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Category:   Application (Generic)  >   curl Vendors:
libcurl FTP PWD Response Processing Buffer Overread Lets Remote Users Deny Service
SecurityTracker Alert ID:  1039509
SecurityTracker URL:
CVE Reference:   CVE-2017-1000254   (Links to External Site)
Date:  Oct 5 2017
Impact:   Denial of service via network
Fix Available:  Yes  Vendor Confirmed:  Yes  Exploit Included:  Yes  
Version(s): 7.7 - 7.55.1
Description:   A vulnerability was reported in libcurl. A remote user can cause denial of service conditions on the target system.

A remote server can return a specially crafted response to a PWD command issued by the target connected client library to trigger a parsing error and buffer overread and cause libcurl to crash.

Max Dymond reported this vulnerability.

Impact:   A remote server can cause the target connected client library to crash.
Solution:   The vendor has issued a fix (7.56.0).

The vendor advisory is available at:

Vendor URL: (Links to External Site)
Cause:   Boundary error
Underlying OS:  Linux (Any), UNIX (Any), Windows (Any)

Message History:   This archive entry has one or more follow-up message(s) listed below.
Oct 11 2017 (Ubuntu Issues Fix) libcurl FTP PWD Response Processing Buffer Overread Lets Remote Users Deny Service
Ubuntu has issued a fix for Ubuntu Linux 14.04 LTS, 16.04 LTS, and 17.04.
Oct 24 2017 (Ubuntu Issues Fix) libcurl FTP PWD Response Processing Buffer Overread Lets Remote Users Deny Service
Ubuntu has issued a fix for Ubuntu Linux 12.04 ESM.

 Source Message Contents

Subject:  [oss-security] [SECURITY ADVISORY] curl: FTP PWD response parser out of bounds read

FTP PWD response parser out of bounds read

Project curl Security Advisory, October 4th 2017 -


libcurl may read outside of a heap allocated buffer when doing FTP.

When libcurl connects to an FTP server and successfully logs in (anonymous or
not), it asks the server for the current directory with the `PWD` command. The
server then responds with a 257 response containing the path, inside double
quotes. The returned path name is then kept by libcurl for subsequent uses.

Due to a flaw in the string parser for this directory name, a directory name
passed like this but without a closing double quote would lead to libcurl not
adding a trailing NUL byte to the buffer holding the name. When libcurl would
then later access the string, it could read beyond the allocated heap buffer
and crash or wrongly access data beyond the buffer, thinking it was part of
the path.

A malicious server could abuse this fact and effectively prevent libcurl-based
clients to work with it - the PWD command is always issued on new FTP
connections and the mistake has a high chance of causing a segfault.

The simple fact that this issue has remained undiscovered for this long could
suggest that malformed PWD responses are rare in benign servers.

We are not aware of any exploit of this flaw.


This bug was introduced in commit
[415d2e7cb7](, March 2005.

In libcurl version 7.56.0, the parser always zero terminates the string but
also rejects it if not terminated properly with a final double quote.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2017-1000254 to this issue.


- Affected versions: libcurl 7.7 to and including 7.55.1
- Not affected versions: libcurl < 7.7 and >= 7.56.0

curl is used by many applications, but not always advertised as such.


A [patch for CVE-2017-1000254]( is


We suggest you take one of the following actions immediately, in order of

  A - Upgrade curl to version 7.56.0

  B - Apply the patch to your version and rebuild

  C - Switch off FTP in `CURLOPT_PROTOCOLS`


It was reported to the curl project on September 24, 2017.  We contacted
distros@openwall on September 25.

curl 7.56.0 was released on October 4 2017, coordinated with the publication
of this advisory.


Reported by Max Dymond. Patch by Daniel Stenberg.

Thanks a lot!



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