Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx skills add ByamB4/find-cve-agent --skill entity-expansiongit clone --depth 1 https://github.com/ByamB4/find-cve-agentWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/byamb4/find-cve-agent/entity-expansion)<a href="https://agentmods.dev/skills/byamb4/find-cve-agent/entity-expansion"><img src="https://agentmods.dev/badge/skills/byamb4/find-cve-agent/entity-expansion/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/byamb4/find-cve-agent/entity-expansion"><img src="https://agentmods.dev/badge/skills/byamb4/find-cve-agent/entity-expansion.svg" alt="Reviewed on agentmods" width="80" height="20"></a>What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00029 | $0.01497 |
| Opus 5 | $0.00015 | $0.00749 |
| Sonnet 5 | $0.00006 | $0.00299 |
| Haiku 4.5 | $0.00003 | $0.00150 |
Grade A, and why
entity-expansion scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 10d ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 154 lines — stays where its author put it; the contents beside it link to each section on GitHub.
Entity Expansion (Billion Laughs) Detection
When to Use
Audit any package that parses XML, SVG, HTML with entity support, or YAML with alias/anchor support. This includes:
- XML/SVG parsing libraries
- Document processors (DOCX, XLSX, RSS, Atom, SOAP)
- YAML parsers with alias expansion
- Configuration file parsers
~90% CVE acceptance rate when confirmed.
Key Insight
Many parsers have NO default entity expansion limit. A 1KB XML payload with recursive entity definitions can expand to 1GB+ in memory, crashing the process with an OOM kill (uncatchable — process dies).
Entity Expansion Types
1. Billion Laughs (Internal Entity Recursion)
<?xml version="1.0"?>
<!DOCTYPE lolz [
<!ENTITY lol "lol">
<!ENTITY lol2 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
<!ENTITY lol3 "&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;&lol2;">
...
<!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
]>
<root>&lol9;</root>
Each level multiplies by 10. Level 9 = 10^9 = 1 billion "lol" strings.
2. Quadratic Blowup (Single Entity Repeated)
<!DOCTYPE foo [
<!ENTITY a "AAAAAAAAAA..."> <!-- 50KB entity -->
]>
<root>&a;&a;&a;&a;&a;...&a;</root> <!-- 50000 references -->
Less dramatic but still effective — 50KB entity × 50000 refs = 2.5GB.
3. YAML Alias Expansion
a: &anchor
x: *anchor
y: *anchor
Recursive alias references can cause exponential expansion in some YAML parsers.
Process
Step 1: Find XML/YAML Parsing
# JavaScript
grep -rn "xml2js\|fast-xml-parser\|xmldom\|sax\|DOMParser\|cheerio" .
grep -rn "\.parseString\|\.parse(" . --include="*.js" --include="*.ts"
grep -rn "yaml\.load\|yaml\.parse\|YAML\.parse" .
# Python
grep -rn "xml\.etree\|lxml\|minidom\|xml\.sax\|defusedxml" .
grep -rn "yaml\.load\|yaml\.safe_load\|yaml\.unsafe_load" .
# Go
grep -rn "xml\.Decoder\|xml\.Unmarshal\|encoding/xml" .
grep -rn "yaml\.Unmarshal\|gopkg.in/yaml" .
# Ruby
grep -rn "Nokogiri\|REXML\|Ox\.\|LibXML" .
# PHP
grep -rn "simplexml\|DOMDocument\|XMLReader\|xml_parse" .
What ships with it
3 files beside SKILL.md in the same directory: the scripts, references and assets a skill reads on demand. Not counted in the per-session cost; read them before you install if any of them is executable.
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 10d ago First seen · 154 lines · 29 tokens per session scan A 9461ea0b4f95
entity-expansion is a skill published in the GitHub repository ByamB4/find-cve-agent (48 stars, last pushed 5mo ago), licensed Apache-2.0. It adds 29 tokens to every session and 1,497 once invoked, about $0.0001 per session on Opus 5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
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