bitlbee-libpurple: Use after free when expiring file transfer requests

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Description

Pending file transfer requests expire after 120 seconds, which may result in use after free if the corresponding account is disconnected. A malicious remote server could force this disconnection.

Impact

This results in denial of service (remote crash of the BitlBee instance), or remote code execution (theoretically).

For BitlBee servers configured in ForkDaemon mode (default) or inetd mode, the crash is limited to one user connection, who may just reconnect.

• Access Vector: Network
• Access Complexity: High
• Authentication: None
• Confidentiality Impact: None
• Integrity Impact: None
• Availability Impact: Partial
• Exploitability: Functional Exploit Exists
• Remediation Level: Official Fix
• Report Confidence: Confirmed
• Target Distribution: Medium
• CVSS v2 score: 1.6

Affected versions

bitlbee-libpurple 3.4.2 or older

Unaffected versions

bitlbee (non-libpurple builds), any version

bitlbee-libpurple 3.5

Resolution

• Upgrade to 3.5 (released 2017-01-08)

• For 3.4.2 see the attached

​0001-purple-fix-file-transfer-memory-management-3.4.2.patch

• For 3.4.1 and 3.4 see the attached

​0001-purple-fix-file-transfer-memory-management-3.4-3.4.1.patch

• For earlier versions upgrading is strongly recommended because of

the amount of accumulated bugfixes, but the following line may be removed from protocols/purple/purple.c to prevent any processing of incoming file transfers:

purple_xfers_set_ui_ops(&bee_xfer_uiops);

Discussion

This affects any libpurple protocol when used through BitlBee. It does not affect other libpurple-based clients such as pidgin.

This is a very visible issue - all file transfer request attempts and all disconnections will be logged in the control channel and visible by the targeted user. File transfer requests look like this:

<@root> [account] - File transfer request from [username] for
[filename] (0 kb).
<@root> Accept the file transfer if you'd like the file. If you
don't, issue the 'transfer reject' command.

Cancelling the file transfer request using the "transfer reject" command before the disconnection happens can prevent this. However, using that command after the account is disconnected will result in an immediate crash.

References

Original bugfix commit:

​https://github.com/bitlbee/bitlbee/commit/ea902752503fc5b356d6513911081ec932d804f2

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Made public.

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Added attachment links, fixed code block formatting.