ANALYSIS · 7fbb6677· FINISHED 2026-09-05 · PATCHDIFF

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CVE-2026-66755

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tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-microsoft-module/src/test/java/org/apache/tika/parser/microsoft/ooxml/MetadataExtractorTest.java

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CVE-2026-66755

Security writeup for CVE-2026-66755

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## The Exploit

An attacker with the ability to supply a malicious OOXML file (e.g., a `.docx` document) to Apache Tika for parsing can inject an XML External Entity (XXE) payload into the `docProps/custom.xml` component.

```xml
<?xml version="1.0" encoding="UTF-8"?>
<!DOCTYPE foo [
  <!ENTITY xxe SYSTEM "file:///etc/passwd">
]>
<Properties xmlns="http://schemas.openxmlformats.org/officeDocument/2006/custom-properties" xmlns:vt="http://schemas.openxmlformats.org/officeDocument/2006/docPropsVTypes">
  <property fmtid="{D5CDD505-2E9C-101B-9397-08002B2CF9AE}" pid="2" name="test">
    <vt:lpwstr>&xxe;</vt:lpwstr>
  </property>
</Properties>
```

When Tika's `MetadataExtractor.CustomPropertiesHandler` parses this XML via the undefended `SAXParser`, the entity resolves to the contents of `/etc/passwd`, which is either written to the document's extracted metadata or disclosed in error messages. The attacker observes the file contents leaking into the parsed metadata output or error logs.

## What the Patch Did

**Before**

**After**

```java
private static Metadata parseCustomProperties(String xml) throws Exception {
    MetadataExtractor.CustomPropertiesHandler handler =
            new MetadataExtractor.CustomPropertiesHandler();
    SAXParserFactory factory = SAXParserFactory.newInstance();
    factory.setNamespaceAware(true);
    factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
    factory.setFeature("http://xml.org/sax/features/external-general-entities", false);
    factory.setFeature("http://xml.org/sax/features/external-parameter-entities", false);
    factory.setFeature("http://apache.org/xml/features/nonvalidating/load-external-dtd", false);
    SAXParser parser = factory.newSAXParser();
    parser.parse(new InputSource(new ByteArrayInputStream(
            xml.getBytes(StandardCharsets.UTF_8))), handler);
```

The patch adds four `SAXParserFactory.setFeature()` calls that collectively disable external entity resolution in the parser. The controls are:

1. `http://apache.org/xml/features/disallow-doctype-decl` — rejects any DOCTYPE declaration entirely.
2. `http://xml.org/sax/features/external-general-entities` — disables external general entity resolution (the primary XXE vector).
3. `http://xml.org/sax/features/external-parameter-entities` — disables parameter entity injection.
4. `http://apache.org/xml/features/nonvalidating/load-external-dtd` — prevents loading external DTD subsets.

## Root Cause

**CWE-611: Improper Restriction of XML External Entity Reference** ("XXE Injection").

The attacker controls the contents of the OOXML file's `docProps/custom.xml` entry, which is extracted and passed as the `xml` parameter to `parseCustomProperties()`. The `SAXParserFactory` is instantiated with no XXE defences; by default, both general and parameter entities are resolved. When `parser.parse()` is called on the untrusted XML stream, the SAX parser follows entity declarations and fetches resources from the filesystem or network on behalf of the application. The trust boundary is crossed when user-supplied OOXML enters the parsing pipeline without entity resolution guards.

## Why It Works

The load-bearing line is:

```java
factory.setFeature("http://apache.org/xml/features/disallow-doctype-decl", true);
```

Removing it alone would still leave the parser vulnerable: an attacker could craft a well-formed XML document *without* a DOCTYPE but leverage parameter entity tricks or namespace-based attacks. However, this line is the highest-impact single control because it **rejects the attack at the grammar level** — no DOCTYPE means no entity declarations can be introduced.

The engineer added the three remaining features (`external-general-entities`, `external-parameter-entities`, `load-external-dtd`) as **defence in depth**. Together, they form a layered barrier:

- If DOCTYPE somehow gets through, general entity resolution is already off.
- If an attacker crafts a parameter entity attack, that vector is blocked separately.
- If a legacy parser version has a bypass, the external DTD loader is still disabled.

Each feature targets a different XXE attack surface; removing any one reduces the margin of safety. The patch as written ensures no single feature regression will re-open the vulnerability.

## Hardening Checklist

- **Disable DOCTYPE declarations** in all `SAXParserFactory` or `XMLInputFactory` instances handling untrusted XML by setting `http://apache.org/xml/features/disallow-doctype-decl` to `true`.
- **Disable external entity resolution** by setting both `http://xml.org/sax/features/external-general-entities` and `http://xml.org/sax/features/external-parameter-entities` to `false` on every parser factory before parsing.
- **Audit all XML parsing code** (OOXML, ODP, SVG, or any embedded XML) — use the OWASP XXE Prevention Cheat Sheet to verify all sinks are protected, including DOM parsers (`DocumentBuilderFactory`), StAX (`XMLInputFactory`), and XPath evaluators.
- **Use an allowlist approach**: if the XML format is known, disable schema validation and only enable the specific XML features actually needed (namespace awareness, etc.); disable everything else by default.
- **Add XXE-payload test cases** to your test suite — craft OOXML files with XXE payloads in each structural component (core properties, custom properties, document relationships) and verify they do not leak file contents or cause out-of-band requests.

## References

- https://nvd.nist.gov/vuln/detail/CVE-2026-66755
- OWASP XXE Prevention Cheat Sheet: https://cheatsheetseries.owasp.org/cheatsheets/XML_External_Entity_Prevention_Cheat_Sheet.html
- Apache Xerces2 SAX Parser Feature Documentation

CVE CVE-2026-66755

PRODUCT tika

AFFECTED ≤ 3.3.1

FIXED IN 3.3.2

STATUS Generated

Security writeup · CVE-2026-66755

Source files · 1

tika-parsers/tika-parsers-standard/tika-parsers-standard-modules/tika-parser-microsoft-module/src/test/java/org/apache/tika/parser/microsoft/ooxml/MetadataExtractorTest.java Linked

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