CVE-2026-5187: wolfSSL Buffer Overflow Vulnerability

CVE-2026-5187 Overview

Two potential heap out-of-bounds write vulnerabilities exist in the DecodeObjectId() function within wolfcrypt/src/asn.c in wolfSSL. The first flaw occurs when a bounds check only validates one available slot before writing two OID arc values (out[0] and out[1]), enabling a 2-byte out-of-bounds write when outSz equals 1. The second vulnerability stems from multiple callers passing sizeof(decOid) (64 bytes on 64-bit platforms) instead of the element count MAX_OID_SZ (32), causing the function to accept crafted OIDs with 33 or more arcs that write past the end of the allocated buffer.

Critical Impact

Attackers with network access and low privileges can exploit this heap-based buffer overflow to corrupt memory, potentially leading to denial of service or limited integrity compromise through crafted OID values in ASN.1 data structures.

Affected Products

• wolfSSL wolfcrypt library (versions containing vulnerable DecodeObjectId() implementation)
• Applications utilizing wolfSSL for TLS/SSL or cryptographic operations
• Embedded systems and IoT devices using wolfSSL for certificate parsing

Discovery Timeline

• 2026-04-09 - CVE-2026-5187 published to NVD
• 2026-04-09 - Last updated in NVD database

Technical Details for CVE-2026-5187

Vulnerability Analysis

This vulnerability is classified as CWE-122 (Heap-based Buffer Overflow), a dangerous memory corruption flaw that occurs in the ASN.1 Object Identifier decoding functionality of wolfSSL's cryptographic library. The vulnerability manifests in the DecodeObjectId() function, which is responsible for parsing and decoding Object Identifiers (OIDs) from ASN.1-encoded data structures commonly found in X.509 certificates, PKCS standards, and other cryptographic protocols.

The core issue involves two distinct but related buffer overflow conditions. The first occurs due to an insufficient bounds check that validates only a single available slot in the output buffer before the function attempts to write two OID arc values. When the output buffer size (outSz) is exactly 1, the function proceeds to write both out[0] and out[1], resulting in a 2-byte heap overflow.

The second vulnerability arises from a semantic mismatch between callers and the function implementation. Multiple code paths pass sizeof(decOid) as the buffer size parameter, which on 64-bit platforms equals 64 bytes. However, the actual array can only safely hold MAX_OID_SZ (32) elements. This discrepancy allows attackers to craft malicious OIDs containing 33 or more arcs, causing writes beyond the allocated buffer boundary.

Root Cause

The root cause of this vulnerability is improper input validation and incorrect size calculations in the DecodeObjectId() function. The bounds checking logic fails to account for the dual-write operation in the first case, and caller functions incorrectly use byte-based sizeof() instead of element count when specifying buffer capacity. This represents a classic C programming error where the distinction between buffer size in bytes versus element count is not properly maintained, compounded by insufficient validation before multi-element write operations.

Attack Vector

The vulnerability is exploitable over the network by authenticated users with low privileges. An attacker can craft malicious ASN.1-encoded data containing specially constructed Object Identifiers and send them to applications using wolfSSL for certificate parsing or cryptographic operations. When the vulnerable DecodeObjectId() function processes this data, the heap buffer overflow occurs.

Exploitation requires the attacker to control or influence ASN.1 data being parsed by the target application, such as through certificate exchanges during TLS handshakes, PKCS message processing, or other cryptographic protocol interactions. The attack has prerequisites that make exploitation more complex, limiting the overall impact despite the network accessibility.

The vulnerability mechanism involves crafting an OID with either:

1. Minimal encoded data that triggers the 2-byte overflow when outSz equals 1
2. An OID containing 33 or more arcs to exceed the MAX_OID_SZ limit

When decoded, the function writes beyond the allocated heap buffer, potentially corrupting adjacent memory structures. This can lead to denial of service through application crashes or limited integrity violations if adjacent data structures can be controllably overwritten.

Detection Methods for CVE-2026-5187

Indicators of Compromise

• Unexpected application crashes or segmentation faults during certificate parsing or TLS handshake operations
• Abnormal memory consumption patterns in processes using wolfSSL libraries
• Core dumps or crash logs showing memory corruption in ASN.1 decoding routines

Detection Strategies

• Deploy memory sanitizers (ASan, MSan) in development and testing environments to detect heap overflow attempts
• Monitor for malformed or unusually long OIDs in network traffic during TLS negotiations
• Implement application-level logging for ASN.1 parsing failures and exceptions

Monitoring Recommendations

• Enable verbose logging for wolfSSL certificate validation and ASN.1 parsing operations
• Configure intrusion detection systems to alert on malformed certificate exchanges
• Monitor process stability for applications using wolfSSL, tracking crash frequency and patterns

How to Mitigate CVE-2026-5187

Immediate Actions Required

• Identify all applications and systems using wolfSSL cryptographic libraries
• Review the wolfSSL GitHub project for security patches addressing this vulnerability
• Apply vendor patches as soon as they become available
• Consider temporarily restricting certificate parsing from untrusted sources until patches are applied

Patch Information

Organizations should monitor the official wolfSSL GitHub repository for security updates addressing CVE-2026-5187. The fix should address both the insufficient bounds check before dual-write operations and correct the caller functions to pass element counts rather than byte sizes when invoking DecodeObjectId().

Workarounds

• Implement additional input validation on ASN.1 data before passing to wolfSSL parsing functions
• Restrict network exposure of affected applications until patches are available
• Deploy web application firewalls or network filters to inspect and reject malformed certificate data
• Consider using alternative cryptographic libraries for critical operations until the vulnerability is patched

The mitigation configuration will depend on your specific deployment environment and application architecture. Consult wolfSSL documentation and your security team to implement appropriate controls for your use case.