Summary
Protego has exponential backtracking ReDoS in robots.txt URL wildcard matching
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.
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
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.
Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service. Typical impact: denial of service.
Affected versions
Security releases
Kodem intelligence
Severity tells you how bad this could be in the worst case. It does not tell you whether you are exposed. Exploitability and impact are functions of runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A vulnerable package can sit in your dependency tree and never run.
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Frequently Asked Questions
- What is CVE-2026-55520? CVE-2026-55520 is a high-severity uncontrolled resource consumption vulnerability in Protego (pip), affecting versions <= 0.6.1. It is fixed in 0.6.2. Crafted input forces the application to consume excessive CPU, memory, or other resources, degrading or denying service.
- Which versions of Protego are affected by CVE-2026-55520? Protego (pip) versions <= 0.6.1 is affected.
- Is there a fix for CVE-2026-55520? Yes. CVE-2026-55520 is fixed in 0.6.2. Upgrade to this version or later.
- Is CVE-2026-55520 exploitable, and should I be worried? Whether CVE-2026-55520 is exploitable in your environment depends on whether the vulnerable code is present and reachable. A CVSS score is a worst-case rating; it does not account for your specific deployment, configuration, or usage patterns. Kodem, an Intelligent Application Security platform, uses runtime intelligence to show which vulnerabilities actually execute in production, so you can focus on the ones that represent real risk. Get a demo
- What actually determines whether CVE-2026-55520 is exploitable, and how bad it is? Exploitability and impact are not fixed properties of a CVE. They depend on runtime truth: whether the vulnerable code is present, reachable, and actually executes in your application. A high CVSS score on a dependency that never runs is not the same as real risk. Kodem, an Intelligent Application Security platform, uses runtime intelligence to reveal which vulnerabilities actually execute in production, so teams prioritize the ones that genuinely matter.
- How do I fix CVE-2026-55520? Upgrade
Protegoto 0.6.2 or later.