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CurlApplication · Haxx

CVE-2021-22947

MEDIUM · 5.9 CVSS v3.1 Published 2021-09-29
Fix available
A fix is available. Upgrade to 7.79.0 or later.
See remediation →
68/100
Remediation priority · Elevated
Public exploit Remotely reachable No privileges Zero-click Patch available

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 · unedited
When curl >= 7.20.0 and <= 7.78.0 connects to an IMAP or POP3 server to retrieve data using STARTTLS to upgrade to TLS security, the server can respond and send back multiple responses at once that curl caches. curl would then upgrade to TLS but not flush the in-queue of cached responses but instead continue using and trustingthe responses it got *before* the TLS handshake as if they were authenticated.Using this flaw, it allows a Man-In-The-Middle attacker to first inject the fake responses, then pass-through the TLS traffic from the legitimate server and trick curl into sending data back to the user thinking the attacker's injected data comes from the TLS-protected server.

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
How this class of weakness works · CWE-310

A legacy NVD category for weaknesses in how cryptography is chosen or used — weak algorithms, poor key handling, missing integrity protection. It is a broad bucket rather than a specific bug. The remedy depends on the detail, but it comes down to strong modern algorithms, disciplined key management, and validated implementations rather than home-grown schemes.

General guidance for the cryptographic issues class — the official description and references above are authoritative for this specific CVE. Want a bespoke review and a reviewed fix? Ask our team →

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
CurlApplication
Affected:>= 7.20.0, < 7.79.0
FedoraOperating system
Affected:= 33= 35
Debian LinuxOperating system
Affected:= 9.0= 10.0= 11.0
Cloud BackupApplication
Affected:all versions
Clustered Data OntapApplication
Affected:all versions
H300s FirmwareOperating system
Affected:all versions
H500s FirmwareOperating system
Affected:all versions
H700s FirmwareOperating system
Affected:all versions

CVSS 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
High
Privileges
None
User interaction
None
Scope
Unchanged
Confidentiality
None
Integrity
High
Availability
None

CVSS:3.1/AV:N/AC:H/PR:N/UI:N/S:U/C:N/I:H/A:N

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.

dbcve · scoped
Upgrade available Upgrade to 7.79.0 or later
Fixed in 7.79.0
Vendor patch cert-portal.siemens.com →
Recommended fix High confidence

curl 7.79.0 or later

  1. Check current curl version using 'curl --version'
  2. Upgrade curl to version 7.79.0 or later from the official curl website (https://curl.se/download.html) or your OS package manager
  3. Verify the upgrade was successful by running 'curl --version' and confirming the version is 7.79.0 or higher
  4. Test IMAP/POP3 connections using STARTTLS to confirm the fix works correctly
Caveat As with any software upgrade, test in a non-production environment first; minor behavioral changes are possible but unlikely for this security fix

Generated from the published advisory — verify against the referenced sources before acting.

We can perform the upgrade in your staging environment and verify nothing breaks — typical engagement from $1,600. Get the upgrade done

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References Go to the primary sourcePrimary sources — vendor advisories, patches and trackers. Where our summary and a reference disagree, the reference wins.

Primary sources

Practitioner notes

Contributed

Peer-ranked notes from engineers who’ve handled CVE-2021-22947 in production — separate from our analysis above.

No notes yet

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What this is

A place for practitioners to share what actually worked: a mitigation you’ve tested, a configuration change, a version- or environment-specific caveat, or a link to a verified patch. The most useful notes rise to the top as peers upvote them, so the signal stays high.

What belongs here
  • 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
  • No spam, self-promotion, credentials, or personal data