curl / Docs / Security Problems / FTP-KRB double-free

FTP-KRB double-free

Project curl Security Advisory, September 11th 2019 - Permalink

VULNERABILITY

libcurl can be told to use kerberos over FTP to a server, as set with the CURLOPT_KRBLEVEL option.

During such kerberos FTP data transfer, the server sends data to curl in blocks with the 32 bit size of each block first and then that amount of data immediately following.

A malicious or just broken server can claim to send a very large block and if by doing that it makes curl's subsequent call to realloc() to fail, curl would then misbehave in the exit path and double-free the memory.

In practical terms, an up to 4 GB memory area may very well be fine to allocate on a modern 64 bit system but on 32 bit systems it will fail.

Kerberos FTP is a rarely used protocol with curl. Also, Kerberos authentication is usually only attempted and used with servers that the client has a previous association with.

We are not aware of any exploit of this flaw.

INFO

This bug was introduced in November 2016 in commit 0649433da53c7165f839e2.

The Common Vulnerabilities and Exposures (CVE) project has assigned the name CVE-2019-5481 to this issue.

CWE-415: Double Free

Severity: 6.3 (Medium)

AFFECTED VERSIONS

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

THE SOLUTION

A fix for CVE-2019-5481

RECOMMENDATIONS

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

A - Upgrade curl to version 7.66.0

B - Apply the patch to your version and rebuild

C - do not use CURLOPT_KRBLEVEL

TIMELINE

The issue was reported to the curl project on September 3, 2019. The fix was done, verified and communicated with the reporter on September 3, 2019.

We contacted distros@openwall on September 5.

This advisory was posted on September 11th 2019.

CREDITS

Reported by Thomas Vegas. Patch by Daniel Stenberg.

Thanks a lot!