PT-2025-48530 · Julia · Mbedtls Jll

Published

2025-11-21

·

Updated

2025-11-21

None

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Vulnerability

Mbed TLS's modular inversion routine and GCD routine are vulnerable to local timing attacks in a number of settings discussed below.
These functions are used in RSA, making the following operations vulnerable in all configurations:
  • RSA key generation with any API (mbedtls rsa gen key() and all psa generate key*() functions).
  • Use of mbedtls rsa complete() to import RSA private keys that are incomplete or not in the standard format. (The only exception is when MBEDTLS RSA NO CRT is enabled and all of N, E, D, P and Q have been set before calling mbedtls rsa complete().) (Note: internal uses of mbedtls rsa complete() in the library are always safe.)
Additionally, if MBEDTLS RSA NO CRT is enabled, the following operations are also vulnerable:
  • Import of RSA key pairs with psa import key().
  • Export of RSA key pairs with any key export API (PSA, PK).
Additionally, if MBEDTLS RSA NO CRT and MBEDTLS USE PSA CRYPTO are both enabled, the following operations are also vulnerable:
  • Signature generation with PK (mbedtls pk sign(), mbedtls pk sign ext()).
  • Decryption with PK (mbedtls pk decrypt()).
Elliptic curve cryptography (ECDSA, ECDH, EC-JPAKE) is not affected as the vulnerable functions are used in a safe way (with blinding).
Finite-field Diffie-Hellman (FFDH) is not affected as it does not use the vulnerable functions.
Direct use of mbedtls mpi inv mod() by applications on secret data is most likely vulnerable unless you use proper blinding.
Direct use of mbedtls mpi gcd() by applications on secret data is most likely vulnerable, unless the compiler used has builtin ctz (GCC 10 and above has it, earlier versions don't), and it is not implemented using a loop (most architectures have instructions that are more efficient than using a loop).

Impact

When one of the vulnerable RSA functions mentioned above is used, the vulnerability allows the attacker to fully recover the RSA private key.
When mbedtls mpi mod inv() is directly called by an application (or mbedtls mpi gcd() in a build where it is affected, see above), the vulnerability allows the attacker to recover both inputs.

Attack settings

The side channels in GCD and modular inversion can be exploited by a local attacker in a number of circumstances. Two teams independently developed tools allowing to trace a victim process, recover the inputs to mbedtls mpi mod inv() and break RSA key generation:
  1. SSBleed: this uses the Memory Dependence Predictor (MDP) on some Arm-v9 CPUs. The attacker only needs to be able to run code on the same core as the victim but does not need elevated privileges. The proof of concept fully recovers RSA private keys from mbedtls rsa gen key() and the same principle applies to other unblinded uses of mbedtls mpi inv mod().
  2. M-Step: in a setting where TrustZone-M is used, the M-Step framework allows the non-secure world to abuse timer interrupts to effectively single-step the secure world and trace its execution flow. The proofs of concepts fully recover RSA private keys from mbedtls rsa gen key() and mbedtls rsa complete(). Several other exploitation paths in RSA are identified. In particular, mbedtls mpi gcd() is found to be vulnerable when Mbed TLS is compiled with a compiler that does not have builtin ctz.
The same functions are most probably also vulnerable in other circumstances using similar attack techniques: SGX-Step, SEV-Step, microarchitectural attacks similar to SSBleed...

Credits

SSBleed: Chang Liu from Tsinghua University and Trevor E. Carlson from National University of Singapore
M-Step: Cristiano Rodrigues (University of Minho), Marton Bognar (DistriNet, KU Leuven), Sandro Pinto (University of Minho), Jo Van Bulck (DistriNet, KU Leuven)

Affected versions

All versions of Mbed TLS up to 3.6.4 are affected.

Work-around

Applications that do not generate RSA keys and do not import private RSA keys with mbedtls rsa import()+mbedtls rsa complete(), but use other RSA functions that are only vulnerable with MBEDTLS RSA NO CRT, can recompile without MBEDTLS RSA NO CRT.
Applications that do not use RSA private keys and do not directly call mbedtls mpi inv mod() or mbedtls mpi gcd() are not affected.

Resolution

Affected users should upgrade to Mbed TLS 3.6.5 or TF-PSA-Crypto 1.0.
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Related Identifiers

JLSEC-2025-232

Affected Products

Mbedtls Jll