In a memo sent to flight crews on Saturday (13 May), the airlines said it learned that its "flight desk procedures may have been compromised". It also asked pilots to review and follow existing procedures until a "corrective action plan" has been implemented.
It reminded employees that "the risk of a breach of the flight deck door is strongly mitigated by carefully following the flight deck security procedures".
"The safety of our customers and crew is our top priority and United utilizes a number of measures to keep our flight decks secure beyond door access information," United said. "United utilizes a number of measures to keep our flight decks secure beyond door-access information."
The source told WSJ that United regularly changes the codes that need to be entered into the keypad outside the cockpit door. The system also allows pilots to override any person's attempts to enter the cockpit even if they do enter the correct access codes.
However, United noted that such information is "sensitive security information and sharing this with anyone not authorised or who does not have a need to know is strictly prohibited".
In a statement obtained by CNN affiliate KCAL/KCBS, United spokeswoman Maddie King clarified that the information was not made public due to a breach or hack. She declined to give any additional details saying United does not discuss security procedures.
United Airlines operates around 4,500 flights to more than 330 destinations across the globe. The airline did not report any flight delays or schedule issues caused by the incident.
The Air Line Pilots Association, a union that represents about 55,000 pilots in the US and Canada, said on Sunday that the issue has been fixed.
US airlines beefed up their cockpit security following the 9/11 terrorist attacks in 2001. Airlines also re-examined cockpit security protocols following the crash of Germanwings Flight 9525 in March 2015. It emerged that the co-pilot locked himself in the cockpit after the captain left momentarily. The suicidal pilot crashed the plane into the French Alps killing all 150 people on board.
The latest incident comes as United drew fierce criticism and calls to boycott the airlines after passenger David Dao was dragged by security off an overbooked flight. He suffered a concussion, a broken nose and two broken teeth in the violent incident that was captured in multiple videos that quickly went viral. Dao sued the airline and received an undisclosed financial settlement.
The airline also came under fire over multiple other incidents involving its passengers, triggering severe backlash on social media.
(Hyacinth Mascarenhas - International Business Times)