CVE-2019-6690: Improper Input Validation in python-gnupg
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We discovered a way to inject data through the passphrase property of
the gnupg.GPG.encrypt() and gnupg.GPG.decrypt() methods when symmetric
encryption is used.
The supplied passphrase is not validated for newlines, and the library
passes --passphrase-fd=0 to the gpg executable, which expects the
passphrase on the first line of stdin, and the ciphertext to be
decrypted or plaintext to be encrypted on subsequent lines.
By supplying a passphrase containing a newline an attacker can
control/modify the ciphertext/plaintext being decrypted/encrypted.
Vulnerable
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python-gnupg 0.4.3, and maybe earlier versions.
Mitigation
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Users should upgrade to 0.4.4.
Timeline
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2019-01-19: Vulnerability discovered during Insomni’hack teaser 2019
2019-01-20: PoC created
2019-01-22: Applied for CVE, vendor notified
2019-01-23: CVE-2019-6690 assigned
2019-01-23: Vendor responded, fix committed
2019-01-24: Vendor released 0.4.4
References
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https://pypi.org/project/python-gnupg/
https://cve.mitre.org/cgi-bin/cvename.cgi?name=CVE-2019-6690
https://blog.hackeriet.no/cve-2019-6690-python-gnupg-vulnerability/
https://github.com/hackeriet/CVE-2019-6690-python-gnupg-vulnerability
https://ctftime.org/task/7458
Proof of Concept
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Hypothetical application using successful decryption of data to authenticate a
user, and a way to exploit it is available here:
https://blog.hackeriet.no/assets/CVE-2019-6690-python-gnupg-vulnerability.tgz
https://blog.hackeriet.no/assets/CVE-2019-6690-python-gnupg-vulnerability.tgz.sig
Credits
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Vulnerability discovered by Alexander Kjäll and Stig Palmquist.
Thanks to @dewaelethom who wrote the CTF challenge.
