The Tor Project is preparing a fix for a bug that has been abused for the past years to launch distributed denial of service (DDoS) attacks against dark web (.onion) websites.
Barring any unforeseen problems, the fix is scheduled for the upcoming Tor protocol 0.4.2 release, according to a bug report seen by ZDNet.
How the DoS bug works
In information security (infosec) terms, the bug is a "denial of service" (DoS) issue that crashes the Onion service running on a web server hosting a .onion website.
More specifically, in a simplified explanation of what happens during this bug, an attacker can initiate thousands of connections to a targeted website hosted on the dark web, but leave the connections hanging.
For each connection, the remote Onion service must negotiate a complex circuit through the Tor network that secures the connection between the remote user and its server. This process is CPU intensive, and with enough connections, the server processor is maxed out at 100% and can't accept new connections.
This is an old bug that has been known to Tor developers for years but has not been fixed because of a lack of human power, but also because there's no simple and straightforward way -- because the bug also exploits the same process which needs to happen to establish a legitimate user's connection.
There is no viable way to identify if any incoming connection requests are from an attacker or a legitimate user until the connection is established, at which point, it is already too late.
DDoS attacks on dark web portals
The vulnerability has plagued the operators of dark web portals for years. Initially, legitimate sites reported being attacked, but in recent months, the primary target of DDoS attacks exploiting this bug have been dark web marketplaces that sell illegal products, such as drugs, weapons, malware, and hacked user data.
In March 2019, the Dream Market, the largest dark web marketplace for illegal funds at the time, announced it was shutting down after the site had been under a barrage of DDoS attacks for months.
"The Tor network is not suitable for hidden services due to flaws in the network which allow denial of service attacks," administrators of the Libertas Market wrote on their now-closed Tor portal.
The efforts to migrate to I2P failed, and the Libertas Market eventually shut down for good.
Tor DDoS tool readily available on GitHub
In the meantime, DDoS attacks on dark web portals have continued unabated, because there's nothing that Onion site operators can do to protect themselves.
Furthermore, a tool uploaded on GitHub more than four years ago, and named Stinger-Tor, can allow anyone to carry out one of these attacks just by running a Python script.
Furthermore, there are also groups who are selling similar Tor DDoS tools on underground forums, also exploiting this same bug.
In an attempt to stop the almost constant attacks, members of the Dread community have been urging their userbase to donate to the Tor Project to help developers patch the bug.
Their efforts appear to have worked and one of the 14 bug reports where this vulnerability has been reported now lists a "sponsor" spot, meaning Tor supporters have managed to prioritize the bug fixing process via their donations.
The planned fix won't completely fix the bug because this implies breaking some of Tor's main privacy and security features, but it will make the attacks less effective.
Onion site operators will have a new option available inside the Tor service, which they can enable to activate defenses for their sites.
When this defense is enabled, Tor users will be able to access Onion sites attacked via this vulnerability, but connections will take longer to establish.
However, the good news is that sites will continue to be operational, and a threat actor can't pummel a dark web portal into oblivion and take it offline for days or weeks.