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[SECURITY ADVISORY] curl: TFTP small blocksize heap buffer overflow

From: Daniel Stenberg via curl-users <>
Date: Wed, 11 Sep 2019 08:01:09 +0200 (CEST)

TFTP small blocksize heap buffer overflow

Project curl Security Advisory, September 11th 2019 -


libcurl contains a heap buffer overflow in the function
(`tftp_receive_packet()`) that receives data from a TFTP server. It can call
`recvfrom()` with the default size for the buffer rather than with the size
that was used to allocate it. Thus, the content that might overwrite the heap
memory is controlled by the server.

This flaw is only triggered if the TFTP server sends an OACK without the BLKSIZE
option, when a BLKSIZE smaller than 512 bytes was requested by the TFTP client.
OACK is a TFTP extension and is not used by all TFTP servers.

Users choosing a smaller block size than default should be rare as the primary
use case for changing the size is to make it larger.

It is rare for users to use TFTP across the Internet. It is most commonly used
within local networks. TFTP as a protocol is always inherently insecure.

This issue was introduced by the add of the TFTP BLKSIZE option handling. It
was previously incompletely fixed by an almost identical issue called

We are not aware of any exploit of this flaw.


This bug was introduced in January 2009 in [commit
The Common Vulnerabilities and Exposures (CVE) project has assigned the name
CVE-2019-5482 to this issue.
CWE-122: Heap-based Buffer Overflow
Severity: 5.2 (Medium)
- Affected versions: libcurl >= 7.19.4 to and including 7.65.3
- Not affected versions: libcurl < 7.19.4
libcurl is used by many applications, but not always advertised as such.
A [fix for CVE-2019-5482](
We suggest you take one of the following actions immediately, in order of
  A - Upgrade curl to version 7.66.0
  B - Apply the patch to your version and rebuild
  C - do not use TFTP with curl with smaller than the default BLKSIZE
The issue was reported to the curl project on August 29, 2019. The fix was done,
verified and communicated with the reporter on September 2, 2019.
We contacted distros_at_openwall on September 5.
This advisory was posted on September 11th 2019.
Reported and patched by Thomas Vegas.
Thanks a lot!
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Received on 2019-09-11