Improper MAC Verification (non-constant-time / unauthenticated)

← Insufficient Cryptography

improper_mac_verification   MEDIUM

Description

A signed token’s HMAC is verified with a short-circuiting string equals (==) instead of a constant-time compare, leaking a timing oracle; a companion path skips MAC verification entirely and trusts the payload, so a forged tag is accepted.

How it works

A signed token has the form “.”. The verifier compares the presented tag with a short-circuiting == , which is a timing oracle that leaks the correct tag byte by byte, and a second path skips MAC verification entirely for tokens with no tag, trusting a forged, unsigned payload.

How to exercise it. DVMA is the harness - open this module from the home index and tap the demo action. The screen ships the malicious input and simulates the attacker (e.g. the companion app, crafted intent, or scanned payload) in-process, and the evidence panel prints the proof. The Tools (optional) and Attack inputs below are only needed to reproduce the exploit end-to-end on a real device.

Exploit steps

1. Set up. Build DVMA with a flavor that enables the Insufficient Cryptography category (e.g. --dart-define-from-file=config/flavors/dev.json) and run on an emulator/simulator you control. The demo needs no external tooling; for the optional on-device reproduction the relevant tools are: frida, frida-trace, r2.
2. Locate the target. From the home index, open Improper MAC Verification (non-constant-time / unauthenticated) (improper_mac_verification). The How it works section above describes this module’s specific weakness; the screen states the intended-secure behavior and exposes the vulnerable action.
3. Exploit. Capture the algorithm/key/IV the module uses (the evidence panel prints it), then reproduce the weakness offline - e.g. recompute the digest, brute the short key, or decrypt a captured blob - to show the protection is ineffective.
4. Observe the evidence. Trigger the vulnerable action and read the evidence panel - it prints the concrete proof (leaked value, accepted replay, executed payload, or unauthorized result).
5. Contrast with the secure path. Run the module’s secure/hardened action (where provided) and confirm the same attack is rejected - this is what a correct implementation should do.

Tools (optional)

• frida
• frida-trace
• r2

Attack inputs

Payloads/artifacts you author for the on-device attack. The demo already ships and simulates these in-process (e.g. the malicious companion app / crafted intent is emulated inside the screen), so you only need to craft them to reproduce the exploit on a real device:

• forged tag