Tool Security

SSRF guard, tool policy profiles, content scanning, sanitization pipeline, and audit wrapper reference

Tool security encompasses four subsystems that protect against malicious input from external sources: SSRF prevention for outbound HTTP, tool policy enforcement for access control, content scanning for skill body inspection, and sanitization for hidden content defense.

SSRF Guard

The SSRF guard prevents server-side request forgery by validating every outbound URL before any HTTP request is made. The validateUrl() function performs DNS-pinned validation: it resolves the hostname, then checks the resolved IP against blocked ranges.

Source: packages/core/src/security/ssrf-guard.ts — requirement: every web-facing tool must pass through validateUrl() before fetch.

Validation Steps

validateUrl() performs 5 checks in order:

Step	Check	Behavior on Failure
1	URL parsing	Returns err("Invalid URL: ...")
2	Protocol check	Returns err("Blocked protocol: ...")
3	DNS resolution	Returns err() with DNS error
4	Cloud metadata IP blocklist	Returns err("Blocked: resolved IP ... is a cloud metadata service address")
5	IP range classification	Returns err("Blocked: resolved IP ... is in {range} range")

The function returns Result<ValidatedUrl, Error> where ValidatedUrl contains hostname, ip (resolved address), and url (parsed URL object).

Allowed Protocols

Only two protocols are permitted:

• http:
• https:

All other protocols (e.g., file:, ftp:, gopher:) are rejected.

Blocked IP Ranges

Six IP range categories are blocked using ipaddr.js range classification:

Range Name	IPv4	IPv6	Purpose
loopback	127.0.0.0/8	::1	Localhost access
linkLocal	169.254.0.0/16	fe80::/10	Link-local addresses
private	10.0.0.0/8, 172.16.0.0/12, 192.168.0.0/16	—	RFC 1918 private networks
uniqueLocal	—	fc00::/7	IPv6 private addresses
unspecified	0.0.0.0	::	Unspecified address
reserved	IANA reserved ranges	IANA reserved ranges	Future-allocated blocks

Cloud Metadata IPs

Three explicit cloud metadata service IPs are blocked regardless of their range classification:

IP	Cloud Provider
169.254.169.254	AWS, GCP, Azure instance metadata
169.254.170.2	AWS ECS task metadata
100.100.100.200	Alibaba Cloud metadata

DNS Resolution

The SSRF guard resolves hostnames before checking IPs. This prevents DNS rebinding attacks where a hostname initially resolves to a safe IP but later resolves to a blocked one. IPv6 literal hostnames (bracketed like [::1]) have brackets stripped before DNS lookup.

ValidatedUrl Interface

On successful validation, validateUrl() returns a ValidatedUrl object:

interface ValidatedUrl {
  hostname: string;  // Original hostname from the URL
  ip: string;        // DNS-resolved IP address
  url: URL;          // Parsed URL object
}

The caller uses the resolved ip for the actual HTTP request, ensuring the fetched IP matches the validated one (DNS pinning).

Usage Example

import { validateUrl } from "@comis/core";

const result = await validateUrl("https://api.example.com/data");
if (!result.ok) {
  // URL is blocked -- do not fetch
  logger.warn({ err: result.error }, "SSRF validation failed");
  return;
}

// Safe to fetch -- use result.value.url
const response = await fetch(result.value.url.toString());

Tool Policy

Tool policies control which tools are available to an agent. Policies use named profiles as a baseline, with allow/deny overrides for fine-grained control.

Source: packages/skills/src/policy/tool-policy.ts — config-driven tool filtering with group expansion.

Built-in Profiles

Built-in profiles define baseline tool sets. The five general-purpose profiles plus two narrow opt-in presets for non-interactive workloads:

Profile	Tools Included
minimal	read, write
coding	read, edit, write, grep, find, ls, apply_patch, exec, process
messaging	message, session_status
supervisor	agents_manage, obs_query, sessions_manage, memory_manage, channels_manage, tokens_manage, models_manage, skills_manage, mcp_manage, heartbeat_manage
full	All tools (empty array = unrestricted)
cron-minimal	web_search, message, read_file, write_file, list_dir, memory_store, memory_search, cron, discover
heartbeat-minimal	message, memory_store, memory_search, discover

The *-minimal presets are opt-in via toolPolicy.profile on a CronJob or heartbeat config respectively. They are never applied as a silent default — operators are expected to widen them per-job via allow.

