REDHAT-BUG-2529697: Use After Free

SUMMARY A double-free / use-after-free exists in the SAX attributeDecl callback handler (pythonAttributeDecl in python/libxml.c). When parsing XML containing a DTD <!ATTLIST> declaration with enumerated attribute values, each value string is freed twice: PyListSetItem() steals the reference, and an explicit PyDECREF() then drops the refcount to zero and frees the object, leaving a dangling pointer in the list.

AFFECTED COMPONENT

libxml2 with Python bindings enabled (python3-libxml2 / libxml2-python)

The defect is long-standing; the affected code in pythonAttributeDecl has been unchanged for many years.

Reproduced on upstream commit 4b35628e97472eaf23d8a841d2f711f7c2f96255 (2026-02-24).

IMPACT

Denial of service: 100% reproducible crash (SIGSEGV) in any Python application that uses the libxml2 SAX bindings (libxml2.createPushParser), registers an attributeDecl handler, and parses untrusted XML with a DTD <!ATTLIST> containing enumerated values. Verified 10/10 in isolated processes.

The use-after-free is also potentially exploitable for code execution: I was able to demonstrate hijacking the freed object's tpdealloc function pointer in-process. Full remote code execution would require additional heap grooming and is not demonstrated. I'd defer to your team on final severity scoring; I'd characterize the reliably demonstrated impact as DoS, with code execution as a credible but conditional escalation.

ROOT CAUSE (python/libxml.c, pythonAttributeDecl)

for (node = tree; node != NULL; node = node->next) { newName = PYIMPORTSTRING((char ) node->name); PyListSetItem(nameList, count, newName); / steals reference / PyDECREF(newName); / double-free / count++; }

PyListSetItem() does not increment the refcount, so the subsequent PyDECREF() over-decrements. Because of CPython pymalloc free-list reuse, list entries can end up pointing at the same freed address; cleanup after the SAX callback then decrefs dangling pointers and corrupts allocator state.

PROOF OF CONCEPT (minimal DoS)

import libxml2 class Handler: def attributeDecl(self, args): pass def startElement(self, a): pass def endElement(self, a): pass def characters(self, a): pass xml = b'''<?xml version="1.0"?> <!DOCTYPE r [ <!ELEMENT r EMPTY> <!ATTLIST r a (xx|yy|zz|ww|qq) "xx"> ]> <r a="xx"/>''' h = Handler() c = libxml2.createPushParser(h, "", 0, "t") c.parseChunk(xml, len(xml), 1) # SIGSEGV

UPSTREAM FIX (already merged and closed)

Fix: remove the erroneous PyDECREF(newName) since PyListSetItem already takes ownership.

Merge request: https://gitlab.gnome.org/GNOME/libxml2/-/mergerequests/397 ("python: Do not decref string after adding to the list")

Fix commit: 046931e6

Issue report: https://gitlab.gnome.org/GNOME/libxml2/-/workitems/1076

Upstream maintainer Nick Wellnhofer confirmed it appears to be a security issue and asked that a CVE be requested; the maintainer who merged the fix indicated CVE assignment is not handled by the project itself.

SUGGESTED CLASSIFICATION

CWE-415 (Double Free), leading to CWE-416 (Use After Free)

ENVIRONMENT

OS: Ubuntu 22.04 x8664; Python 3.10.12; GCC 11.4.0; libxml2 built from source with -fsanitize=address.

I'm happy to provide the full ASan trace or the code-execution PoC on request. Please let me know if you need anything else to proceed.

Thank you, Adnan Jakati!https://mailtrack.io/trace/mail/773e02eeb348f92b41f2a5d93b4c46af0007ab1f.png?u=12519192!