Null pointer dereference when setting a distinguished name (CVE-2026-34874)

Title	Null pointer dereference when setting a distinguished name
CVE	CVE-2026-34874
Date	31 March 2026
Affects	All versions of Mbed TLS from 3.5.0 up to 3.6.5, Mbed TLS 4.0.0
Not affected	Mbed TLS 3.6.6 and later 3.6 versions, Mbed TLS 4.1.0 and later 4.x versions
Impact	Arbitrary code execution
Severity	HIGH
Credits	Haruto Kimura (Stella)

Vulnerability

An attacker who can cause a memory allocation failure during the execution of mbedtls_x509_string_to_names() can cause a call to memcpy() with a null pointer as the destination address.

mbedtls_x509_string_to_names() is also called indirectly via the following functions:

• mbedtls_x509write_csr_set_subject_name()

• mbedtls_x509write_crt_set_subject_name()

• mbedtls_x509write_crt_set_issuer_name()

Impact

On a platform with memory protection this is likely to result in a segmentation fault. On a microcontroller this may write to an interrupt vector at address 0 and thereby allow arbitrary code execution.

Affected versions

All versions of Mbed TLS from 3.5.0 up to 3.6.5 and Mbed TLS 4.0.0 are affected.

Work-around

Ensure that Mbed TLS has ample memory when calling mbedtls_x509_string_to_names() so that mbedtls_calloc() will not fail.

On systems with memory protection where address 0 is not writable, this vulnerability will generate a segmentation fault or a memory protection error. As a result it will lead to a denial of service (DoS) on such systems rather than arbitrary code execution.

Resolution

Affected users should upgrade to Mbed TLS 3.6.6 or 4.1.0.

Fix commits

We recommend that users upgrade to a release including the fix. However, if you are maintaining a branch with backported bug fixes, here are the most relevant commits. Please note that these commits may not apply cleanly to older versions of the library, and may not provide a complete fix even if they do apply. The Mbed TLS development team does not provide support outside of maintained branches.

Branch	Mbed TLS 3.6.x	TF-PSA-Crypto 1.x	Mbed TLS 4.x
Basic fix	bfaf4a47fd33da860796feaba6235847acb71127	N/A	f549fc7bdcc9b435236de5594bec0ed8e587988c
With tests and documentation	branch up to 4704b6b4bd963f1331582374e881184addf8f523	N/A	branch up to fe2599ea8238bdf5340a2f76ff2f3e3df095524b