CVE-2026-31789: OpenSSL Buffer Overflow Vulnerability

CVE-2026-31789 Overview

CVE-2026-31789 is a heap buffer overflow vulnerability in OpenSSL affecting 32-bit platforms. The vulnerability occurs when converting an excessively large OCTET STRING value to a hexadecimal string, specifically within X.509 certificate processing. When extensions such as Subject Key Identifier (SKID) or Authority Key Identifier (AKID) contain oversized OCTET STRING values, the buffer size calculation can overflow due to integer multiplication, leading to allocation of an insufficient buffer and subsequent heap corruption.

Critical Impact

Applications processing untrusted X.509 certificates on 32-bit systems may experience crashes or potentially attacker-controlled code execution through heap buffer overflow exploitation.

Affected Products

• OpenSSL versions prior to security patches (excluding FIPS modules in 3.6, 3.5, 3.4, 3.3, and 3.0)
• Applications and services that print or log contents of untrusted X.509 certificates
• 32-bit platforms running vulnerable OpenSSL versions

Discovery Timeline

• 2026-04-07 - CVE-2026-31789 published to NVD
• 2026-04-08 - Last updated in NVD database

Technical Details for CVE-2026-31789

Vulnerability Analysis

This vulnerability (CWE-787: Out-of-Bounds Write) exists in the buffer-to-hexadecimal conversion function within OpenSSL's crypto/o_str.c file. The core issue lies in how the required buffer size is calculated when converting binary OCTET STRING data to its hexadecimal representation.

When processing X.509 certificate extensions like SKID or AKID, the function computes the output buffer size by multiplying the input length by 3 (when using separators) or by 2 plus 1 (without separators). On 32-bit platforms where size_t is limited to 32 bits, providing input data larger than approximately 1.4 GB causes this multiplication to wrap around due to integer overflow. The result is a drastically undersized buffer allocation, and subsequent write operations overflow the heap buffer.

While exploitation requires certificates exceeding 1 GB in size—making practical attacks rare—the vulnerability could enable memory corruption, denial of service, or potentially arbitrary code execution in affected applications.

Root Cause

The root cause is the absence of overflow checking in the buffer size calculation within the buf2hex conversion routine. The calculation buflen * 3 (with separator) or 1 + buflen * 2 (without separator) can overflow the size_t data type on 32-bit platforms when buflen exceeds SIZE_MAX / 3 or (SIZE_MAX - 1) / 2 respectively. This results in allocating a buffer much smaller than required for the actual data conversion operation.

Attack Vector

An attacker could exploit this vulnerability by crafting a malicious X.509 certificate containing an oversized OCTET STRING value in certificate extensions. When a vulnerable application attempts to print, log, or otherwise convert these extension values to hexadecimal format, the integer overflow triggers, leading to heap corruption. The attack requires the target application to:

1. Accept and process untrusted X.509 certificates
2. Convert certificate extension OCTET STRING values to hexadecimal representation
3. Run on a 32-bit platform with a vulnerable OpenSSL version

// Security patch in crypto/o_str.c - Avoid possible buffer overflow in buf2hex conversion
// Source: https://github.com/openssl/openssl/commit/364f095b80601db632b0def6a33316967f863bde

    int has_sep = (sep != CH_ZERO);
    size_t i, len = has_sep ? buflen * 3 : 1 + buflen * 2;

+    if (buflen > (has_sep ? SIZE_MAX / 3 : (SIZE_MAX - 1) / 2)) {
+        ERR_raise(ERR_LIB_CRYPTO, CRYPTO_R_TOO_MANY_BYTES);
+        return 0;
+    }
+
    if (len == 0)
        ++len;
    if (strlength != NULL)

Detection Methods for CVE-2026-31789

Indicators of Compromise

• Unexpected application crashes in OpenSSL-linked applications on 32-bit systems when processing certificates
• Memory corruption errors or segmentation faults during X.509 certificate parsing operations
• Abnormally large X.509 certificate files (exceeding 1 GB) appearing in logs or traffic

Detection Strategies

• Monitor for applications processing unusually large X.509 certificates on 32-bit platforms
• Implement file size checks for certificate inputs, flagging any exceeding reasonable thresholds
• Use memory sanitizers (ASan, MSan) in development environments to detect heap overflow conditions
• Deploy SentinelOne Singularity to detect memory corruption exploitation attempts in real-time

Monitoring Recommendations

• Enable crash reporting and memory fault logging for applications handling certificate processing
• Monitor OpenSSL-dependent services for unusual memory allocation patterns or allocation failures
• Set up alerts for X.509 certificate processing failures with error codes related to oversized inputs

How to Mitigate CVE-2026-31789

Immediate Actions Required

• Update OpenSSL to the latest patched version that includes overflow checking in buf2hex conversion
• Audit applications running on 32-bit platforms that process untrusted X.509 certificates
• Consider migrating critical certificate-processing services to 64-bit platforms where this overflow is not exploitable
• Implement input validation to reject excessively large certificate files before processing

Patch Information

OpenSSL has released security patches addressing this vulnerability. The fix adds explicit bounds checking before the buffer size calculation to prevent integer overflow. Multiple commits address the issue across different OpenSSL branches:

• OpenSSL commit 364f095b
• OpenSSL commit 7a9087ef
• OpenSSL commit 945b935a
• OpenSSL commit a2421601
• OpenSSL commit a91e537d

For full details, see the OpenSSL Security Advisory.

Workarounds

• Limit the maximum size of X.509 certificates accepted by applications to prevent processing of oversized inputs
• Migrate certificate-processing workloads to 64-bit platforms where this integer overflow cannot occur
• Implement application-level input validation to reject certificates with extensions exceeding reasonable sizes
• Note that FIPS modules in OpenSSL 3.6, 3.5, 3.4, 3.3, and 3.0 are not affected as the vulnerable code is outside the FIPS boundary

# Configuration example - Check OpenSSL version and architecture
openssl version -a
# Verify if running 32-bit (vulnerable) or 64-bit (not vulnerable to this specific issue)
getconf LONG_BIT
# Update OpenSSL on Debian/Ubuntu systems
sudo apt-get update && sudo apt-get upgrade openssl
# Update OpenSSL on RHEL/CentOS systems
sudo yum update openssl