Unauthenticated Encryption (malleable ciphertext / bit-flipping)

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unauthenticated_encryption_malleable_ciphertext   MEDIUM

Description

A token is encrypted with AES-CBC and no MAC, so an attacker who never learns the key flips a ciphertext byte to predictably change the decrypted plaintext (bit-flipping); the secure path uses authenticated encryption (AES-GCM / encrypt-then-MAC) and rejects the tampered ciphertext.

How it works

A grant token is encrypted with AES-CBC and shipped with no integrity tag, so the ciphertext is malleable: flipping a byte in one CBC block flips the corresponding byte of the next decrypted block in a predictable way. An attacker who never learns the key edits the ciphertext to change the decrypted role from user to admin, and the app decrypts the tampered blob and acts on it without noticing. The secure path uses authenticated encryption (AES-GCM, or encrypt-then-MAC) whose tag check fails on the edited ciphertext and rejects it before decryption is trusted.

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, r2.
2. Locate the target. From the home index, open Unauthenticated Encryption (malleable ciphertext / bit-flipping) (unauthenticated_encryption_malleable_ciphertext). 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
• 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:

• flipped ciphertext byte