CVE-2026-55520 Protego: Exponential backtracking ReDoS in robots.txt URL wildcard matching
Attackers can cause denial of service in Protego by crafting robots.txt files that trigger exponential backtracking during URL matching.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- A user has to click or open something
- Fix
- Available
What is CVE-2026-55520?
CVE-2026-55520 is a high-severity denial of service vulnerability in scrapy protego, published on 28 Aug 2026. An attacker can exploit it over the internet without logging in, and only if a user opens a file or follows a link.
Official description
Protego is a pure-Python robots.txt parser with support for modern conventions. Prior to 0.6.2, protego._urlpattern._URLPattern._prepare_pattern_for_regex translates every asterisk in an Allow or Disallow directive into a lazy regular-expression wildcard, so a directive containing many asterisks creates exponential backtracking. After protego.Protego.parse processes a crafted robots.txt file, protego.Protego.can_fetch can spend an attacker-controlled period matching a near-miss URL and deny service to the crawler. The vulnerable path is src/protego/_urlpattern.py in the _URLPattern match logic. This issue is fixed in version 0.6.2.
Which versions are affected by CVE-2026-55520?
3 open source packages ship an affected version, including protego and Protego.
| Package | Affected | Fixed in |
|---|---|---|
protego PyPI | All versions | 0.6.2 |
protego PyPI | All versions | 0.6.2 |
Protego PyPI | All versions | 0.6.2 |
Linux distribution packages (1)
python-Protego openSUSE:Tumbleweed | All versions | 0.6.2-1.1 |
How do I fix CVE-2026-55520?
Upgrade protego to 0.6.2 or later.
Upgrade, then put limits on request size, rate and nesting depth so one client cannot exhaust the service. More about Denial of service.
Is CVE-2026-55520 being exploited?
There is no known exploitation of CVE-2026-55520 in the wild and no public exploit code.
Sources
Also known as DEBIAN-CVE-2026-55520, GHSA-wjmf-p669-5m5p, PYSEC-2026-3906, UBUNTU-CVE-2026-55520, openSUSE-SU-2026:11202-1.