Published:2007/04/19  Last Updated:2008/05/21

APOP password recovery vulnerability


POP3 is a protocol for receiving email from mail servers. APOP is an authentication mechanism used by the POP3 protocol.

It is reported that APOP passwords could be recovered by third parties.

In its successful attack, the attacker spoofs itself as the mail server, provides challenge strings to the client, and collects the responses from the client. The attacker should repeat this process for a certain period of time without alerting the user of the attack.

Products Affected

  • Mail clients with an APOP implementation
As this is a protocol issue, software fixes cannot solve the issue essentially. Encrypted communications such as POP over SSL are recommended. Moreover, users should use different passwords for different services or accounts to minimize the risk of their accounts to be compromised.



APOP passwords may be compromised. When the same password is used for other systems, those systems could be compromised as well.


Vendor Status


  1. IETF
    RFC1939:Post Office Protocol - Version 3
  2. FSE2007 rump session
    Practical Password Recovery on an MD5 Challenge-Response such as APOP (pdf)
  3. FSE2007 rump session
    Extended APOP Password Recovery Attack (pdf)

JPCERT/CC Addendum

Vulnerability Analysis by JPCERT/CC

Analyzed on 2007.04.19

Measures Conditions Severity
Access Required Routed - can be attacked over the Internet using packets
  • High
Authentication None - anonymous or no authentication (IP addresses do not count)
  • High
User Interaction Required Simple - the user must be convinced to take a standard action that does not feel harmful to most users, such as click on a link or view a file
  • Medium
Exploit Complexity High - large amount of expertise and/or luck required (BIOS expertise, guessing correctly in a large space)
  • Low

Description of each analysis measures


Yu Sasaki, Lei Wang, Kazuo Ohta, and Noboru Kunihiro of The University of Electro-Communication reported this vulnerability to IPA.
JPCERT/CC coordinated with the vendors under Information Security Early Warning Partnership.

Other Information

JPCERT Reports
CERT Advisory
CPNI Advisory
CVE CVE-2007-1558
JVN iPedia JVNDB-2007-000295

Update History