capability-system
Skill17 starsChanged 9 months agoApache-2.0View on GitHub
Implement capability-based security for fine-grained access control and least privilege.
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The file
We split this skill into the rules it gives an agent
3 rules. About 0 are common, 3 are distinctive, and 0 are very specific to this project.
≈0
Common · estimate
Many files like this have a rule of this kind.
≈3
Distinctive · estimate
This author's own choices. Worth a look.
0
Very specific
Tied to their project's names, files or tools. Probably skip.
- Common
- Distinctive
- Very specific
- Distinctive: line 44
Use unforgeable tokens (cryptographic or memory-safe references)
- Distinctive: line 46
Consider revocation carefully (may be impossible without tracking)
- Distinctive: line 47
Use object capabilities for fine-grained control
≈ “Common” is an estimate: at least 1 in 5 of the 350,495 skill files we read have a rule of the same kind, not the same words. “Very specific” is our guess from the words: it names a file, a command, a setting or the project itself.
Files
Capability System
Capability-based security uses unforgeable tokens (capabilities) to represent authority. Instead of checking "who you are" (ACLs), it checks "what you hold" (capabilities), enabling fine-grained least privilege.
When to Use This Skill
- Building secure operating systems
- Implementing sandboxing
- Designing least-privilege systems
- Distributed object systems
- Secure browsers (seL4, Capsicum)
What This Skill Does
- Capability Tokens: Unforgeable references to resources
- Authority Delegation: Transfer capabilities between principals
- Attenuation: Reduce capability powers
- Revocation: Revoke delegated capabilities
- Confinement: Restrict capability propagation
Key Concepts
| Concept | Description |
| Capability | Unforgeable token granting authority |
| Attenuation | Reduce capability permissions |
| Delegation | Transfer capability to another principal |
| Revocation | Invalidate capability and descendants |
| Confinement | Restrict capability propagation |
| Facet | Specific interface to an object |
Tips
- Use unforgeable tokens (cryptographic or memory-safe references)
- Implement proper attenuation
- Consider revocation carefully (may be impossible without tracking)
- Use object capabilities for fine-grained control
- Log capability usage for auditing
Common Use Cases
- Secure operating systems (seL4, Fuchsia)
- Browser sandboxing
- Distributed object systems
- Cloud access control
- IoT device security
Related Skills
information-flow-analyzer- Track information flowlinear-type-implementer- Linear capabilitiessandbox-builder- Use capabilities for sandboxingownership-type-system- Ownership and capabilities
Canonical References
| Reference | Why It Matters |
| Levy "Capability-Based Computer Systems" | Classic text |
| Miller "Capability Myths Demolished" | Clarifies misconceptions |
| Shapiro et al. "EROS: A Capability System" | Practical system |
Tradeoffs and Limitations
Approach Tradeoffs
| Approach | Pros | Cons |
| Pure capabilities | Composable, no ambient authority | Requires language support |
| Hybrid (ACL + caps) | Practical | Complex |
| Cryptographic caps | Distributed | Performance |
When NOT to Use This Skill
- Simple access control needs
- When ACLs suffice
- Performance-critical paths (crypto overhead)
Limitations
- Revocation is complex
- Confinement is hard
- Lost capabilities are hard to recover
Assessment Criteria
A high-quality implementation should have:
| Criterion | What to Look For |
| Unforgeability | Capabilities cannot be forged |
| Attenuation | Permissions can be reduced |
| Delegation | Capabilities can be shared |
| Revocation | Capabilities can be revoked |
Quality Indicators
✅ Good: Unforgeable, supports attenuation and revocation ⚠️ Warning: Forgery possible, no revocation ❌ Bad: No security guarantees
Research Tools & Artifacts
Capability systems:
| System | What to Learn |
| EROS | Capability OS |
| CapROS | Modern capability OS |
| Fuchsia | Capability-based |
Research Frontiers
1. Distributed Capabilities
- Goal: Capabilities across trust boundaries
- Approach: Cryptographic capabilities
Implementation Pitfalls
| Pitfall | Real Consequence | Solution |
| Forgery | Security breach | Cryptographic seals |
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