Callfuscated: Reverse Engineering a Stack-Based VM Protected by MBA, Opaque Predicates and Call-Based Jumps
The HackTheBox challenge Callfuscated presents one of the most layered crackmes published on the platform, combining virtualization, Mixed Boolean-Arithmetic (MBA), opaque predicates and junk instructions inside a single binary.
Initial analysis shows hundreds of functions that appear to receive arguments through mysterious arg_* variables. Disassembly reveals that every call is immediately followed by a pop r8 instruction; the return address is discarded and only r8 is clobbered. Unicorn emulation of the entire binary confirms that r8 is never subsequently read, proving that each call-pop pair is a hand-written jump.
A lightweight Python script using Capstone walks the code, replaces the call opcode (0xE8) with jmp (0xE9) and overwrites the pop r8 bytes with NOPs. After patching, Ghidra produces a readable main function containing the characteristic dispatch loop of a stack-based virtual machine.
Each handler invokes a separate function protected by MBA. The author employs Triton to symbolically execute these routines and extract an AST for the return value in RAX. The resulting expressions, sometimes exceeding 7 000 characters, are fed to the CoBRA MBA solver, which reduces them to the original operations: addition, subtraction, multiplication, bitwise AND, OR and XOR. Two handlers that receive random values as arguments simplify to expressions that ignore those arguments, confirming the presence of opaque predicates.
With opcode semantics recovered, the author uses Unicorn again to dump the bytecode at the point where it has been fully initialized. A small C emitter translates the bytecode into ordinary C statements that operate on a 256-element stack. The generated file is compiled with optimizations and loaded into a decompiler, yielding a clean routine whose final comparison against zero determines whether the flag is correct.
Related articles
uCreator Spyware Infiltrates Modified Telegram, WeChat and GCash Builds on Android
Security researcher Ovi has published a detailed analysis of uCreator, also known as YingGai, a commercial Android spyware platform designed for surveillance and remote app control. The malware intercepts messages, verification codes and account data directly inside legitimate applications without requiring root privileges. Infection vectors include tampered Telegram, WeChat and GCash APKs distributed through unofficial sources, as well as the Onion Box container that launches the target app inside a controlled environment. Once active, uCreator hooks into the host process, reads Telegram’s local message database through client interfaces, captures two-factor authentication passwords and hides its own session from the user. The operation is supported by cloud-based command panels and exposed GitLab repositories that revealed server-side code and development history. Compatibility problems have been noted on Android 17, and the total number of victims remains unknown.
Researchers Deploy Cowrie Honeypot Disguised as NVIDIA H100 Server to Study SSH Botnet Tactics
A cybersecurity team set up a realistic Cowrie honeypot on a fake server equipped with four NVIDIA H100 GPUs, 128 AMD EPYC cores, and terabytes of storage to observe what SSH brute-force bots actually do after gaining access. The trap used a custom StickyAuth module that allowed entry only after several failed attempts and locked in the successful credential pair per IP address. Over the first 12.5 hours the system recorded 1,846 connections from 126 unique IPs, 175 successful logins, and 1,524 commands executed, with the majority of activity traced to a single botnet linked to the Outlaw group. The attackers deployed SSH keys signed mdrfckr, changed the root password, killed competing processes, installed cron jobs, and downloaded a mining archive called dota3 containing an IRC backdoor and its own SSH scanner. The researchers also identified three realism issues in the base Cowrie setup—limited grep flags, outdated filesystem paths in Debian 12, and inconsistent hostname reporting—and patched them to keep sophisticated bots engaged longer. An auxiliary LLM service later summarized each session in Telegram for easier analysis.
Hirudo Android Banking Trojan Uninstalls Legitimate Bank Apps and Installs Attacker-Controlled Fakes
Researchers at ThreatFabric have detailed Hirudo, a sophisticated Android banking trojan that also serves as a remote access tool and replaces genuine bank applications with attacker-controlled copies. The malware spreads through decoy apps themed around shopping, adult video, and short series, including Shopora, 1V1 X, ZoyaTV, and KoiShort. Once installed, it uses accessibility services to detect target banking apps, verify their signatures, display a fake update prompt, and force uninstallation of the real application before installing a malicious replica signed with a publicly known test key. The trojan employs components from the open-source YAHFA and Dobby projects to intercept signature checks, hide accessibility services, bypass screenshot restrictions, and forward Play Integrity attestations to the attacker’s server. Eight ready-made replicas target seven Indonesian banks, while credential theft focuses on users in China, India, Mexico, Morocco, and Thailand. Server logs collected in August 2026 contained thousands of credentials, device data, and financial profiles, with the attestation key set confirmed as publicly leaked and partially revoked by Google.
Over 300 Tor Exit Nodes Hijacked to Intercept SSH Passwords and Deploy Ebury Backdoor
Attackers compromised hundreds of Tor exit nodes and repurposed them to perform man-in-the-middle attacks against SSH connections. Instead of reaching the intended server, users were redirected to attacker-controlled infrastructure where they could be tricked into accepting a forged host key and revealing their credentials. Researcher kaba identified 333 malicious exit nodes routing traffic through the same 12 interception servers after noticing an unexpected host-key warning on October 2. The campaign also attempted to install the Ebury Linux backdoor designed to steal authentication data. The Tor Project confirmed that operators' servers had their configurations altered and some middle relays were converted into exit nodes. The initial access vector remains unknown, but the activity window was limited to October 1-5. Users who connected via Tor during this period, accepted an unfamiliar key, and entered a password should treat their credentials as compromised.