CVE-2026-78253 Overview
CVE-2026-78253 is an uncontrolled recursion vulnerability [CWE-674] in the QXmlStreamReader::readElementText() function within the Qt framework maintained by Qt Group. A remote attacker can deliver a crafted XML document that triggers unbounded recursion during element text parsing. The recursion exhausts the thread stack and crashes the host application, producing a denial-of-service (DoS) condition. Exploitation requires the target application to parse attacker-influenced XML input. No code execution or data disclosure results from the flaw.
Critical Impact
A crafted XML document parsed by an affected Qt application causes stack exhaustion and application termination, disrupting availability of any service that consumes untrusted XML through QXmlStreamReader.
Affected Products
- Qt Group Qt framework (qtbase) — versions incorporating the vulnerable QXmlStreamReader::readElementText() implementation
- Downstream applications and services that link against the affected QtCore XML parsing components
- Cross-platform Qt deployments on Windows, Linux, and macOS that expose XML parsing to untrusted input
Discovery Timeline
- 2026-09-23 - CVE-2026-78253 published to NVD
- 2026-09-23 - Last updated in NVD database
Technical Details for CVE-2026-78253
Vulnerability Analysis
The defect lives in QXmlStreamReader::readElementText(), a convenience API that reads all text inside an XML element, including text from nested child elements. When the caller selects a mode that recurses into child elements, the implementation invokes itself for each descendant without enforcing a depth limit. A malicious XML document with deeply nested elements forces the parser into recursive calls proportional to the nesting depth.
Each recursive frame consumes native thread stack space. Once nesting exceeds the available stack, the process receives a stack-overflow signal and terminates. Because Qt is embedded in a wide range of desktop, mobile, embedded, and server-side applications, any code path that hands untrusted XML to readElementText() inherits this crash primitive. The vulnerability is limited to availability impact and does not corrupt memory in an exploitable manner.
Root Cause
The root cause is missing recursion-depth validation during element traversal. The parser recurses on every nested child element without tracking the current depth against a configurable maximum. This maps directly to [CWE-674: Uncontrolled Recursion], where user-controlled input drives the recursion count.
Attack Vector
An attacker crafts an XML document containing thousands of nested elements and delivers it through any channel the target application accepts, such as a file upload, network message, configuration import, or clipboard paste. When the application calls QXmlStreamReader::readElementText() on the document, the recursion depth tracks the XML nesting depth and exhausts the stack. Exploitation requires user or application interaction to load the document but no authentication.
The upstream fix is tracked in the Qt Project Code Review, which introduces bounded traversal in the affected parsing path.
Detection Methods for CVE-2026-78253
Indicators of Compromise
- Unexpected termination of Qt-based applications immediately after ingesting XML input, with crash reports referencing stack overflow in QXmlStreamReader frames.
- Repeated crash-dump generation on hosts that process XML from network or user-supplied sources, correlated with inbound XML payloads.
- Web or application logs showing XML uploads containing pathological element nesting depth prior to a service restart.
Detection Strategies
- Inspect XML payloads at ingress points for element nesting depth beyond application requirements and flag documents exceeding a defined threshold.
- Enable operating-system crash telemetry (Windows Error Reporting, systemd-coredump, macOS CrashReporter) and alert on faults inside Qt XML parsing symbols.
- Instrument Qt applications with structured logging around XML parsing entry points to record document size, nesting depth, and outcome.
Monitoring Recommendations
- Forward application crash events and web application firewall logs to a centralized analytics platform for correlation with XML input sources.
- Track process restart frequency for services that expose XML endpoints and investigate abnormal spikes.
- Monitor egress and ingress traffic for repeated malformed XML from the same source, which may indicate probing.
How to Mitigate CVE-2026-78253
Immediate Actions Required
- Identify applications and services that link against Qt and parse untrusted XML through QXmlStreamReader.
- Deploy input validation at trust boundaries to reject XML documents exceeding a reasonable element nesting depth.
- Restrict XML ingestion endpoints to authenticated users where feasible and rate-limit repeated failures.
Patch Information
The fix is available in the upstream qtbase repository through the Qt Project Code Review. Rebuild and redistribute affected applications against a Qt release that incorporates the patch. Downstream Linux distributions and application vendors should track their respective Qt package updates and apply them across managed endpoints.
Workarounds
- Pre-validate XML input with a lightweight parser that enforces a maximum nesting depth before handing the document to QXmlStreamReader.
- Wrap XML parsing calls in a worker thread with a constrained stack size and a supervisor that restarts on failure, containing the impact of a crash.
- Where the API contract allows, avoid the recursive form of readElementText() and iterate element-by-element to control traversal depth explicitly.
# Example: reject XML documents with excessive nesting at the WAF or proxy layer
# Pseudocode rule — adapt to your inspection engine
if xml_max_depth(request.body) > 100:
deny(request, reason="xml-nesting-depth-exceeded")
Disclaimer: This content was generated using AI. While we strive for accuracy, please verify critical information with official sources.
