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Vulnerability

|< < Prev #3314 (October 21, 2026)

Title text: Other numbers are largely unaffected, but anyone using '4' for their certificate key is urged to upgrade to '5'.

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Explanation

This comic is a math-themed parody of software security vulnerabilities, flaws within a piece of software (such as an app or website) which can be exploited by hackers. Security vulnerabilities come in many forms and their severity varies, from exposure of sensitive user data to catastrophic software bugs which allows remote code execution on a user's system. xkcd, as is typical, riffs on this by treating numbers as something susceptible to security vulnerabilities. This is reminiscent of 3137: Cursed Number.

The joke is that there is a critical vulnerability in the numberline, which can never actually happen in real life.^{[citation needed]} It is somewhat common to find critical vulnerabilities in things like operating systems or programming language, but the number line is more abstract than these things and should not have a vulnerability. It is also comical that the vulnerability is the whole number between two and four, because it's unclear how the number is creating a vulnerability.

Cueball then implies that people should avoid even naming the aforementioned number, referring to it as "the integer between two and four", "half six" or "baker's two". These first two euphemisms are simply correct mathematics, whereas a "baker's two" borrows from the phrase "baker's dozen". A dozen means 12, but a baker's dozen is 13. One of several reasons commonly given for this is that if a customer orders a dozen loaves, for example, then by providing 13 loaves the customer would still have a dozen loaves if one of them was bad. A baker's two then, by the same logic, is one more than two. Some variations of Baker's Dozens were previously shown 3092: Baker's Units.

This comic was released at the same time as two extremely high-profile security vulnerabilities became public, these being the compromised eMudhra Intermediate TLS certificates and the PHP RCE vulnerability.

The Intermediate TLS certificates of the Indian certification authority eMudhra were found to be insecure. Despite using 384-bit keys, the actual values of the private keys for the 8 certificates were 4, 56, 66, 115, 155, 156, 230 and 237.

A major PHP vulnerability was announced on the same day. PHP is a scripting language used in a massive number of websites across the internet, many of which (including explainxkcd itself) were temporarily shut down before the vulnerability was patched.

The title text is an explicit reference to the eMudhra security vulnerability, specifically the certificate key "4".

The idea of hard-coding the number 4 appeared in 221: Random Number.

The concept of avoiding explicitly naming this specific number appeared previously in 1275: int(pi).

Not using an area of the coordinate plane was mentioned in 2735: Coordinate Plane Closure

Transcript

[Cueball is sitting at a desk, apparently as a news anchor.]

Cueball: Breaking: Mathematicians have announced a critical vulnerability in the number line affecting the integer between 2 and 4.

Cueball: Until the patch is applied, if you must count small numbers of objects, please use the alternatives "half six" or "baker's two".

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Edit this week's pick!

This Week's Explanation for Improvement

As part of the Explanation Improvement Project, each week the community selects one older, stagnant, or otherwise neglected explanation to expand and polish together. This week's pick is comic 976: Sail, you can help us by editing its explanation!

To get involved with the project, you can also nominate an explanation that you think should be improved, or vote on current nominations!

Sail

|< < Prev #976 (November 11, 2011) Next > >|

Title text: It only works a few times before you have to capsize the boat in a soap lagoon again.

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Explanation

Welcome to This Week's Explanation for Improvement! Anyone is encouraged to help explain comic 976: Sail; click here to get started! If you are new to wiki editing, you can read our editing FAQ for guidance. Here are some suggestions for improving this explanation:   Explain whether this idea could actually work in real life, (with the right weather conditions)   How fast would the wind need to be to form a bubble?   What is Cueball pulling? Is he trying to create the bubble or govern the ship?   Would a boat be able to float in a "soap lagoon"?

A billowing sail sometimes looks like the first stage of blowing a bubble. A strong gust of wind blows by and fills the sail, which Cueball is clearly surprised by and fights to keep the sail under control. As the wind continues, the sail warps and begins acting like a film of bubble soap, eventually separating into its own bubble and floating off and the sail reverting to the original position, as the gust of wind dies down. Sails do not typically act like this under strong winds^{[citation needed]}.

If the booms and mast were actually attached by a rope on the far side, the sail frame could function as a large bubble wand. Commercial 'bubble ropes' can have an area of at least a couple of square metres, but it would need the right velocity and consistency of wind to sustain the bubble long enough to separate cleanly from the sail. This could, theoretically, be possible under extreme wind conditions, but it would be rare to find such winds.

The title text suggests that the water the boat is sailing in is a soap solution that can be used to create the bubbles. If the boat was overturned (capsized) in such water, a film would form between the mast and the boom. The film would become like the sail in the comic, filling with air and forming bubbles. However, with each bubble that is released the remaining film on the boat would become thinner, until it can no longer sustain itself, at which point it would need to be refilled by capsizing again. While this arrangement could be used to produce large bubbles with a sufficient film, Cueball would likely not have to pull on the mainsheet as much as is depicted in the comic, as the 'sail' isn't mechanically attached to the vessel for long, since it is a bubble film, not a piece of cloth.

Transcript

[Cueball is sailing a lateen rigged sailboat.]

[The wind picks up, pushing the sail forward.]

[The wind builds strength. Cueball has to stand up to hold on to the main sheet, with shaking depicting his tugging.]

[The distorted sail balloons out. Cueball is still holding on to the main sheet. More curves depict a stronger tugging.]

[Part of the sail separates into a bubble. Cueball falls backwards into the boat. The sail begins to return to its normal shape.]

[Cueball lands on his rear and scratches his head in confusion as the sail-sized bubble floats away.]

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