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Are there defenses based on browser cache or temporary files for CSAM?
Executive summary
There is reporting and legal guidance showing that browser cache and temporary files can and have been treated as evidence in CSAM cases — courts and prosecutors treat cached browser artifacts as potential “possession” and investigators routinely recover artifacts from caches and temp storage [1] [2]. At the same time, privacy and defense advocates warn that automated scanning and hashing systems — many operating at CDN/cache layer like Cloudflare’s CSAM Scanning Tool — change where and how images are detected and reported, raising new legal and technical questions for defenses that rely on transient or cached artifacts [3] [4] [5] [6].
1. Cached images are tangible forensic leads — prosecutors and guides treat them as evidence
Digital‑forensics and prosecutorial materials note explicitly that “possession” can include electronic files such as browser cache files and that known CSAM is often identified by hash matching; law enforcement uses search warrants to seize devices and examine caches for image files and hashes [1] [2]. Forensic tool vendors and investigative write‑ups describe scanning large numbers of files (including recovered fragments) with block‑hash or fuzzy‑hash techniques to locate CSAM in device storage, which implicitly includes temporary files and remnants [7] [2].
2. Defense strategies often target provenance, intent and artifact volatility
Criminal defense resources advise challenging evidence handling, arguing lack of intent, or pointing to contamination — for example, claiming files were planted by malware, downloaded automatically, or existed only in transient cache without the defendant’s knowledge — and note that deleted files may still be recoverable, which creates contestable chains of custody [8] [9]. Legal guides specifically warn that possession requires proof the defendant knew of and controlled the files; cached or temporary files therefore form a common battleground in defense briefs [1] [9].
3. Malware and infostealers undermine neat narratives about ownership of cached artifacts
Independent research shows infostealer malware can exfiltrate credentials, browser cookies, saved autofill data and screenshots — and researchers have used malware logs to link users to CSAM sites — demonstrating that artifacts found on a device or in a browser profile can reflect compromise rather than deliberate possession [10] [11]. Defense teams can point to such compromises to argue that cached artifacts are the product of infection or remote activity rather than user intent [10] [11].
4. CDNs and cache‑layer scanning shift detection upstream — complicating defenses based on transient local caches
Cloudflare’s CSAM Scanning Tool and related CDN cache scanning programs compute fuzzy hashes of cached images as content “enters the Cloudflare cache” and flag matches to partner lists; platforms can then block access and report to authorities or NCMEC [4] [3] [5]. That means an image might be identified at the network/cache layer even if a local browser cache has been cleared — defenses premised on “no persistent copy” may face new technical evidence originating far from the end‑user device [3] [5].
5. Hashing and fuzzy‑hash limits create room for dispute
Investigations and legal summaries emphasize hash matching as a primary detection tool, but also note differences between strict hashes (byte‑identical) and fuzzy/block hashing (which tolerates edits); this technical detail matters because a hash hit can be used to justify reporting or warrants, while defenses can challenge the reliability and interpretation of fuzzy matches or testing methodology [2] [7] [3]. Where reporting is automated (platforms sending hits to NCMEC without human review), defense lawyers can press on how matches were produced and whether files were actually viewed [12] [3].
6. Competing policy perspectives: public safety vs. privacy and unintended legal exposure
Civil‑liberties groups warn that laws or requirements pushing platforms to scan more aggressively — or to break end‑to‑end encryption — risk undermining privacy and will create new litigation risks for providers and users; they argue that defenses rooted in technical impossibility of detection (e.g., encryption) are limited under some proposals [13] [6]. Conversely, law‑enforcement and forensic vendors emphasize that scanning, hashing and caching tools provide essential leads and scalability to find victims and abusers [7] [14]. Those opposing viewpoints frame different incentives: platform operators may want to avoid liability and thus scan more of their caches [4] [5], while privacy advocates stress the broader harms of pervasive scanning [13] [6].
7. What reporting does not say — limits and open questions
Available sources do not mention specific case law examples where a defense based solely on the ephemerality of browser cache prevailed in a CSAM prosecution; sources note that cached files “can” constitute possession and are routinely examined, but do not catalogue appellate outcomes tied to that single argument [1] [2]. Similarly, while sources describe malware and infostealer use to link users to CSAM, they do not provide statistics on how often malware‑based defenses succeed in court [10] [11].
Conclusion: For defendants and counsel, transient browser cache and temporary files are real evidentiary vectors that investigators use; but they are also contestable. Practical defenses documented in guidance include challenging chain of custody, asserting lack of intent, and demonstrating device compromise or automated downloads — all supported in the sources above [8] [1] [9] [10]. At the same time, the migration of scanning into CDN and cache layers changes where evidence is generated and may limit some arguments based on local ephemerality [3] [4] [5].