Alert

Alert missions are one-time, temporary missions which appear randomly in place of any mission on a planet's map which a player has previously unlocked (though not necessarily completed). They can be of any mission type (noted in top of description window) and often have vague references to the mission type explained under "description." If the type selected is an assassination, the boss will have a chance to drop a blueprint. The level of the boss will often be different from the standard. While hovering the mouse over an alert node, it will display the name of the boss to be fought (most often met are Kril & Tyl Regor).

Every alert that is available to a player is visible in-game in the upper-left corner (or the right if playing on the PlayStation 4) of the solar map; however, all alerts are also displayed on the WarframeAlerts Twitter, displayed below.

Alerts are temporary, usually lasting between 30 and 70 minutes(doubled if the bonus award is a special item) and issued at intervals of 20 to 40 minutes. They spawn linked to a specific mission, and are available to any given player until the time passes or until the player completes the alert. The timer restricts only the time in which the alert can be started; there is no restriction on when the alert must be completed. There is almost always at least one alert available, and as many as 5 alerts can be available at once.

Alerts also give bonus credits almost always ranging from 2,000 to 20,000 depending on the difficulty of the mission. They also sometimes award special Blueprints and Auras that players cannot obtain anywhere else. The credits reward at the end of alert missions is increased by 10% for each additional squad member. The difficulty of the enemies has little relation to the standard difficulty of the attached standard mission. Occasionally a long alert is released with a rare blueprint for up to 24 or 48 hours.

Because they are linked to missions, a player can only select an alert if they have unlocked the mission that the alert is associated with. However, another player who can select an alert can invite another player who cannot, allowing the latter to still complete the alert and receive the rewards. They is commonly referred to as "taxi-ing" in-game. Currently, a player cannot play the original mission of the level until the alert expires or he/she completes the Alert. 

As of Update 10, enemies in Alert Missions have their levels increased.

Alert Rewards

• Cosmetic Weapon-Skin Blueprints
• Alternate Helmet Blueprints
• Ceramic Dagger Blueprint
• Dark Dagger Blueprint
• Dark Sword Blueprint
• Heat Dagger Blueprint
• Heat Sword Blueprint
• Jaw Sword Blueprint
• Pangolin Sword Blueprint
• Plasma Sword Blueprint
• Glaive Blueprint
• Orokin Catalyst Blueprint
• Orokin Reactor Blueprint
• Auras
• Vauban Components Blueprints
• Forma Blueprint
• Any Resources

See the special award missions on twitter. 

Bugs

• There is a bug where a total of 5 players end up connected to an alert mission.

Over 20 Grineer at once.

• Sometimes when you load up an alert that is Grineer mission, the Grineer soldier can spawn in mass groups surpassing normal amounts.
• Bosses spawned by assassination alerts will frequently be renamed in their health bar, either to the name of a generic enemy, (e.g.: Captain Vor and Tyl Regor often become 'Grineer Flameblades') or to the name of the boss of the planet the alert appeared on.
• When clicking on an alert mission on the planet view, then clicking the "Cancel" button, if a level is located under the area you clicked you will be put into the lobby for that level. Can be very annoying when all you do is click cancel and you are thrown into a game. Use Esc. instead to avoid this issue.
• After Update 11.5 on the PS4, alert missions sometimes will have no enemies at all. This literally makes Exterminate alerts impossible to do, and other missions very easy. This seems to happen most often with missions against the Infested.(Unconfirmed as to if this keeps Capture targets from spawning as well)