CVE-2026-3548: 
wolfSSL vulnerability analysis and mitigation

CVE-2026-3548 describes two buffer overflow vulnerabilities in the wolfSSL CRL (Certificate Revocation List) parser affecting all versions before 5.9.0. The first is a heap-based buffer overflow triggered when the CRL number is improperly stored as a hexadecimal string using an undersized buffer; the second is a stack-based overflow triggered by CRL numbers larger than 56 octets. Both flaws only affect builds compiled with CRL support enabled and require loading a CRL from an untrusted source. The vulnerability was published on March 19, 2026, and carries a CVSS v3.1 base score of 9.8 (Critical) (Red Hat Advisory, Feedly).

The root cause involves two distinct out-of-bounds write flaws (CWE-122, CWE-787) in the wolfSSL CRL parser's ParseCRL_Extensions function. For the heap overflow (PR #9628): the crlNumber field in CRL_Entry stores the CRL number as a hex string via mp_toradix, but the buffer is sized by CRL_MAX_NUM_SZ (max octets), not the hex string length — which requires CRL_MAX_NUM_SZ * 2 + 1 bytes, causing an overflow for any non-trivial CRL number (wolfSSL PR #9628). For the stack overflow (PR #9873): in the ASN_TEMPLATE code path, GetInt() calls mp_init() which overwrites the size field with SP_INT_DIGITS (e.g., 130), then sp_read_unsigned_bin() writes past the end of a 64-byte stack allocation for CRL numbers exceeding 56 bytes (wolfSSL PR #9873). Both vulnerabilities are network-reachable with no authentication required, provided the application loads CRLs from an attacker-controlled source.

Successful exploitation allows an unauthenticated remote attacker to corrupt heap or stack memory, potentially achieving arbitrary code execution with the privileges of the wolfSSL process. This could result in full confidentiality, integrity, and availability compromise of the affected system. The scope is limited to applications that explicitly enable CRL support and load CRLs from untrusted sources, but such deployments — common in PKI-heavy environments — could expose sensitive cryptographic material, enable man-in-the-middle attacks, or serve as a foothold for lateral movement (Feedly, ENISA EUVD).

1. Identify target: Locate applications using wolfSSL compiled with --enable-crl that load CRLs from network or user-supplied sources (e.g., TLS clients performing certificate revocation checks against attacker-controlled infrastructure).
2. Craft malicious CRL: Generate a CRL with an oversized CRL number extension — either a number whose hex representation exceeds the CRL_MAX_NUM_SZ-sized buffer (heap overflow), or a CRL number larger than 56 octets (stack overflow) — using standard ASN.1/DER encoding tools such as OpenSSL or a custom script.
3. Serve the malicious CRL: Host the crafted CRL at a URL or LDAP endpoint that the target application is configured to fetch for revocation checking, or perform a man-in-the-middle attack to intercept and replace a legitimate CRL response.
4. Trigger parsing: Cause the target application to initiate a TLS handshake or certificate validation that fetches and parses the malicious CRL, triggering the out-of-bounds write in ParseCRL_Extensions.
5. Achieve code execution: Exploit the memory corruption (heap or stack overflow) to redirect execution flow, potentially deploying a reverse shell or other payload with the privileges of the wolfSSL process (wolfSSL PR #9628, wolfSSL PR #9873).

• Network: Unusual or unexpected CRL fetch requests (HTTP/LDAP) to external or untrusted endpoints from applications using wolfSSL; anomalous outbound connections from TLS-enabled services following certificate validation events.
• Process: Unexpected crashes or segmentation faults in processes linked against wolfSSL with CRL support enabled; unusual child processes spawned by wolfSSL-linked services (e.g., shells, network utilities).
• Logs: Application logs showing CRL parsing errors or crashes in ParseCRL_Extensions; address sanitizer (ASAN) reports referencing sp_read_unsigned_bin or mp_toradix stack/heap overflows.
• File System: Core dump files generated by wolfSSL-linked processes; unexpected binaries or scripts written to disk by the affected process.

Upgrade wolfSSL to version 5.9.0 or later, which incorporates both fixes: PR #9628 (merged January 11, 2026) addressing the hex string heap overflow, and PR #9873 (merged March 7, 2026) addressing the stack overflow for CRL numbers exceeding 56 octets (wolfSSL PR #9628, wolfSSL PR #9873). As an interim workaround, disable CRL support at build time if it is not required by the application (omit --enable-crl from the configure flags). Additionally, ensure that any CRLs loaded by the application originate exclusively from trusted, controlled sources to eliminate the untrusted-input attack vector (Feedly).

The vulnerability was assigned by wolfSSL and disclosed on March 19, 2026. Red Hat has tracked the CVE in their security advisory system. A Korean-language threat intelligence report (Argus AI) referenced both fix PRs shortly after disclosure. No significant broader media coverage or notable researcher commentary beyond the fix PRs themselves has been identified (Red Hat Advisory, wolfSSL PR #9873).