For the last four months, Chinese hackers have persistently attacked The New York Times, infiltrating its computer systems and getting passwords for its reporters and other employees.
After surreptitiously tracking the intruders to study their movements and help erect better defenses to block them, The Times and computer security experts have expelled the attackers and kept them from breaking back in.
The timing of the attacks coincided with the reporting for a Times investigation, published online on October 25, that found that the relatives of Wen Jiabao, China's prime minister, had accumulated a fortune worth several billion dollars through business dealings.
Security experts hired by The Times to detect and block the computer attacks gathered digital evidence that Chinese hackers, using methods that some consultants have associated with the Chinese military in the past, breached The Times's network. They broke into the e-mail accounts of its Shanghai bureau chief, David Barboza, who wrote the reports on Mr. Wen's relatives, and Jim Yardley, The Times's South Asia bureau chief in India, who previously worked as bureau chief in Beijing.
"Computer security experts found no evidence that sensitive e-mails or files from the reporting of our articles about the Wen family were accessed, downloaded or copied," said Jill Abramson, executive editor of The Times.
The hackers tried to cloak the source of the attacks on The Times by first penetrating computers at United States universities and routing the attacks through them, said computer security experts at Mandiant, the company hired by The Times. This matches the subterfuge used in many other attacks that Mandiant has tracked to China.
The attackers first installed malware — malicious software — that enabled them to gain entry to any computer on The Times's network. The malware was identified by computer security experts as a specific strain associated with computer attacks originating in China. More evidence of the source, experts said, is that the attacks started from the same university computers used by the Chinese military to attack United States military contractors in the past.
Security experts found evidence that the hackers stole the corporate passwords for every Times employee and used those to gain access to the personal computers of 53 employees, most of them outside The Times's newsroom. Experts found no evidence that the intruders used the passwords to seek information that was not related to the reporting on the Wen family.
No customer data was stolen from The Times, security experts said.
Asked about evidence that indicated the hacking originated in China, and possibly with the military, China's Ministry of National Defense said, "Chinese laws prohibit any action including hacking that damages Internet security." It added that "to accuse the Chinese military of launching cyberattacks without solid proof is unprofessional and baseless."
The attacks appear to be part of a broader computer espionage campaign against American news media companies that have reported on Chinese leaders and corporations.
Last year, Bloomberg News was targeted by Chinese hackers, and some employees' computers were infected, according to a person with knowledge of the company's internal investigation, after Bloomberg published an article on June 29 about the wealth accumulated by relatives of Xi Jinping, China's vice president at the time. Mr. Xi became general secretary of the Communist Party in November and is expected to become president in March. Ty Trippet, a spokesman for Bloomberg, confirmed that hackers had made attempts but said that "no computer systems or computers were compromised."
Signs of a Campaign
The mounting number of attacks that have been traced back to China suggest that hackers there are behind a far-reaching spying campaign aimed at an expanding set of targets including corporations, government agencies, activist groups and media organizations inside the United States. The intelligence-gathering campaign, foreign policy experts and computer security researchers say, is as much about trying to control China's public image, domestically and abroad, as it is about stealing trade secrets.
Security experts said that beginning in 2008, Chinese hackers began targeting Western journalists as part of an effort to identify and intimidate their sources and contacts, and to anticipate stories that might damage the reputations of Chinese leaders.
In a December intelligence report for clients, Mandiant said that over the course of several investigations it found evidence that Chinese hackers had stolen e-mails, contacts and files from more than 30 journalists and executives at Western news organizations, and had maintained a "short list" of journalists whose accounts they repeatedly attack.
While computer security experts say China is most active and persistent, it is not alone in using computer attacks for a variety of national purposes, including corporate espionage. The United States, Israel, Russia and Iran, among others, are suspected of developing and deploying cyberweapons.
The United States and Israel have never publicly acknowledged it, but evidence indicates they released a sophisticated computer virus in 2012 that attacked and caused damage at Iran's main nuclear enrichment plant. Iran is believed to have responded with computer attacks on targets in the United States, including American banks and foreign oil companies.
Russia is suspected of having used computer attacks during its war with Georgia in 2008.
The following account of the attack on The Times — which is based on interviews with Times executives, reporters and security experts — provides a glimpse into one such spy campaign.
After The Times learned of warnings from Chinese government officials that its investigation of the wealth of Mr. Wen's relatives would "have consequences," executives on Oct. 24 asked AT&T, which monitors The Times's computer network, to watch for unusual activity.
On Oct. 25, the day the article was published online, AT&T informed The Times that it had noticed behavior that was consistent with other attacks believed to have been perpetrated by the Chinese military.
The Times notified and voluntarily briefed the Federal Bureau of Investigation on the attacks and then — not initially recognizing the extent of the infiltration of its computers — worked with AT&T to track the attackers even as it tried to eliminate them from its systems.
But on November 7, when it became clear that attackers were still inside its systems despite efforts to expel them, The Times hired Mandiant, which specializes in responding to security breaches. Since learning of the attacks, The Times — first with AT&T and then with Mandiant — has monitored attackers as they have moved around its systems.
Hacker teams regularly began work, for the most part, at 8 a.m. Beijing time. Usually they continued for a standard work day, but sometimes the hacking persisted until midnight. Occasionally, the attacks stopped for two-week periods, Mandiant said, though the reason was not clear.
Investigators still do not know how hackers initially broke into The Times's systems. They suspect the hackers used a so-called spear-phishing attack, in which they send e-mails to employees that contain malicious links or attachments. All it takes is one click on the e-mail by an employee for hackers to install "remote access tools" — or RATs. Those tools can siphon off oceans of data — passwords, keystrokes, screen images, documents and, in some cases, recordings from computers' microphones and Web cameras — and send the information back to the attackers' Web servers.
Michael Higgins, chief security officer at The Times, said: "Attackers no longer go after our firewall. They go after individuals. They send a malicious piece of code to your e-mail account and you're opening it and letting them in."