diff --git a/NEWS b/NEWS
index 5a865955281d..f32d6714a140 100644
--- a/NEWS
+++ b/NEWS
@@ -204,6 +204,10 @@ PHP                                                                        NEWS
   . Fixed xml_get_current_byte_index() returning negative values for byte
     indexes beyond 2 GiB on Windows x64. (Marc Bennewitz)
 
+- XSL:
+  . Fixed a use-after-free when a transformation was started on a processor
+    that is already transforming. (Ilia Alshanetsky)
+
 - Zlib:
   . Fixed inflate_init() dropping the preset dictionary for raw streams with
     a non-default window. (Ilia Alshanetsky)
diff --git a/UPGRADING b/UPGRADING
index a4df23352639..e0554f2014a7 100644
--- a/UPGRADING
+++ b/UPGRADING
@@ -64,6 +64,13 @@ PHP 8.7 UPGRADE NOTES
     to 16 by default. Set the stream context option max_filter_count to change
     this.
 
+- XSL:
+  . XSLTProcessor::transformToDoc(), XSLTProcessor::transformToUri() and
+    XSLTProcessor::transformToXml() now throw an Error when called while the
+    same processor is already transforming, for example from a php:function()
+    callback or from a suspended Fiber. Use a separate XSLTProcessor instance
+    instead.
+
 ========================================
 2. New Features
 ========================================
diff --git a/ext/xsl/tests/xsltprocessor_nested_transform.phpt b/ext/xsl/tests/xsltprocessor_nested_transform.phpt
new file mode 100644
index 000000000000..cef56cc67925
--- /dev/null
+++ b/ext/xsl/tests/xsltprocessor_nested_transform.phpt
@@ -0,0 +1,100 @@
+--TEST--
+XSLTProcessor refuses a nested transformation on the same processor
+--EXTENSIONS--
+dom
+xsl
+--FILE--
+<?php
+
+function nested($nodes) {
+    static $called = false;
+    if ($called) {
+        return 'v';
+    }
+    $called = true;
+    $proc = $GLOBALS['proc'];
+    $doc = $GLOBALS['doc'];
+    try {
+        var_dump($proc->transformToXml($doc));
+    } catch (Error $e) {
+        echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+    }
+    try {
+        var_dump($proc->transformToDoc($doc));
+    } catch (Error $e) {
+        echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+    }
+    try {
+        var_dump($proc->transformToUri($doc, $GLOBALS['uri']));
+    } catch (Error $e) {
+        echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+    }
+    return 'v';
+}
+
+function suspend($nodes) {
+    Fiber::suspend();
+    return 'f';
+}
+
+function text($nodes) {
+    return $nodes[0]->textContent;
+}
+
+$doc = new DOMDocument();
+$doc->loadXML('<root><a>1</a><b>2</b><c>3</c></root>');
+
+$xsl = new DOMDocument();
+$xsl->loadXML(<<<XML
+<xsl:stylesheet version="1.0" xmlns:xsl="http://www.w3.org/1999/XSL/Transform" xmlns:php="http://php.net/xsl">
+  <xsl:key name="k" match="*" use="name()"/>
+  <xsl:param name="cb" select="'nested'"/>
+  <xsl:template match="/"><xsl:for-each select="/root/*">[<xsl:value-of select="php:function(\$cb, .)"/><xsl:value-of select="name(key('k', name(.)))"/>]</xsl:for-each></xsl:template>
+</xsl:stylesheet>
+XML);
+
+$uri = __DIR__ . '/xsltprocessor_nested_transform.xml';
+
+$proc = new XSLTProcessor();
+$proc->registerPHPFunctions();
+$proc->importStylesheet($xsl);
+
+var_dump($proc->transformToXml($doc));
+
+$proc->setParameter('', 'cb', 'suspend');
+$fiber = new Fiber(fn () => $proc->transformToXml($doc));
+$fiber->start();
+try {
+    var_dump($proc->transformToXml($doc));
+} catch (Error $e) {
+    echo $e::class, ": ", $e->getMessage(), PHP_EOL;
+}
+while (!$fiber->isTerminated()) {
+    $fiber->resume();
+}
+var_dump($fiber->getReturn());
+
+$proc->setParameter('', 'cb', 'text');
+var_dump($proc->transformToXml($doc));
+var_dump($proc->transformToDoc($doc)->textContent);
+
+?>
+--CLEAN--
+<?php
+@unlink(__DIR__ . '/xsltprocessor_nested_transform.xml');
+?>
+--EXPECT--
+Error: Cannot call XSLTProcessor::transformToXml() while a transformation is in progress
+Error: Cannot call XSLTProcessor::transformToDoc() while a transformation is in progress
+Error: Cannot call XSLTProcessor::transformToUri() while a transformation is in progress
+string(35) "<?xml version="1.0"?>
+[va][vb][vc]
+"
+Error: Cannot call XSLTProcessor::transformToXml() while a transformation is in progress
+string(35) "<?xml version="1.0"?>
+[fa][fb][fc]
+"
+string(35) "<?xml version="1.0"?>
+[1a][2b][3c]
+"
+string(12) "[1a][2b][3c]"
diff --git a/ext/xsl/xsltprocessor.c b/ext/xsl/xsltprocessor.c
index 3f6a7b6aa923..4737bf5643f7 100644
--- a/ext/xsl/xsltprocessor.c
+++ b/ext/xsl/xsltprocessor.c
@@ -323,6 +323,13 @@ static xmlDocPtr php_xsl_apply_stylesheet(zval *id, xsl_object *intern, xsltStyl
 		return NULL;
 	}
 
+	if (UNEXPECTED(intern->transform_depth > 0)) {
+		zend_string *name = get_active_function_or_method_name();
+		zend_throw_error(NULL, "Cannot call %s() while a transformation is in progress", ZSTR_VAL(name));
+		zend_string_release(name);
+		return NULL;
+	}
+
 	intern->transform_depth++;
 
 	if (intern->profiling) {