How severe is CVE-2026-55520?

No CVSS score has been assigned to CVE-2026-55520 yet. Review the advisory details and affected package list to assess your exposure.

Which packages are affected by CVE-2026-55520?

CVE-2026-55520 affects the following packages: protego (PyPI). Ecosystems affected: PyPI.

How do I fix CVE-2026-55520?

Update protego to 0.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55520 is resolved across your whole dependency graph.

How do I detect CVE-2026-55520 in my PyPI dependencies?

Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for protego. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.

How do I mitigate CVE-2026-55520 if there is no patch (or I can't update yet)?

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 does O3 Security protect against CVE-2026-55520?

O3 pinpoints whether CVE-2026-55520 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Is CVE-2026-55520 actively exploited in the wild?

No public exploit code has been indexed for CVE-2026-55520 yet. This does not mean the vulnerability cannot be exploited — absence of public exploits does not imply safety. Apply the recommended fix and use O3 Security to monitor your exposure.

What type of vulnerability is CVE-2026-55520?

CVE-2026-55520 is classified as Uncontrolled Resource Consumption (CWE-400), CWE-1333 (CWE-1333). These weakness types describe the underlying flaw category, which helps determine the potential impact and the right class of mitigation.

When was CVE-2026-55520 published, and has it been updated?

CVE-2026-55520 was published on August 28, 2026 and was last updated on August 28, 2026. Advisory data evolves as severity scores, affected ranges, and exploit intelligence are revised — always check the latest version of the advisory before acting.

🐍 PyPI

Not in CISA KEV

CVE-2026-55520

Fix: scrapy/protego@7859401

CVE-2026-55520 is a Uncontrolled Resource Consumption vulnerability in protego. O3 Security confirms whether CVE-2026-55520 is actually reachable in your code before you act, and blocks exploitation at runtime until you patch.

Protego has exponential backtracking ReDoS in robots.txt URL wildcard matching

Published

Aug 28, 2026

Updated

Aug 28, 2026

Affected

1 pkg

Patched

1 / 1

Exploits

None indexed

Exploitation data as of Aug 28, 2026 · OSV.dev, NVD, FIRST.org (EPSS)

Real-World Exposure

1 pkg affected

🐍protego

Real-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

Problem description

Protego constructs regular expressions to match URLs against robots.txt Allow: and Disallow: directives, see protego._urlpattern._URLPattern._prepare_pattern_for_regex(). Every * in the directive value is translated into a lazy .*? regex piece, thus a specially crafted directive value with many asterisks may produce a regex that freezes the parser due to exponential backtracking.

Impact

Parsing a specially crafted robots.txt with protego.Protego.parse() and then trying to match an URL with protego.Protego.can_fetch() results in the latter call not returning for a period dependent on the length of the URL.

Proof of concept

from protego import Protego

robotstxt = f"""
User-agent: *
Disallow: /{"*1" * 12}*Z
"""
rp = Protego.parse(robotstxt)
url = "/" + "1" * 60
rp.can_fetch(url, "mybot")  # freezes

Affected Packages

1 total 1 fixed

Ecosystem	Package	Vulnerable range	Fix
🐍PyPI	`protego`	all versions	0.6.2

Detection & mitigation playbook

Open-source dependency

Detect
Scan your dependency tree (package-lock.json, pnpm-lock.yaml, requirements.txt, go.sum, etc.) for protego. O3's reachability analysis confirms whether the vulnerable code path is actually invoked in your application, so you act on real exposure instead of every transitive match.
Fix
Update protego to 0.6.2 or later, then make sure no transitive (indirect) dependency still pins the vulnerable range — O3 confirms CVE-2026-55520 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 pinpoints whether CVE-2026-55520 is reachable in your code and exactly where to fix it, then blocks exploitation in production at runtime until the patched version is deployed.

Tailored to CVE-2026-55520. Runtime protection reduces exposure until a permanent patch is applied and verified — it complements patching, it doesn't replace it.

Frequently Asked Questions

### Problem description Protego constructs regular expressions to match URLs against `robots.txt` `Allow:` and `Disallow:` directives, see `protego._urlpattern._URLPattern._prepare_pattern_for_regex()`. Every `*` in the directive value is translated into a lazy `.*?` regex piece, thus a specially crafted directive value with many asterisks may produce a regex that freezes the parser due to exponential backtracking. ### Impact Parsing a specially crafted `robots.txt` with `protego.Protego.parse()` and then trying to match an URL with `protego.Protego.can_fetch()` results in the latter call n

O3 Security · Impact-Aware SCA

Is CVE-2026-55520 in your dependencies?

O3 detects CVE-2026-55520 across PyPI dependencies and uses function-level reachability to confirm whether the vulnerable code path is actually reachable — not just present. No false positives.

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