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Introduction:
Forget theoretical cryptanalysis—real‑world crypto breaches almost always stem from flawed implementations, not broken algorithms. This article dissects practical attack vectors like predictable OTPs, misconfigured JWTs, and low‑entropy tokens that expose web and mobile applications. We’ll translate Bugcrowd’s insights into actionable penetration‑testing steps you can execute today.
Learning Objectives:
- Identify and exploit common cryptographic implementation flaws in web and mobile apps.
- Master tools like Burp Suite, hashcat, and John the Ripper to test token randomness and JWT security.
- Implement hardening measures for OTP systems, JWT validation, and mobile secret storage.
You Should Know:
1. Predictable OTPs: Brute‑Forcing the Time‑Based Seed
When a One‑Time Password (OTP) is seeded with a system timestamp or other low‑entropy source, it becomes predictable within its validity window. Attackers can brute‑force all possible 6‑digit codes faster than the typical 30‑60 second expiration.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Identify the OTP Endpoint: Intercept a login or 2FA request that sends an OTP. Note the length (usually 6 digits) and the validity period (often 30 seconds).
Step 2 – Assess Entropy Source: If you have insider knowledge or find information leakage (e.g., server time in headers), suspect time‑based seeding. Use Burp Suite’s “Repeater” to send multiple OTP requests and analyze patterns.
Step 3 – Launch a Brute‑Force Attack: Use `hashcat` or a Python script to generate all possible combinations.
Generate all 1,000,000 possible 6-digit codes hashcat -a 3 --stdout ?d?d?d?d?d?d > otp_list.txt Use a tool like ffuf or a custom script to try each code ffuf -w otp_list.txt -u https://target.com/verify-otp -X POST -d "code=FUZZ" -H "Content-Type: application/json" -mr "success"
Windows Alternative: Use `john` (John the Ripper) in mask mode or PowerShell to generate the wordlist.
0..999999 | ForEach-Object {"{0:D6}" -f $_} | Out-File -FilePath otp_list.txt
- JWT Verification Edge Cases: `alg:none` and `jwk` Header Injection
JSON Web Tokens (JWTs) are often misconfigured. The `alg:none` vulnerability allows tokens to be accepted without verification, while `jwk` (JSON Web Key) header injection lets attackers supply their own verification key.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Decode the JWT: Capture a JWT from your session (often in the `Authorization: Bearer` header). Decode it using `jwt.io` or the command line.
echo -n "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9..." | cut -d '.' -f 1 | base64 -d 2>/dev/null | jq .
Step 2 – Test for alg:none: Modify the header to set the algorithm (alg) to none. Re‑encode the token (remember to remove the signature) and send it to the endpoint.
Step 3 – Test for jwk/x5c Injection: In the JWT header, inject a `jwk` parameter pointing to a RSA public key you control. Sign the token with your corresponding private key. If the server uses your injected key to verify, you’ve compromised the system.
Use jwt_tool to automate testing python3 jwt_tool.py <your_JWT> -T -hc jwk -pk my_public_key.pem
3. Low‑Entropy Tokens: Measuring Randomness with Burp Sequencer
Session tokens, CSRF tokens, and other identifiers must be cryptographically random. Low entropy makes them predictable and susceptible to brute‑force or statistical attacks.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Capture a Token‑Generation Request: Find a request that returns a session cookie, anti‑CSRF token, or similar value.
Step 2 – Load Burp Sequencer: In Burp Suite, send the request to Sequencer.
Step 3 – Configure and Run Analysis: Define the token location in the response (e.g., cookie value, custom header). Configure Sequencer to automatically collect hundreds of token samples.
Step 4 – Analyze Results: Burp will calculate the entropy (bits of randomness) and perform multiple statistical tests (FIPS, Chi‑square). Results below ~64 bits of effective entropy are a serious concern, indicating the token can be guessed.
4. Mobile “Secrets” Hard‑Coded in Applications
Mobile apps often embed API keys, encryption keys, or certificates within the application package. These can be extracted through reverse engineering, enabling attackers to forge requests or decrypt traffic.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Obtain the Application Package: For Android, download the APK. For iOS, obtain the IPA (requires a jailbroken device or App Store extraction tools).
Step 2 – Decompile and Analyze:
Android (APK): Use `apktool` to decompile resources and dex2jar/jd-gui for Java code.
apktool d application.apk -o output_dir
iOS (IPA): Unzip the `.ipa` file and use `class-dump` or Hopper Disassembler on the binary.
Step 3 – Search for Secrets: Use `grep` or `strings` to hunt for keywords.
strings application.apk | grep -i -E "key|secret|password|token|api" find output_dir -type f -name ".smali" -o -name ".xml" | xargs grep -l "secret"
Step 4 – Exploit Extracted Secrets: Use a hard‑coded API key to make unauthorized API calls, or use a hard‑coded encryption key to decrypt local storage or intercepted traffic.
5. Flawed Session Termination: Server‑Side Token Persistence
A logout function that only clears the client‑side token (e.g., deletes a cookie) but leaves the server‑side session valid is a critical flaw. The token remains usable until its natural expiration.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Establish a Session: Log into the application and capture your active session token (JWT, session cookie).
Step 2 – “Logout”: Use the application’s logout function.
Step 3 – Test Token Validity: Immediately reuse the captured token in a request to a protected endpoint (e.g., /api/user/profile). Use Burp Repeater for this.
Step 4 – Verify Exploit: If the request with the “logged‑out” token succeeds, the logout is client‑side only. This means stolen tokens have a much longer, uncontrollable lifespan.
6. Cloud Key Management Service (KMS) Misconfigurations
While not in the original post, cloud‑stored secrets are a critical extension. Misconfigured AWS KMS, Azure Key Vault, or GCP KMS policies can leak keys or allow unauthorized encryption/decryption.
Step‑by‑step guide explaining what this does and how to use it.
Step 1 – Enumerate Cloud Resources: Using compromised credentials or via SSRF, discover what KMS services are in use.
AWS CLI example (with credentials set) aws kms list-keys aws kms list-aliases
Step 2 – Audit Key Policies: Retrieve the policy on a key to see which principals have what permissions.
aws kms get-key-policy --key-id <key_id> --policy-name default
Step 3 – Exploit Over‑Permissive Policies: If a policy grants `kms:Decrypt` to a wide principal (e.g., "") or an insecure identity, you can decrypt any ciphertext encrypted with that key. Craft a request using the CLI or SDK to decrypt stolen data.
What Undercode Say:
- The Devil is in the Details: Modern crypto breaks are about flawed logic, not advanced mathematics. Your most reliable tools are entropy analyzers, brute‑force scripts, and policy auditors.
- Assume Nothing, Verify Everything: Never trust a token’s claimed algorithm, a mobile app’s “secure” storage, or a logout button’s functionality. Active verification through attack simulation is mandatory.
Prediction:
The automation of cryptographic flaw discovery will accelerate. We’ll see a surge in AI‑powered scanners that passively audit JWTs, simulate OTP brute‑force attacks, and statically analyze mobile app binaries for secrets during CI/CD pipelines. This will push vulnerabilities deeper, forcing attackers towards chaining lower‑severity implementation bugs (like low‑entropy tokens) with business‑logic flaws to achieve critical impacts. Defense will require shifting security left, integrating advanced secret scanning and dynamic cryptographic testing directly into developer workflows.
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Reported By: Bugcrowd Hacking – Hackers Feeds
Extra Hub: Undercode MoN
Basic Verification: Pass ✅
