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CVE-2026-7210 is a medium-severity vulnerability affecting CPython’s XML parsing modules, `xml.parsers.expat` and xml.etree.ElementTree. The core issue lies in the insufficient entropy used for the hash-flooding protection mechanism within the Expat library. In computing, a hash table is a data structure that stores key-value pairs. For a hash table to be efficient, it uses a hash function to convert a key (like an XML attribute or tag name) into an index. If many different keys produce the same hash, collisions occur, degrading performance from O(1) to O(n), which can severely impact CPU usage.
Expat includes a protection mechanism to mitigate hash flooding by randomizing its hash function. This randomness is crucial; if an attacker can predict the hash values, they can craft a document where every single key collides. This vulnerability arises because the entropy (randomness) source used to seed the hash function was not sufficiently robust, making it possible for an attacker to calculate or predict the hash values. Consequently, a malicious actor can send a specially crafted XML document designed to trigger a worst-case collision scenario. As the parser processes this document, the CPU consumption spikes dramatically, leading to a denial of service. The attack is particularly dangerous in web applications or APIs that accept XML input, as an attacker could repeatedly send such payloads to exhaust resources across an entire system.
The root cause is cataloged as CWE-331 (Insufficient Entropy). The fix is not straightforward; it requires two separate actions to be fully mitigated. First, the system’s `libexpat` library must be updated to version 2.8.0 or later, which contains a properly seeded hash randomization. Second, a specific patch must be applied to the CPython source code to ensure the interpreter correctly interfaces with the updated library.
DailyCVE Form:
Platform: CPython
Version: up to 3.14.x
Vulnerability : Hash flooding DoS
Severity: Medium (CVSS 6.3)
date: 2026-05-11
Prediction: 2026-06-15 (14 days after publication)
What Undercode Say: Analytics
To determine if a system is vulnerable, an administrator can check the versions of CPython and the system’s libexpat.
Check CPython version python3 --version Check libexpat version on Debian/Ubuntu dpkg -l | grep libexpat Check libexpat version on RHEL/CentOS/Fedora rpm -qa | grep expat
The following Python script demonstrates the entropy weakness by attempting to parse a malicious XML payload, which would lead to excessive CPU consumption:
import xml.etree.ElementTree as ET
import sys
def parse_billion_laughs(xml_content):
try:
root = ET.fromstring(xml_content)
except ET.ParseError as e:
print(f"Parse error: {e}")
Malicious payload with massive hash collisions
(Theoretical example; exact payload not public)
malicious_xml = """<?xml version="1.0"?>
<!DOCTYPE lolz [
<!ENTITY lol "lol">
<!ENTITY lol1 "&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;&lol;">
]>
<lolz>&lol1;</lolz>"""
parse_billion_laughs(malicious_xml)
How Exploit:
While no public exploit code has been released for CVE-2026-7210, the attack vector is understood. An attacker would craft an XML document with tens of thousands of elements or attributes with names specifically chosen to cause maximum hash collisions. For example, generating a large number of XML tags with keys designed to produce the same hash value.
Theoretical exploit: simulate multiple requests to exhaust CPU
for i in {1..100}; do
curl -X POST https://victim.com/api/parse \
-H "Content-Type: application/xml" \
--data-binary @evil_payload.xml &
done
Protection:
Immediate Mitigation: The only complete protection is to apply the fix. This requires:
1. Updating the system’s `libexpat` library to version 2.8.0 or higher.
2. Applying the CPython source code patch (e.g., by updating to CPython 3.15.0 or a backported version).
Configuration Hardening: While not a complete fix, applications can limit the size of XML documents accepted and set strict timeouts for XML processing.
Input Validation: Use an allow-list approach for XML structures and reject documents with a very high number of elements or attributes.
Impact:
Successful exploitation leads to a Denial of Service (DoS). The CPU usage of the target process will spike to 100% while processing the malicious document, causing the application to become unresponsive. This can be used by an attacker to disrupt service availability, potentially leading to significant downtime for any application that parses untrusted XML data. The impact is limited to availability; confidentiality and integrity are not directly affected.
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Sources:
Reported By: nvd.nist.gov
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