Tool Groups

Groups provide convenient bulk operations using group:xxx syntax in allow/deny arrays:

Group	Tools
group:coding	read, edit, write, grep, find, ls, apply_patch, exec, process
group:web	web_fetch, web_search, browser
group:browser	browser
group:memory	memory_search, memory_get, memory_store
group:scheduling	cron
group:messaging	message
group:sessions	sessions_list, sessions_history, sessions_send, sessions_spawn, session_status, session_search, subagents, pipeline
group:platform_actions	discord_action, telegram_action, slack_action, whatsapp_action
group:supervisor	agents_manage, obs_query, sessions_manage, memory_manage, channels_manage, tokens_manage, models_manage, skills_manage, mcp_manage, heartbeat_manage

There are no group:context / group:context_expand groups: the DAG context engine’s on-demand ctx_* recall tools are not yet shipped, so no tool group bundles them.

Resolution Precedence

Tool policy resolution follows a strict order:

1. Profile baseline — start with the profile’s tool set
2. Allow additions — add explicitly allowed tools and expand group references
3. Deny removals — remove denied tools (deny always takes precedence)

Per-skill tool restrictions from skill manifests provide an additional layer of filtering applied after the agent-level policy.

For user-facing configuration guide, see Tool Policy.

MCP Export Policy

Tool policy (above) governs which tools an agent may call. A separate, stricter layer governs which tools an external MCP client may reach over POST /mcp/v1: every platform tool carries an mcpExportPolicy of safe, permission-gated, or never-export (a missing annotation is treated as never-export — default-deny). See the MCP server bucket reference for the full classification.

Source: packages/skills/src/skills/bridge/tool-metadata-registry.ts (the mcpExportPolicy annotations) + packages/daemon/src/api/mcp-server-handlers.ts (the registration filter + per-tool dispatch).

obs_explain — read-only incident report, allowlist-gated

obs_explain is a permission-gated, isReadOnly MCP tool that surfaces the obs.explain IncidentReport (root-cause post-mortem for a session or trace) to an external coding agent. It is reachable only when the operator adds "obs_explain" to that client’s gateway.tokens[].mcpClient.allowlist.

Three properties make this safe to expose with no new privilege:

• The allowlist IS the granted permission — not admin trust. obs_explain does not invoke the admin-gated obs.explain RPC. Its dispatch branch runs the report assembler directly under daemon authority, bypassing the trust-flag indirection entirely. No _trustLevel:"admin" is ever injected, and the admin RPC’s own ["admin"] gate is unchanged for non-MCP callers. A client without obs_explain allowlisted is rejected (default-deny).
• Digest-only and bounded. The report is summary-bounded (no raw tool bodies, no raw message content cross the boundary) and flows through wrapExternalContent — the calling agent receives it wrapped in a SECURITY NOTICE block + untrusted-content markers, like any other external text.
• Read-only + rate-limited. The tool performs no mutation, and the standard per-client per-tool minute-bucket rate limit applies.

Enumeration implication. An allowlisted client can request an IncidentReport for any sessionKey or traceId it can name — there are no per-session ACLs on top of the allowlist grant (out of scope; the report is digest-only/bounded regardless). The operator decides which clients receive obs_explain by editing mcpClient.allowlist; it must not be granted by default. Before flipping /mcp/v1 live on a publicly reachable instance, a security reviewer should confirm the grant list and the digest-only bounding meet the deployment’s threat model.

Content Scanner

The content scanner inspects skill body content at load time for dangerous patterns across six categories. It is a pure function with no side effects — callers handle audit emission and blocking decisions.

Source: packages/skills/src/prompt/content-scanner.ts — iterates CONTENT_SCAN_RULES array against content, producing ContentScanResult.

Scan Categories

exec_injection (CRITICAL)

Targets injection syntax operators combined with dangerous binaries, not mere mentions of command names.

Pattern ID	Description
EXEC_SUBSHELL	Subshell command injection: $(command) with dangerous binary
EXEC_BACKTICK	Backtick command injection with dangerous binary
EXEC_EVAL	eval() with string argument
EXEC_PIPE_BASH	Pipe to shell interpreter

env_harvesting (WARN)

Targets mass-dump patterns, not individual $VAR references which are common in documentation.

Pattern ID	Description
ENV_PRINTENV	printenv command dumps all environment variables
ENV_PROC_ENVIRON	Direct read of process environment via /proc
ENV_MASS_DUMP	Environment dump piped to exfiltration or encoding

crypto_mining (CRITICAL/WARN)

Very low false-positive risk — these terms rarely appear in legitimate skill instructions.

Pattern ID	Severity	Description
CRYPTO_STRATUM	CRITICAL	Mining pool protocol (stratum://)
CRYPTO_MINER_BINARY	CRITICAL	Known cryptocurrency miner binary
CRYPTO_POOL_DOMAIN	WARN	Mining pool domain pattern

network_exfiltration (WARN/CRITICAL)

Focuses on piped execution rather than standalone URL references.

