# CVE-2026-25833: Mbed TLS buffer underflow in x509_inet_pton_ipv6

Source: https://junglewise.ai/threats/cve-2026-25833-mbed-tls-buffer-underflow-in-x509-inet-pton-ipv6  
Publisher: Junglewise Threat Intelligence, https://junglewise.ai/threats  
Published: 2026-04-01. Updated: 2026-06-05  
License: CC BY 4.0 with a link back

CVE-2026-25833 · Severity: high · CVSS 7.5 · Published 2026-04-01

Mbed TLS, a widely used open-source cryptographic library for embedded devices, contains a flaw in how it processes certain network addresses in digital certificates. An attacker could potentially exploit this to cause a system crash or service outage. This issue primarily affects systems where the library's built-in address parsing is used instead of the operating system's native functions.

## Quick answers

**Is CVE-2026-25833 exploited in the wild?** No. Junglewise Threat Intelligence has no report of CVE-2026-25833 being exploited in the wild.

**How severe is CVE-2026-25833?** High severity, with a CVSS score of 7.5, published 1 April 2026.

**Which products and versions are affected?** TrustedFirmware Mbed TLS 3.5.0 to 3.6.5, 4.0.0.

**Is there a fix for CVE-2026-25833?** No fixed version is recorded yet. Check the vendor's advisory in the references below. If you run it, Junglewise drafts a mitigation that fits your setup and applies it once you approve, with senior security engineers behind it.

**How is CVE-2026-25833 attacked?** It is attacked over the network, in a software library.

**Am I affected by CVE-2026-25833?** Check whether anything you run is named in the advisory. 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

A buffer underflow (specifically a buffer underread) exists in the x509_inet_pton_ipv6() function within Mbed TLS. The vulnerability is triggered when the library uses its internal implementation of inet_pton() (typically when the AF_INET6 macro is undefined by the toolchain). When parsing IPv4-mapped IPv6 addresses, a logic error allows the pointer to walk back up to 4 bytes before the start of the buffer. While primarily a memory underread, this can lead to a Denial of Service (DoS) on platforms with strict memory protection if the read crosses a page boundary. The issue is fixed in versions 3.6.6 and 4.1.0.

## Affected products

- TrustedFirmware Mbed TLS 3.5.0 to 3.6.5, 4.0.0

## Timeline

- 2026-03-31: advisory: Vendor advisory published
- 2026-04-01: disclosed: CVE published to NVD

## References

- [https://mbed-tls.readthedocs.io/en/latest/security-advisories/](https://mbed-tls.readthedocs.io/en/latest/security-advisories/)
- [https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-inet-pton/](https://mbed-tls.readthedocs.io/en/latest/security-advisories/mbedtls-security-advisory-2026-03-inet-pton/)