ATTACKER

This article explores how attackers can introduce hidden vulnerabilities through module updates, leading to the compromise of ESP32-based devices and unauthorized access to private keys, affecting billions of devices using this microcontroller. The main issue is the discovered vulnerability CVE-2025-27840 in the ESP32 architecture. To protect the Bitcoin network, the following problems were identified: the possibility of using invalid private keys due to the lack of a lower bound check in the has_invalid_privkey function; the possibility of forging transaction signatures due to incompatibility of the electrum_sig_hash function with BIP-137; a weak random number generator in the random_key function, making private keys predictable; the absence of curve point validation in the multiply function, opening the door for curve attacks; a vulnerability in recovering the Y-coordinate in the ecdsa_raw_sign function, allowing public key substitution; and issues with outdated hashing APIs in the bin_ripemd160 function.

Discovery of CVE-2025-27840 Vulnerability

In early March 2025, Tarlogic Security specialists discovered a vulnerability in the ESP32 microcontroller, which is widely used to connect devices via Wi-Fi and Bluetooth. This vulnerability was assigned the identifier CVE-2025-27840. Attackers can use the ESP32 chip as an entry point for cryptographic attacks on devices operating in popular cryptocurrency networks such as Bitcoin and Ethereum. The problem affects millions of IoT devices using this chip. Exploiting the vulnerability allows attackers to act as legitimate users and infect devices with malware, threatening the security of Bitcoin wallet private keys.

ESP32 is a microcontroller commonly used in IoT devices for wireless communication. Through it, attackers can gain access to Bitcoin wallet private keys.

Main Security Threats

Vulnerabilities in ESP32 can lead to the theft of Bitcoin wallet private keys. The main risks are associated with the presence of backdoors and vulnerabilities that allow memory manipulation, MAC address spoofing, and malware injection.

Attackers can target ESP32-based IoT devices via Bluetooth and Wi-Fi vulnerabilities to attack other devices in the Bitcoin network and steal confidential information, including private keys.

Hidden List of Vulnerabilities

When updating modules, an attacker can introduce the following vulnerabilities into the code:

• In the has_invalid_privkey function, a private key can be obtained due to incorrect validation.
• The electrum_sig_hash function allows forging Bitcoin transaction signatures.
• The random_key function uses a weak random number generator.
• The multiply function lacks curve point validation.
• Vulnerabilities in the ecdsa_raw_sign and bin_ripemd160 functions.

These vulnerabilities allow malicious updates to be embedded in ESP32 devices, bypassing code control and gaining access to private keys. Currently, billions of devices may be at risk due to hidden architectural features described in CVE-2025-27840.

Vulnerability in Private Key Validation Function (has_invalid_privkey)

A bug was found in the Bitcoin private key validation code: there is no check that the key is greater than zero. This allows the use of invalid (e.g., zero or negative) private keys, which can lead to the loss of funds. To fix this, a check should be added to ensure the key is greater than zero.

The code only checks that the key does not exceed the upper bound, but does not check the lower bound. Because of this, invalid (too small) private keys can be used.

Transaction Signature Forgery Vulnerability in the electrum_sig_hash Function

In Electrum, the electrum_sig_hash function uses a non-standard way of hashing messages, making signature forgery possible due to incompatibility with BIP-137. An attacker can exploit this feature, as the function forms the message hash in a special way.

Weak Key Generator Vulnerability in the random_key Function

The problem arises when the standard random module is used to generate private keys, which is not intended for cryptographic purposes. This makes private keys predictable and vulnerable to attacks. For security, one should use the secrets or os.urandom modules, which provide sufficient entropy.

Vulnerability in Y-Coordinate Recovery in the ecdsa_raw_sign Function

In the ecdsa_raw_sign function, when recovering the Y-coordinate, a situation may arise where the public key is substituted. This is due to ambiguity in recovering the coordinate on the elliptic curve, allowing an attacker to create an invalid public key.

Vulnerability in Outdated Hashing APIs in the bin_ripemd160 Function

If RIPEMD-160 support is missing in the standard library, a simplified implementation is used, which may be weak and prone to collisions. This opens the possibility for attacks, transaction forgery, and other abuses. It is recommended to use modern cryptographic libraries and regularly update software.

Vulnerability in Elliptic Curve Multiplication Function (multiply)

The multiply function lacks a full check that the point actually belongs to the main elliptic curve. This allows attacks using invalid (forged) points, which can lead to private key compromise through so-called invalid curve attacks.

Small Subgroup Attack

A small subgroup attack allows extracting private keys from vulnerable transactions using special services for decoding and analyzing signatures.

Private Key Verification and Confirmation

The obtained private key can be checked using specialized tools to ensure it matches the Bitcoin wallet address and to verify the balance.

Conclusions and Recommendations for Reducing Risks

Today, it is extremely important to ensure the security of devices and networks. The article examines vulnerabilities in ESP32 and cryptocurrency software. The main risks are related to private key validation, hashing methods, random number generation, curve point validation, and outdated APIs. Special attention is given to the CVE-2025-27840 vulnerability, which allows malicious updates and low-level system access. To protect against this, it is recommended to use modern cryptographic libraries, regularly update software, thoroughly validate input data, and use only trusted devices.

Relevance

Billions of devices may be vulnerable due to hidden flaws in the ESP32 architecture. This allows attackers to spoof MAC addresses, gain unauthorized memory access, and attack via Bluetooth.

List and Analysis of Vulnerabilities

• Lack of lower bound check for the private key (has_invalid_privkey) can lead to loss of funds.
• Bitcoin transaction signature forgery in the electrum_sig_hash function due to non-standard hashing.
• Use of a weak random number generator in the random_key function.
• Insufficient curve point validation in the multiply function.
• Incorrect Y-coordinate recovery in the ecdsa_raw_sign function.
• Use of outdated hashing APIs in the bin_ripemd160 function.

Benefits of Identifying and Eliminating Vulnerabilities

1. Protection against financial losses.
2. Preservation of data confidentiality.
3. Increased network security.
4. Greater user trust.
5. Compliance with modern security standards.

Conclusion

Identifying and analyzing vulnerabilities highlights the need for continuous monitoring and improving the security of devices and software. Fixing these issues not only prevents attacks and financial losses but also helps build user trust and comply with security standards. It is important to regularly update software, use modern security tools, and carefully check cryptographic operations.

Note

This material was prepared for the CRYPTO DEEP TECH portal to improve the security of financial data and elliptic curve cryptography. The authors are not responsible for the misuse of this information.