# CVE-2026-5188: wolfSSL integer underflow in X.509 SAN extension parsing

Source: https://junglewise.ai/threats/cve-2026-5188-wolfssl-integer-underflow-in-x-509-san-extension-parsing  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-10. Updated: 2026-05-10  
License: CC BY 4.0 with a link back

CVE-2026-5188 · Severity: high · CVSS 8.1 · Published 2026-04-10

wolfSSL is a security library used by applications to establish secure encrypted connections. A vulnerability in how it processes digital certificates could allow an attacker to provide a specially crafted certificate that causes the software to malfunction. This could lead to a service outage or potentially allow an attacker to access sensitive information, though it only affects systems using a specific, non-default configuration.

Technologies: [Wolfssl](https://junglewise.ai/threats/technologies/wolfssl). Vendors: [Wolfssl](https://junglewise.ai/threats/vendors/wolfssl).

## Quick answers

**Is CVE-2026-5188 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-5188 being exploited in the wild.

**How severe is CVE-2026-5188?** High severity, with a CVSS score of 8.1, published 10 April 2026.

**Which products and versions are affected?** wolfSSL wolfSSL up to (excluding) 5.9.1.

**Is there a fix for CVE-2026-5188?** A patch was published on 2026-03-20: Fix merged into master branch via pull request 10024 Junglewise finds every place you run it, drafts the upgrade and applies it once you approve.

**How is CVE-2026-5188 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-5188?** Check whether you run Wolfssl. Junglewise does that check for you around the clock: connect your code, cloud and domains, and it matches new vulnerabilities like this one against what you actually run and tells you when one reaches you.

## Technical details

An integer underflow exists in the `DecodeAltNames()` function within `wolfcrypt/src/asn.c`. When parsing the Subject Alternative Name (SAN) extension of an X.509 certificate, a malformed certificate can specify an entry length larger than the enclosing sequence. This causes the internal length counter to wrap (underflow) during the `length -= strLen` operation. The vulnerability is specifically located in the non-template ASN.1 parsing path, which is enabled via the `--enable-asn=original` flag and is off by default. An attacker can exploit this by presenting a malicious certificate during a TLS handshake or certificate validation process, potentially leading to a denial of service or information disclosure. The issue was addressed in version 5.9.1 by adding proper bounds checks before length subtraction.

## Affected products

- wolfSSL wolfSSL up to (excluding) 5.9.1

## Timeline

- 2026-03-20: patched: Fix merged into master branch via pull request 10024
- 2026-04-10: disclosed: CVE-2026-5188 published

## References

- [https://github.com/wolfSSL/wolfssl/pull/10024](https://github.com/wolfSSL/wolfssl/pull/10024)

## Related threats

- [wolfSSL certificate validation bypass in ConfirmNameConstraints()](https://junglewise.ai/threats/cve-2026-95208-an-issue-in-the-confirmnameconstraints-function-wolfcrypt-src-asn)
- [wolfSSL revocation check bypass with OCSP and CRL](https://junglewise.ai/threats/cve-2026-94417-when-an-application-enables-both-ocsp-and-crl-revocation-checking)
- [wolfSSL TLS 1.2 ChangeCipherSpec message ordering bypass](https://junglewise.ai/threats/cve-2026-93304-a-d-tls-1-2-client-can-accept-a-changecipherspec-message-before)
- [wolfSSL MatchTrustedPeer public key validation bypass](https://junglewise.ai/threats/cve-2026-93302-matchtrustedpeer-ignores-the-public-key-used-leading-to-forged-ca)
- [wolfSSL TLS authentication bypass in Raw Public Key handling](https://junglewise.ai/threats/cve-2026-89136-when-using-rpk-raw-public-key-the-client-side-of-a-tls-1-2-1-3)