CVE-2026-39376 — fastfeedparser
HIGHCVE-2026-39376 is a high-severity (CVSS 7.5) CWE-674 vulnerability in fastfeedparser. A fix is available for fastfeedparser — see the affected versions and patch details below.
FastFeedParser has an infinite redirect loop DoS via meta-refresh chain
Exploitation Status
Proof-of-concept exploit code exists
- CISA’s SSVC triage found public proof-of-concept exploit code for this CVE, though no confirmed active exploitation.
- CISA assesses this as automatable — exploitation doesn’t require manual, per-target effort, which raises the odds of mass scanning and opportunistic attacks.
Exploitation and automatability from CISA’s SSVC triage for CVE-2026-39376.
EPSS Exploitation Probability
EPSS (Exploit Prediction Scoring System) is a daily probability model maintained by FIRST.org. It estimates the likelihood a CVE will be exploited in production environments within the next 30 days, derived from real-world threat intelligence signals.
How urgent is this, really
CVE-2026-39376 plotted by exploitation likelihood (EPSS) against impact (CVSS). The shaded corner — EPSS 50%+ and CVSS 7.0+ — is where this CVE doesn't sit, though severity or exploitability alone can still warrant action.
Where this sits among everything scored
Of 378,156 CVEs with a current EPSS score, this one falls in the < 10% band (highlighted). Real counts from FIRST.org, not a sample — log-scaled since the landscape is heavily right-skewed.
Real-World Exposure
fastfeedparserReal-time download stats are indexed for npm and PyPI packages. This vulnerability affects PyPI packages — download data is not available via public APIs for these ecosystems.
Description
Summary
When parse() fetches a URL that returns an HTML page containing a <meta http-equiv="refresh"> tag, it recursively calls itself with the redirect URL — with no depth limit, no visited-URL deduplication, and no redirect count cap. An attacker-controlled server that returns an infinite chain of HTML meta-refresh responses causes unbounded recursion, exhausting the Python call stack and crashing the process. This vulnerability can also be chained with the companion SSRF issue to reach internal network targets after bypassing the initial URL check.
Details
parse() catches ValueError on XML parse failure, extracts a meta-refresh URL from the HTML response via _extract_meta_refresh_url(), and tail-calls itself with that URL. The recursive call is unconditional — there is no maximum redirect depth, no set of already-visited URLs, and no guard against self-referential or looping redirects.
fastfeedparser/main.py — parse() (recursive sink):
def parse(source: str | bytes, ...) -> FastFeedParserDict:
is_url = isinstance(source, str) and source.startswith(("http://", "https://"))
if is_url:
content = _fetch_url_content(source)
try:
return _parse_content(content, ...)
except ValueError as e:
...
redirect_url = _extract_meta_refresh_url(content, source)
if redirect_url is None:
raise
return parse(redirect_url, ...) # ← unconditional recursion, no depth limit
_extract_meta_refresh_url() uses urljoin(base_url, match.group(1)) so relative, protocol-relative (//host/path), and absolute URLs in the content= attribute are all followed.
PoC
No live server required. The following monkeypatches _fetch_url_content to return an infinite HTML meta-refresh chain and confirms unbounded recursion:
import fastfeedparser.main as m
call_count = 0
_orig = m._fetch_url_content
def mock_fetch(url):
global call_count
call_count += 1
if call_count > 10:
raise RuntimeError(f"Stopped at call {call_count}")
next_url = f"http://169.254.169.254/step{call_count}/"
return f"""<html><head>
<meta http-equiv="refresh" content="0; url={next_url}">
</head><body>not a feed</body></html>""".encode()
m._fetch_url_content = mock_fetch
try:
m.parse("http://attacker.com/loop")
except RuntimeError as e:
print(f"CONFIRMED infinite loop: {e}")
finally:
m._fetch_url_content = _orig
print(f"Total fetches before stop: {call_count}")
# Output:
# CONFIRMED infinite loop: Stopped at call 11
# Total fetches before stop: 11
Each recursive call performs a real HTTP request (30 s timeout), HTML parsing, and a Python stack frame allocation. With Python's default recursion limit of 1000 and a 30 s per-request timeout, a single attacker request can hold a server thread busy for up to ~8 hours before a RecursionError is raised.
SSRF chain variant: The first response can be legitimate HTML redirecting to an internal address (http://192.168.1.1/), letting the redirect loop also serve as an SSRF bypass for targets that would otherwise be blocked by application-level URL validation applied only to the initial URL.
Impact
This is a denial-of-service vulnerability with a secondary SSRF-chaining impact. Any application that accepts user-supplied feed URLs and calls fastfeedparser.parse() is affected — including RSS aggregators, feed preview services, and "subscribe by URL" features. An attacker with no authentication can:
- Hold a server worker thread indefinitely (one request per attacker connection)
- Crash the worker process via
RecursionErrorafter ~1000 redirects - Use the redirect chain to pivot SSRF requests to internal network targets
Affected Packages
| Ecosystem | Package | Vulnerable range | Fix |
|---|---|---|---|
| 🐍PyPI | fastfeedparser | all versions | 0.5.10pip install --upgrade 'fastfeedparser==0.5.10' |
Detection & mitigation playbook
Open-source dependencyDetect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for fastfeedparser, including transitive dependencies — a direct dependency you never call can still pull in a vulnerable version.
Fix
Update fastfeedparser to 0.5.10 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-39376 is resolved across your whole dependency graph.
Workarounds
If you can't upgrade right away: gate or disable the affected feature, validate untrusted input at the boundary, and avoid passing attacker-controlled data into the vulnerable path. O3's runtime protection blocks exploitation in production as an interim safeguard until the upgrade lands.
How O3 protects you
O3 Security's impact-aware SCA analyses which vulnerable code paths your application actually calls, so a match like CVE-2026-39376 can be triaged on real exposure rather than presence alone.
Tailored to CVE-2026-39376. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.
Frequently Asked Questions
Is CVE-2026-39376 in your dependencies?
O3 Security finds CVE-2026-39376 across PyPI dependencies, including transitive ones, and its impact-aware SCA ranks findings by whether your code actually calls the vulnerable path.