Scans Algorand smart contracts for 11 common vulnerabilities including rekeying attacks, unchecked transaction fees, missing field validations, and access control issues. Use when auditing Algorand pr
Install
The skill file stays in the publisher's repository.
Paste the copied command in a terminal. It installs this SKILL.md into that agent.
npx skills add trailofbits/skills --skill algorand-vulnerability-scanner -a cursor -y
Scans Algorand smart contracts for 11 common vulnerabilities including rekeying attacks, unchecked transaction fees, missing field validations, and access control issues. Use when auditing Algorand projects (TEAL/PyTeal).
name: algorand-vulnerability-scanner
description: Scans Algorand smart contracts for 11 common vulnerabilities including rekeying attacks, unchecked transaction fees, missing field validations, and access control issues. Use when auditing Algorand projects (TEAL/PyTeal).
Algorand Vulnerability Scanner
1. Purpose
Systematically scan Algorand smart contracts (TEAL and PyTeal) for platform-specific security vulnerabilities documented in Trail of Bits' "Not So Smart Contracts" database. This skill encodes 11 critical vulnerability patterns unique to Algorand's transaction model.
2. When to Use This Skill
Auditing Algorand smart contracts (stateful applications or smart signatures)
Reviewing TEAL assembly or PyTeal code
Pre-audit security assessment of Algorand projects
Validating fixes for reported Algorand vulnerabilities
Training team on Algorand-specific security patterns
3. Platform Detection
File Extensions & Indicators
TEAL files: .teal
PyTeal files: .py with PyTeal imports
Language/Framework Markers
# PyTeal indicators
from pyteal import *
from algosdk import *
# Common patterns
Txn, Gtxn, Global, InnerTxnBuilder
OnComplete, ApplicationCall, TxnType
@router.method, @Subroutine
Project Structure
approval_program.py / clear_program.py
contract.teal / signature.teal
References to Algorand SDK or Beaker framework
Tool Support
Tealer: Trail of Bits static analyzer for Algorand
Installation: uv tool install tealer (ensure uv's tool bin dir is on PATH)
Usage: tealer contract.teal --detect all
4. How This Skill Works
When invoked, I will:
Search your codebase for TEAL/PyTeal files
Analyze each file for the 11 vulnerability patterns
Report findings with file references and severity, above them a coverage table carrying a verdict for every pattern
Provide fixes for each identified issue
Run Tealer (if installed) for automated detection
5. Example Output
When vulnerabilities are found, you'll get a report like this:
I check for 11 critical vulnerability patterns unique to Algorand. For detailed detection patterns, code examples, mitigations, and testing strategies, see VULNERABILITY_PATTERNS.md.
Pattern Summary:
Rekeying Attack ⚠️ CRITICAL - Unchecked RekeyTo field
Unchecked Transaction Fee ⚠️ HIGH - Fee not validated in smart signatures
Determine contract type (stateful application vs smart signature)
Locate approval and clear state programs
Step 2: Static Analysis with Tealer
# Run Tealer on contract
tealer contract.teal --detect all
# Or specific detectors
tealer contract.teal --detect unprotected-rekey,group-size-check,update-application-check
Step 3: Manual Vulnerability Sweep
For each of the 11 vulnerabilities above:
Search for relevant transaction field usage
Verify validation logic exists
Check for bypass conditions
Validate inner transaction handling
Step 4: Transaction Field Validation Matrix
Create checklist for all transaction types used:
Payment Transactions:
RekeyTo validated
CloseRemainderTo validated
Fee validated (if smart signature)
Asset Transfers:
Asset ID validated
AssetCloseTo validated
RekeyTo validated
Application Calls:
OnComplete validated
Access controls enforced
Group size validated
Inner Transactions:
Fee explicitly set to 0
RekeyTo not user-controlled (Teal v6+)
All fields validated
Step 5: Group Transaction Analysis
For atomic transaction groups:
Validate Global.group_size() checks
Review absolute vs relative indexing
Check for replay protection (Lease field)
Verify OnComplete fields for ApplicationCalls in group
Step 6: Access Control Review
Creator/admin privileges properly enforced
Update/delete operations protected
Sensitive functions have authorization checks
8. Reporting Format
Coverage Table
Report on every pattern in §6, whether or not it turned anything up. Emit this table above the findings, with
all 11 rows present:
#
Pattern
Verdict
Evidence
1
Rekeying Attack
found
approval.py:45 -- no Txn.rekey_to() assertion on the payment branch
2
Unchecked Transaction Fee
3
Closing Account (CloseRemainderTo)
4
Closing Asset (AssetCloseTo)
5
Group Size Check
6
Time-Based Replay Attack
7
Access Controls
8
Asset ID Verification
9
Denial of Service (Asset Opt-In)
10
Inner Transaction Fee
11
Clear State Transaction
Each verdict is one of:
found — cite file:line and write the finding up in full below.
clear — the pattern applies to this contract and the contract handles it. Name the field, opcode, or
check you searched for, so a reader can repeat the search.
n/a — the pattern cannot apply here. Give the reason in one clause ("no inner transactions in this
contract"). Not having looked is not n/a.
