Integral Ad Science (IAS), a global technology and data firm, came to this conclusion after studying 20m ad impressions from 75 brands in the second half of 2016.
Covering six countries – Indonesia, Malaysia, the Philippines, Singapore, Thailand and Vietnam – IAS's Media Quality Report flagged impressions that appeared alongside inappropriate subjects, such as adult content, alcohol content, hate speech, illegal downloads, illegal drugs, offensive language and violence.
Based on these checks, IAS found that Thailand's online environment presented the lowest risk for brand safety in the region, with just 1.6% of display and 2.2% of video ad impressions appearing on unsafe websites.
Singapore came second, with only 2.7% of display ads and 4.6% of video ads appearing on unsafe websites, but Indonesia and Vietnam were judged to present the highest risk.
Some 15.3% of the video ad impressions served in Indonesia were flagged for appearing alongside unsafe content, with Vietnam following close behind (13.2%), although Vietnam's display ad risk was relatively low at 4.2%.
And 5% of Indonesia’s display ads were also found on risky sites, the second highest rate in the region after Malaysia (6.7%). The brand safety risk for Malaysia's video ads was also found to be relatively high (7.1%).
Meanwhile, IAS said the video ad environment in the Philippines posed a high risk to brand safety, with 8.6% of ad inventories featuring against unsafe content, while 4.4% of display ads in the country were found to be at risk.
Commenting on the risk to brand safety in the region, IAS Managing Director Niall Hogan said: "Southeast Asia data revealed that a majority of sites flagged for brand risk were illegal file sharing sites. With the current socio-political environment, brands should more than ever be vigilant about where their ads appear."
IAS also examined ad viewability across the six countries and, more positively, found some scored better than in the US, where 53% of display ads and 58.2% of video ads are viewable.
Ads in Thailand, for example, had viewability rates of 61.7% for display and 63.3% for video. By contrast, Singapore had the region's worst video viewability (37.6%), although its display viewability stood at a respectable 56.3%.
Data sourced from Integral Ad Science; additional content by WARC staff