Pattern ID	Severity	Description
NET_CURL_PIPE	WARN	curl output piped to interpreter
NET_WGET_EXEC	WARN	wget output to stdout piped elsewhere
NET_REVERSE_SHELL	CRITICAL	Reverse shell pattern

obfuscated_encoding (WARN/CRITICAL)

Only flags long encoded blocks or decode-and-execute chains, not short base64 examples.

Pattern ID	Severity	Description
OBF_BASE64_LONG	WARN	Long base64-encoded string (80+ chars)
OBF_HEX_LONG	WARN	Long hex-escaped string (20+ sequences)
OBF_BASE64_DECODE_PIPE	CRITICAL	base64 decode piped to another command

xml_breakout (CRITICAL)

Detects attempts to escape skill XML structure and inject system-level instructions.

Pattern ID	Description
XML_SKILL_CLOSE	Closing tag for skill XML structure (breakout attempt)
XML_SYSTEM_TAG	System-level message tag (breakout attempt)

Severity Levels

• CRITICAL: Patterns that indicate active exploitation attempts (exec injection, reverse shells, crypto mining, base64 decode piping, XML breakout)
• WARN: Patterns that are suspicious but may have legitimate uses (environment harvesting, curl piping, long encoded strings, mining pool domains)

Scan Result

The scanSkillContent() function returns a ContentScanResult:

interface ContentScanResult {
  clean: boolean;       // true if no patterns matched
  findings: ContentScanFinding[];
}

interface ContentScanFinding {
  ruleId: string;       // e.g., "EXEC_SUBSHELL"
  category: ScanCategory;
  severity: "CRITICAL" | "WARN";
  description: string;
  matchedText: string;  // truncated to 100 chars
  position: number;     // character offset in content
  lineNumber: number;   // 1-based line number in the original content
}

For user-facing guide, see Security Scanning.

Sanitization Pipeline

The sanitization pipeline processes skill body content through four ordered steps before it reaches the system prompt.

Source: packages/skills/src/prompt/sanitizer.ts — pure functions, no side effects. Audit metadata returned for caller to emit events.

Pipeline Steps

1. Strip HTML Comments

   Removes all <!-- ... --> sequences using non-greedy regex to handle multiple separate comments correctly. Returns the count of comments removed for audit logging.

2. NFKC Normalization

   Applies Unicode NFKC normalization (compatibility decomposition + canonical composition). This decomposes fullwidth characters and ligatures into their standard equivalents, preventing homoglyph-based obfuscation.

3. Strip Invisible Characters

   Removes zero-width and invisible characters that could hide malicious content:

   • Zero-width joiner (U+200D)
   • Zero-width non-joiner (U+200C)
   • Zero-width space (U+200B)
   • Soft hyphen (U+00AD)
   • Left-to-right mark (U+200E)
   • Right-to-left mark (U+200F)
   • Unicode tag block characters (U+E0000-U+E007F)
4. Size Limit Enforcement

   Truncates the final sanitized output at maxBodyLength characters (default: 20,000). Size enforcement applies to the output after all other sanitization steps, preventing unnecessary truncation when HTML comments inflate raw size. Appends [TRUNCATED] marker when truncation occurs.

Sanitization Result

interface SanitizeResult {
  body: string;                  // Final sanitized content
  htmlCommentsStripped: number;  // Count for audit
  truncated: boolean;            // Whether size limit was hit
  tagBlockDetected: boolean;     // Unicode tag block bypass detected
}

Audit Wrapper

Tool executions generate audit events through the TypedEventBus. Each tool call emits an audit:event with the action type, classification (from the Action Classifier), outcome, and timing metadata. This provides a complete audit trail of all agent actions.

The audit event structure:

{
  timestamp: number;
  agentId: string;
  tenantId: string;
  actionType: string;        // e.g., "file.write", "web.fetch"
  classification: string;    // "read" | "mutate" | "destructive"
  outcome: "success" | "failure";
  metadata: {
    durationMs: number;
    toolName?: string;
    // Additional context per tool
  };
}

Every tool execution — whether built-in, prompt skill, or MCP — generates an audit event. The classification field comes from the Action Classifier registry. Failed executions still emit audit events with outcome: "failure" and the error message in metadata.

Action Classifier

Complete registry of action classifications

Security Model

Defense-in-depth security architecture

Sandbox

Skill sandboxing implementation

Tool Policy Guide

User-facing tool policy configuration