A table with fewer than 11 rows is an incomplete scan and must be reported as one. A row whose Verdict cell is empty is incomplete in the same way: row 1 above is filled in to show the shape, and every row is filled in the same way before the report is done. Eleven clear verdicts is
a result a reader can act on. A report that covers four patterns and says nothing about the other seven reads
exactly like a clean contract, and that is the failure this table exists to prevent.
Finding Template
## [SEVERITY] Vulnerability Name (e.g., Missing RekeyTo Validation)
**Location**: `contract.teal:45-50` or `approval_program.py:withdraw()`
**Description**:
The contract approves payment transactions without validating the RekeyTo field, allowing an attacker to rekey the account and bypass future authorization checks.
**Vulnerable Code**:
```python
# approval_program.py, line 45
If(Txn.type_enum() == TxnType.Payment,
Approve() # Missing RekeyTo check
)
```
**Attack Scenario**:
1. Attacker submits payment transaction with RekeyTo set to attacker's address
2. Contract approves transaction without checking RekeyTo
3. Account authorization is rekeyed to attacker
4. Attacker gains full control of account
**Recommendation**:
Add explicit validation of the RekeyTo field:
```python
If(And(
Txn.type_enum() == TxnType.Payment,
Txn.rekey_to() == Global.zero_address()
), Approve(), Reject())
```
**References**:
- building-secure-contracts/not-so-smart-contracts/algorand/rekeying
- Tealer detector: `unprotected-rekey`
9. Priority Guidelines
Critical (Immediate Fix Required)
Rekeying attacks
CloseRemainderTo / AssetCloseTo issues
Access control bypasses
High (Fix Before Deployment)
Unchecked transaction fees
Asset ID validation issues
Group size validation
Clear state transaction checks
Medium (Address in Audit)
Inner transaction fee issues
Time-based replay attacks
DoS via asset opt-in
10. Testing Recommendations
Unit Tests Required
Test each vulnerability scenario with PoC exploit
Verify fixes prevent exploitation
Test edge cases (group size = 0, empty addresses, etc.)
Tealer Integration
# Add to CI/CD pipeline
tealer approval.teal --detect all --json > tealer-report.json
# Fail build on critical findings
tealer approval.teal --detect all --fail-on critical,high
Scenario Testing
Submit transactions with all critical fields manipulated
Test atomic groups with unexpected sizes
Attempt access control bypasses
Verify inner transaction fee handling
11. Additional Resources
Building Secure Contracts: building-secure-contracts/not-so-smart-contracts/algorand/
Before completing Algorand audit, verify ALL items checked:
RekeyTo validated in all transaction types
CloseRemainderTo validated in payment transactions
AssetCloseTo validated in asset transfers
Transaction fees validated (smart signatures)
Group size validated for atomic transactions
Lease field used for replay protection (where applicable)
Access controls on Update/Delete operations
Asset ID validated in all asset operations
Asset transfers use pull pattern to avoid DoS
Inner transaction fees explicitly set to 0
OnComplete field validated for ApplicationCall transactions
Tealer scan completed with no critical/high findings
Unit tests cover all vulnerability scenarios
Coverage table emitted with all 11 rows, each carrying a verdict of found, clear or n/a with a reason
13. Rationalizations to Reject
"The contract is small, so most patterns obviously don't apply." Obvious to whom? An n/a costs one
clause and makes the judgment reviewable. Silence records nothing, and a reader cannot tell it apart from
not having checked.
"Tealer reported nothing, so the contract is clean." Tealer covers a subset of these 11 patterns and
does not reach the logic-level ones at all. A clean tool run is one row of evidence, not a verdict on the
patterns it never examined. Say which patterns it covered.
"I checked the patterns that matter for this contract." Deciding which patterns matter is the scan,
not a precondition for starting it. Rank by severity after the table is complete, not by leaving rows out.
"No findings, so there is nothing to report." A zero-finding scan still emits the full coverage table.
That table is the deliverable: it is what distinguishes a contract that was examined from one that was
glanced at.
"PyTeal/Beaker handles this." Name the version and the mechanism. Framework defaults change between
releases, and a framework that covers a pattern on one call path often does not on another.
"The RekeyTo check is in the other program." Then cite it. A validation you believe exists elsewhere is
an assumption until you have the file:line, and split-program contracts are where these checks go missing.
How do I install algorand-vulnerability-scanner?
Download plugins/building-secure-contracts/skills/algorand-vulnerability-scanner/SKILL.md from the publisher, or copy the path and open the repository. PlugMCP lists the skill and does not host the file.
Where does algorand-vulnerability-scanner live?
The publisher keeps it at https://github.com/trailofbits/skills.
Which agents can use algorand-vulnerability-scanner?
Claude, Cursor, and Codex can read a SKILL.md you add from the publisher's repository.