounds
Cut exposure now: restrict the management/admin interface to trusted networks, segment the device, and apply the vendor's recommended configuration mitigations and any WAF/IPS signature. O3's runtime protection blocks the exploit chain at execution, holding the line on unpatched or end-of-life systems until you can patch.
How O3 protects you
O3 detects and blocks CVE-2026-7383 exploitation at runtime: eBPF exploit-chain detection, plus L7 egress monitoring that catches the post-exploitation callback and severs the attacker's outbound channel.
Tailored to CVE-2026-7383. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-7383 being exploited in your environment?
O3's eBPF runtime sensors and L7 egress monitoring detect and block the CVE-2026-7383 exploit chain at execution — protecting unpatched and end-of-life systems until the vendor patch is applied.