ComrakApplication · Comrak Project
CVE-2021-27671
Official description Straight from the sourceThe vendor's or NVD's own wording, published unedited. Authoritative, but often terse — it says what broke, rarely what to do.
NVD · uneditedAn issue was discovered in the comrak crate before 0.9.1 for Rust. XSS can occur because the protection mechanism for data: and javascript: URIs is case-sensitive, allowing (for example) Data: to be used in an attack.
Technical summary Written by usOur analysis, written from the advisory, the CVSS vector and the affected-version data. It adds context the advisory leaves out, and never invents facts that are not in the source.
dbcve analysis · high confidenceThe comrak Rust crate (a Markdown parser) before version 0.9.1 has an XSS vulnerability due to case-sensitive filtering of dangerous URI schemes (data:, javascript:). Attackers can bypass the protection by using mixed-case variants like 'Data:' or 'JAVASCRIPT:' in links, allowing script injection through crafted Markdown content.
Verify against the referenced sources before acting — the references below are authoritative for this CVE, this summary is not.
Affected products & versions What the vendor confirmedThe version ranges the vendor confirmed as vulnerable. If your version sits inside a range here, treat yourself as exposed until you have upgraded.
NVD · CPE data
ComrakApplication
< 0.9.1CVSS breakdown How the score is builtThe industry scoring standard. It rates how the flaw is reached, what it takes to exploit, and what an attacker gains — the score is derived from those, not the other way round.
From the vector- Attack vector
- Network
- Complexity
- Low
- Privileges
- None
- User interaction
- Required
- Scope
- Changed
- Confidentiality
- Low
- Integrity
- Low
- Availability
- None
CVSS:3.1/AV:N/AC:L/PR:N/UI:R/S:C/C:L/I:L/A:N
Am I affected? How to checkSteps we derive from the advisory and the affected-version data, so you can decide whether this CVE reaches your setup. They are a guide, not a scan — your own configuration is the authority.
dbcve checksWork through these to decide whether this CVE applies to you.
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Identify comrak versionRun `cargo show comrak` or inspect your Cargo.lock/Cargo.toml for the comrak dependency version numberAffected if The reported version is lower than 0.9.1
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Confirm Markdown link rendering is activeVerify your code calls functions that render Markdown links, such as `markdown_to_html` or similar output functions that process href attributesAffected if Your application processes and renders Markdown content containing hyperlinks
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Check URI scheme filtering implementationInspect your codebase or comrak configuration for any custom link filtering logic; examine how `allow_url` or similar options are setAffected if Filtering relies on case-sensitive matching of URI schemes (data:, javascript:) or has no filtering configured at all
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Audit existing rendered outputSearch any stored or cached rendered Markdown for patterns like `Data:`, `JAVASCRIPT:`, `JaVaScRiPt:` or other mixed-case variants in href attributesAffected if Rendered output contains hyperlinks using mixed-case URI schemes
You are affected if comrak version is below 0.9.1 and your application renders Markdown links from untrusted sources.
Generated from the published advisory. Verify against your own configuration.
Remediation Closing itWhat it takes to close this. Where a vendor fix exists we point at it; where none exists we say so plainly, and can build one. Effort estimates are scoped from the advisory, not from your codebase.
From vendor data0.9.1
Upgrade comrak to version 0.9.1 or later, which implements case-insensitive URI scheme validation. Review existing rendered Markdown for any injected scripts using mixed-case URI schemes.
Fix this in Comrak
Scoped from the published advisory
- Consultation1.0 h
- Implementation1.0 h
- Testing4.0 h
- Review / QA1.0 h
An estimate, not a bill — we confirm scope with you before any work starts. Need it this week? Rush from $1,856.
Scan for this in your stack
Free · runs locallyCheck whether your project pulls in CVE-2021-27671 — or any other known-vulnerable package — straight from your lock files. Free and open source; it runs locally and uploads nothing.
References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.
Primary sourcesPractitioner notes
ContributedPeer-ranked notes from engineers who’ve handled CVE-2021-27671 in production — separate from our analysis above.
The advisory tells you what broke. It rarely tells you what actually worked. If you’ve dealt with this one, that detail is what the next engineer is searching for.
- The version that genuinely resolved it — not the one the vendor claimed
- A config change or rule that shut the vector down
- A gotcha in the upgrade path that cost you an afternoon
No notes yet
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- Verified mitigations, workarounds, and config changes
- Version or environment caveats, and links to real fixes
- No weaponised exploit code, or anything meant to cause harm
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