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CVE-2026-15308 exposes a critical resource exhaustion vulnerability within Python’s standard library, specifically in the incremental HTML parser implemented by the `html.parser.HTMLParser` class. The flaw arises from the parser’s handling of markup declarations—such as comments, processing instructions, and doctype definitions—that lack proper termination sequences. When the parser processes input containing repeated unterminated declarations, it enters an inefficient state where it continuously attempts to parse and reparse these malformed constructs. This behavior triggers a CPU-intensive loop that rapidly consumes available processor cycles, effectively leading to a denial‑of‑service (DoS) condition for any application that relies on this parser for untrusted data.
From a technical standpoint, the vulnerability is rooted in the parser’s state machine logic. Normally, when the parser encounters a markup declaration (e.g., `` to finalize the token. If the closing sequence is missing or malformed, the parser may stall in a waiting state, but the real danger lies in the incremental nature of the `feed()` method. An attacker can supply a stream of data that repeatedly triggers these unterminated declarations, forcing the parser to repeatedly allocate and re‑evaluate parsing contexts without making progress. This results in an exponential or linear blow‑up in CPU usage relative to the input size, especially when the declarations are crafted to be long or nested.
The attack vector is particularly dangerous because no authentication or special privileges are required; the vulnerability is remotely exploitable over the network. An attacker can send a carefully crafted HTML payload to any service that uses `HTMLParser` to parse user‑supplied content—common in web applications, web scrapers, email filters, and document converters. The impact is severe: a single malicious request can saturate a CPU core, and with multiple concurrent requests, the entire system can become unresponsive. The vulnerability has been assigned a CVSS base score of 7.5 (High) , reflecting the low attack complexity, network accessibility, and total loss of availability.
The Python Security Response Team has acknowledged the issue and backported a fix. The patch introduces additional validation logic to detect and safely abort parsing when unterminated declarations are encountered, preventing the CPU exhaustion loop. System administrators and developers are strongly advised to update to Python 3.15.0 or apply the relevant security patches to their distribution’s Python packages.
DailyCVE Form:
Platform: ……. CPython
Version: …….. < 3.15.0
Vulnerability :.. CPU DoS
Severity: ……. High (7.5)
date: ……….. 2026‑07‑09
Prediction: ….. Patch 2026‑07‑15
What Undercode Say:
Analytics & Detection
Monitor CPU spikes and parsing errors in logs. Use the following commands to detect abnormal `HTMLParser` activity:
Watch for high CPU usage by Python processes top -b -n 1 | grep python Check system load averages uptime Search for repeated "unterminated" or "invalid declaration" errors in application logs grep -i "unterminated|markup declaration" /var/log/myapp/.log
Vulnerability Test Script (Python)
import html.parser
import time
class ExploitParser(html.parser.HTMLParser):
def feed(self, data):
try:
super().feed(data)
except Exception as e:
pass
Malicious payload: repeated unterminated comments
payload = "<!--" 100000 + "\n"
parser = ExploitParser()
start = time.time()
parser.feed(payload)
end = time.time()
print(f"Parsing time: {end - start:.2f} seconds")
Expected output: excessive CPU time (e.g., > 60 sec) on vulnerable versions
Exploit:
An attacker can deliver the malicious payload via HTTP POST, file upload, or any input channel that feeds data into `HTMLParser.feed()`. For example:
curl -X POST http://victim:8080/parse \ -H "Content-Type: text/html" \ --data-binary "@exploit_payload.txt"
where `exploit_payload.txt` contains a large sequence of `. On vulnerable systems, this single request will consume 100% CPU for an extended period, degrading or halting service.
<h2 style="color: blue;">Protection:</h2>
- Upgrade Python to version 3.15.0 or later, which includes the official fix.
- Apply distribution patches (e.g., RHEL, Ubuntu, Buildroot) that backport the security fix.
- Implement input validation to reject or sanitize markup containing suspicious patterns (e.g., excessive ``).
- Set timeouts on parsing operations to abort long‑running tasks.
- Deploy rate limiting to restrict the number of parsing requests per client.
- Monitor CPU usage and automatically restart or scale affected services under heavy load.
Impact:
- Availability: Complete denial of service; CPU exhaustion renders the application and potentially the host system unresponsive.
- Confidentiality: None – the vulnerability does not leak data.
- Integrity: None – no data corruption or modification occurs.
- Attack Complexity: Low – simple payload can be generated with minimal effort.
- Privileges Required: None – the attack is unauthenticated.
- User Interaction: None – the attack can be automated.
- Scope: Unchanged – the vulnerability affects only the vulnerable component.
- Business Impact: High – services relying on HTML parsing (e.g., web mail, CMS, API gateways) become unavailable, leading to revenue loss and reputational damage.
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Sources:
Reported By: nvd.nist.gov
Extra Source Hub:
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