CVE-2026-7383: OpenSSL ASN1 Signed Integer Overflow Leading to Heap Corruption

Technical summary

The vulnerability resides in ASN1_mbstring_copy() and ASN1_mbstring_ncopy(), functions that convert character strings to ASN.1 Unicode formats (BMPSTRING/UTF-16, UNIVERSALSTRING/UTF-32, UTF8STRING). The destination buffer size is computed using a signed 32-bit integer. For BMPSTRING and UNIVERSALSTRING, the calculation uses left-shift operations that overflow when the input character count approaches 2^30. In the worst case (UNIVERSALSTRING with ~2^30 characters), the size wraps to zero, triggering OPENSSL_malloc(1) and followed by an out-of-bounds heap write of several gigabytes. UTF8STRING calculates size by summing per-character byte counts, which can also overflow. X.509 certificate processing is protected by input validation at higher layers (ASN1_STRING_set_by_NID), so the vulnerability is not exploitable through standard certificate handling. Exploitation requires direct function calls or custom ASN1 type registration with attacker-controlled input exceeding roughly 500 MB in size.

Business impact

The severity classification reflects the restricted attack surface: X.509 certificate processing—OpenSSL's most common use case in production—cannot trigger this flaw due to input validation at higher layers. However, applications that directly call the vulnerable ASN1 functions or implement custom ASN1 string types remain at risk. A successful exploit could lead to denial of service (application crash) or, in principle, code execution. Organizations using OpenSSL in standard TLS/certificate validation workflows face minimal direct risk; those integrating OpenSSL's ASN1 parsing for custom or niche use cases should assess whether untrusted data flows to the affected functions.

Affected systems

OpenSSL versions 3.0 through 4.0 are affected. The FIPS modules within OpenSSL 4.0, 3.6, 3.5, 3.4, and 3.0 are not affected because the vulnerable code lies outside the FIPS module boundary. Any application statically or dynamically linking OpenSSL and directly invoking ASN1_mbstring_copy() or ASN1_mbstring_ncopy() with untrusted input is potentially vulnerable. This is less common than standard certificate or TLS processing, which use higher-level APIs with built-in guards.

Exploitability

Exploitability is constrained by several factors. First, triggering the overflow requires input on the order of 500 MB to several gigabytes—a substantial payload that is impractical in many network contexts. Second, most common OpenSSL use cases (TLS, certificate validation) do not route untrusted data through the vulnerable functions; they use ASN1_STRING_set_by_NID(), which enforces per-NID length limits and excludes UNIVERSALSTRING. Third, successful exploitation requires either direct calls to the vulnerable functions or custom ASN1 type registration, neither of which is typical. The CVSS 3.1 score of 8.1 (HIGH) reflects potential for code execution if exploitation were possible; the actual CVSS temporality is tempered by the low probability in practice.

Remediation

Update OpenSSL to a patched version that corrects the signed integer overflow in buffer size computation. Verify the specific patched version against the official OpenSSL security advisory. Applications should review whether they directly call ASN1_mbstring_copy() or ASN1_mbstring_ncopy() or register custom ASN1 string types; if so, ensure that input size and encoding are strictly validated before passing to these functions. Standard TLS and certificate processing workflows need not be re-architected.

Patch guidance

Apply the security update released by the OpenSSL project in response to CVE-2026-7383. Consult the official OpenSSL advisory for specific patched version numbers and upgrade paths for your branch (e.g., 3.0.x, 3.4.x, 3.6.x, 4.0.x). FIPS-validated deployments should note that the FIPS module boundary is not crossed by this vulnerability, so module re-validation may not be required; confirm with your FIPS provider or vendor. Testing should confirm that applications linking the updated OpenSSL library function correctly, especially if custom ASN1 processing is in use.

Detection guidance

Monitor for unexpected process crashes or memory violations in applications using OpenSSL, particularly those that process or parse ASN1 structures from untrusted sources. Memory sanitizers (ASAN, MSAN) and heap corruption detectors will flag the out-of-bounds write if the vulnerability is triggered during testing. Review application logs for calls to ASN1_mbstring_copy() or ASN1_mbstring_ncopy() with unusually large input sizes (>100 MB). Intrusion detection can be of limited value here because the payload size and direct function calls are hard to observe at the network level in typical TLS scenarios.

Why prioritize this

Although the CVSS score is 8.1 (HIGH), the actual exploitability in production is low because standard OpenSSL use cases are protected by input validation at higher layers. Prioritize patching if your organization uses OpenSSL for custom ASN1 parsing, direct calls to vulnerable functions, or accepts untrusted ASN1 input in sizes exceeding 100 MB. For standard TLS and certificate validation use cases, patching should follow your normal patch cycle but is not an emergency; the risk is real but indirect.

Risk score, explained

The CVSS 3.1 score of 8.1 reflects the technical severity of a heap buffer overflow that could lead to code execution (HIGH impact across confidentiality, integrity, and availability). The attack vector is network-accessible (AV:N), and no privileges or user interaction are required (PR:N, UI:N). However, the attack complexity is high (AC:H) because the attacker must supply half a gigabyte or more of input, which is impractical in many network contexts, and because standard OpenSSL processing paths are guarded by length limits. The score does not fully discount the restricted real-world attack surface; organizations using only standard TLS/certificate workflows should treat this as a moderate-priority patch rather than a critical one.

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