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TFTP sends more than buffer size

Project curl Security Advisory, August 9th 2017 - Permalink

VULNERABILITY

When doing a TFTP transfer and curl/libcurl is given a URL that contains a very long file name (longer than about 515 bytes), the file name is truncated to fit within the buffer boundaries, but the buffer size is still wrongly updated to use the untruncated length. This too large value is then used in the sendto() call, making curl attempt to send more data than what is actually put into the buffer. The sendto() function will then read beyond the end of the heap based buffer.

A malicious HTTP(S) server could redirect a vulnerable libcurl-using client to a crafted TFTP URL (if the client hasn't restricted which protocols it allows redirects to) and trick it to send private memory contents to a remote server over UDP. Limit curl's redirect protocols with --proto-redir and libcurl's with CURLOPT_REDIR_PROTOCOLS.

We are not aware of any exploit of this flaw.

INFO

This flaw also affects the curl command line tool.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2017-1000100 to this issue.

CWE-126: Buffer Over-read

AFFECTED VERSIONS

This bug has been present in curl since TFTP support was added, in September 2005 (commit 56d9624b566).

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

THE SOLUTION

The function now returns error if attempting to send a file name that is too long to fit in the TFTP packet.

A patch for CVE-2017-1000100 is available.

RECOMMENDATIONS

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

A - Upgrade curl and libcurl to version 7.55.0

B - Apply the patch to your version and rebuild

C - Disable TFTP or otherwise restrict TFTP transfers

TIME LINE

It was reported to the curl project on July 11, 2017. We contacted distros@openwall on August 1.

libcurl 7.55.0 was released on August 9 2017, coordinated with the publication of this advisory.

CREDITS

Reported by Even Rouault. Discovery: credit to OSS-Fuzz. Patch by Daniel Stenberg.

Thanks a lot!