When something's weird in your ImageMagick upload, who ya gonna call? Ghostbusters!

Evil files bypass GhostScript sandbox, run malicious code

GhostScript's security sandbox is so weak, website admins, developers, and users should block ImageMagick and other tools from using the software altogether.

Returning from a sabbatical, noted Google Project Zero researcher Tavis Ormandy this week emitted new ways to execute arbitrary code on vulnerable servers and similar machines that process incoming files using GhostScript. He said exploitation is possible against pic wrangling toolkit ImageMagick, file viewer Evince, image editor Gimp, and other programs that pass data to GhostScript to process.

Earlier this week, Ormandy tweeted the bottom line: “This is your annual reminder to disable all the GhostScript coders in policy.xml.” He was referring back to an ImageMagick-related vuln disclosed in April 2017, and the policy.xml configuration file for controlling access to ImageMagick handlers.

Essentially, if you run a website, or app or some other online service, that uses ImageMagick to process user-submitted pictures – such as photos to turn into account profile pics – you should update your policy.xml to block any GhostScript code from running. What can happen is, a miscreant can upload a file – say, myface.png – and embed some PostScript in it that, when passed to a GhostScript handler via ImageMagick, triggers the execution of a smuggled-in command. That would allow scumbags to submit files to run malicious commands and hijack your computer systems.

Blame GhostScript

Crucially, the problem is not within ImageMagick, Evince, GIMP, etc – it's in the GhostScript backend they rely on. Right now, the best way to protect yourself is to disable any GhostScript handlers.

This CERT advisory also summarizes the issue: “Ghostscript contains multiple -dSAFER sandbox bypass vulnerabilities, which may allow a remote, unauthenticated attacker to execute arbitrary commands on a vulnerable system.”

Ormandy explained the technical details in this mailing list post, which leads with: “I strongly suggest that distributions start disabling PS, EPS, PDF and XPS coders in policy.xml by default.”

According to the Googler, an attacker can get past the sandbox by abusing a broken error handler, /invalidaccess; a bug in setcolor; a type confusion problem because LockDistillerParams isn't type-checked; and .tempfile permissions were broken at some point, so an attacker can create files at an arbitrary location.

And those were just what Ormandy found with a manual scan of the code. He's writing a fuzzer for Ghostscript, which should provide plenty of entertainment in the future.

“IMHO, -dSAFER is a fragile security boundary at the moment, and executing untrusted postscript should be discouraged, at least by default”, he added.

The CERT advisory provides an example of how to block GhostScript processing, assuming a Red Hat system. The sysadmin should add the following lines to the <policymap> section of /etc/ImageMagick/policy.xml:

<policy domain="coder" rights="none" pattern="PS" />
<policy domain="coder" rights="none" pattern="EPS" />
<policy domain="coder" rights="none" pattern="PDF" />
<policy domain="coder" rights="none" pattern="XPS" />

Get busy, sysadmins. As far as we're aware, no patches are available short of altering your policy.xml file, or removing any code that passes untrusted data to GhostScript. ®

Similar topics

Broader topics

Other stories you might like

  • Lonestar plans to put datacenters in the Moon's lava tubes
    How? Founder tells The Register 'Robots… lots of robots'

    Imagine a future where racks of computer servers hum quietly in darkness below the surface of the Moon.

    Here is where some of the most important data is stored, to be left untouched for as long as can be. The idea sounds like something from science-fiction, but one startup that recently emerged from stealth is trying to turn it into a reality. Lonestar Data Holdings has a unique mission unlike any other cloud provider: to build datacenters on the Moon backing up the world's data.

    "It's inconceivable to me that we are keeping our most precious assets, our knowledge and our data, on Earth, where we're setting off bombs and burning things," Christopher Stott, founder and CEO of Lonestar, told The Register. "We need to put our assets in place off our planet, where we can keep it safe."

    Continue reading
  • Conti: Russian-backed rulers of Costa Rican hacktocracy?
    Also, Chinese IT admin jailed for deleting database, and the NSA promises no more backdoors

    In brief The notorious Russian-aligned Conti ransomware gang has upped the ante in its attack against Costa Rica, threatening to overthrow the government if it doesn't pay a $20 million ransom. 

    Costa Rican president Rodrigo Chaves said that the country is effectively at war with the gang, who in April infiltrated the government's computer systems, gaining a foothold in 27 agencies at various government levels. The US State Department has offered a $15 million reward leading to the capture of Conti's leaders, who it said have made more than $150 million from 1,000+ victims.

    Conti claimed this week that it has insiders in the Costa Rican government, the AP reported, warning that "We are determined to overthrow the government by means of a cyber attack, we have already shown you all the strength and power, you have introduced an emergency." 

    Continue reading
  • China-linked Twisted Panda caught spying on Russian defense R&D
    Because Beijing isn't above covert ops to accomplish its five-year goals

    Chinese cyberspies targeted two Russian defense institutes and possibly another research facility in Belarus, according to Check Point Research.

    The new campaign, dubbed Twisted Panda, is part of a larger, state-sponsored espionage operation that has been ongoing for several months, if not nearly a year, according to the security shop.

    In a technical analysis, the researchers detail the various malicious stages and payloads of the campaign that used sanctions-related phishing emails to attack Russian entities, which are part of the state-owned defense conglomerate Rostec Corporation.

    Continue reading

Biting the hand that feeds IT © 1998–2022