entity-expansion

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" .

Step 2: Check Entity/DTD Configuration

For each parser found, check:

1. Does it process DTD declarations by default?
2. Is there a maxExpansion or maxEntitySize option?
3. Is DTD processing explicitly disabled?
4. Does it support custom entity resolution?

Step 3: Check for Expansion Limits

grep -rn "maxExpansion\|maxEntitySize\|entityExpansion\|ENTITY_EXPANSION" .
grep -rn "disableDTD\|forbidDTD\|dtd.*false\|FEATURE_SECURE_PROCESSING" .
grep -rn "noent\|resolve_entities\|processEntities" .

Step 4: Check YAML Alias Limits

grep -rn "maxAliasCount\|maxAliases\|aliasLimit\|MAX_ALIAS" .
grep -rn "anchorLimit\|maxAnchors" .

Step 5: Verify Exploitability

1. Does the parser accept untrusted input? (user uploads, API requests, webhook payloads)
2. Is there a file size limit that would prevent the payload from being processed?
3. Is the process memory-limited (cgroups, ulimit)?
4. Does the parser use streaming that could limit memory usage?

Known Parser Default Safety

Parser	Language	DTD/Entity Default	Safe?
fast-xml-parser	JS	Entities processed, no limit	UNSAFE
xml2js	JS	Entities processed, no limit	UNSAFE
xmldom	JS	Entities processed, no limit	UNSAFE
sax	JS	No entity expansion	SAFE
cheerio (htmlparser2)	JS	No DTD support	SAFE
xml.etree.ElementTree	Python	Entities processed, no limit	UNSAFE
lxml	Python	DTD disabled by default	SAFE (default)
defusedxml	Python	All dangerous features disabled	SAFE
xml.sax	Python	Entities processed	UNSAFE
PyYAML yaml.load	Python	Aliases processed, no limit	UNSAFE
PyYAML yaml.safe_load	Python	Aliases processed, no limit	UNSAFE (aliases)
encoding/xml	Go	No entity support	SAFE
go-yaml v3	Go	Aliases processed, limited	CHECK VERSION
Nokogiri	Ruby	DTD disabled by default	SAFE (default)
REXML	Ruby	Entities processed	UNSAFE
simplexml_load_string	PHP	Entities processed by default	UNSAFE
DOMDocument	PHP	Entities processed by default	UNSAFE

CVSS Guidance

• Unauthenticated OOM crash (process dies): HIGH 7.5 (AV:N/AC:L/PR:N/UI:N/S:U/C:N/I:N/A:H)
• Authenticated OOM crash: MEDIUM 6.5 (PR:L)
• CPU exhaustion (recoverable): MEDIUM 5.3

References

• Sinks — XML/YAML parser safety status by language
• False Positive Indicators — When this isn't exploitable
• PoC Skeleton — Billion Laughs payload templates