CRIMINAL LAW DIVISION
I. Introductory Considerations
The progressive digitalization of personal relationships has brought into judicial proceedings a type of evidentiary source that is now commonplace: communications conducted through instant messaging systems. WhatsApp, as the prime example, along with other similar applications, has come to play a central role in reconstructing the facts of countless disputes.
However, this same ease of use and dissemination is precisely what makes these media especially problematic from an evidentiary perspective. Their extreme malleability, the possibility of manipulating digital files, and the ease of identity impersonation require caution from the courts.
Since Supreme Court Judgment (STS) 300/2015 of 19 May, the Spanish Supreme Court has progressively developed a solid doctrine that has later been refined and clarified. A clear principle emerges from this body of case law: instant messaging evidence is fully admissible, but its evidentiary value depends on sufficiently proving its authenticity, integrity, and origin, as well as the lawfulness of how it was obtained.
II. The Nature of Messages: Beyond Documentary Evidence
One of the most important contributions of STS 300/2015 is the conceptual definition of this type of evidence. The Supreme Court expressly rejects the idea that so-called “screenshots” can be regarded as documents for cassation purposes. They are not documents in the technical legal sense, but rather communications that have later been fixed onto a medium.
This clarification is not merely terminological. It moves these materials away from the realm of documentary evidence — characterized by its self-sufficiency — and into the sphere of documented personal evidence. This means they cannot be assessed in isolation or automatically, but rather in connection with the rest of the available evidence.
Ultimately, the medium in which the messages are presented does not alter their original nature: what matters is not the paper or image itself, but the communication reflected therein.
III. Risks Inherent in Digital Evidence
The 2015 judgment also introduced a warning that has become a structural element of later doctrine: digital communications must be examined “with all due caution.” This statement is based on an empirical reality: digital files can be altered relatively easily, and messaging systems allow the creation of fictitious identities without major difficulty.
Thus, it is not uncommon for a nonexistent conversation to be presented as genuine, or for a real dialogue to be selectively fragmented so as to distort its meaning. The outward appearance of authenticity — the chat format, usernames, or apparent chronology — does not by itself guarantee the authenticity of the message.
Aware of these risks, the case law does not exclude digital evidence, but requires it to be evaluated with methodological prudence.
IV. Authenticity and Integrity as the Core of Evidentiary Validity
Case law has increasingly focused on two fundamental requirements, clearly stated in STS 7/2023 of 19 January: authenticity of origin and integrity of content. In other words, the court must be able to conclude, beyond reasonable doubt, that the message genuinely came from the alleged sender and that its content has not been altered.
These are not rigid formal requirements, but rather reliability criteria. They may be proven through different means, but their absence prevents the evidence from being given incriminating value.
At the same time, the Supreme Court has rejected any automatic exclusionary rule. Merely challenging authenticity does not automatically expel the evidence from the proceedings. Instead, it triggers a more demanding assessment of credibility in light of the body of evidence as a whole.
V. Challenging the Evidence and the Shift in the Burden of Proof
STS 300/2015 introduced the principle that, when the authenticity of a conversation is contested, the burden of proving its evidentiary reliability shifts to the party seeking to rely on it. This does not amount to a radical reversal of the burden of proof, but rather a logical consequence of the adversarial principle.
Subsequent case law, however, has refined the scope of this rule. STS 116/2025 of 13 February warns that the challenge must be real and substantiated, not merely generic or strategic denial. Only when concrete reasons are alleged that raise doubts about authenticity or integrity does the need for stronger evidentiary support arise.
This prevents both the uncritical acceptance of digital evidence and its systematic, unfounded rejection.
VI. Computer Forensic Evidence and Its Non-Absolute Nature
The statement in STS 300/2015 regarding the need for expert forensic evidence when authenticity is challenged has since been interpreted flexibly. STS 7/2023 clarifies that, although computer forensic analysis is the most appropriate means of verifying the origin and integrity of messages, it cannot be treated as an indispensable requirement.
Authenticity may also be established through a convergence of evidentiary elements: testimony from the participants, submission of the original device, verification of content before a public official, or consistency of the messages with the overall factual narrative. In some cases, this collective corroboration may suffice to dispel doubts.
Thus, forensic analysis is the strongest instrument, but not the only possible one. Its necessity depends on the circumstances of the case and the seriousness of the challenge raised.
VII. Procedural Timing of the Challenge
A particularly relevant issue concerns the stage at which authenticity must be challenged. STS 116/2025, relying on STS 332/2019 of 27 June, establishes that a genuine challenge may be raised in the statement of defence, without needing to have been anticipated during the investigation phase.
This doctrine has practical consequences, as it allows the defence to react to the final configuration of the accusation without being burdened with excessive anticipatory obligations. At the same time, it obliges the prosecution to strengthen its evidentiary activity when authenticity is challenged at that stage, and prevents objections based on the alleged lateness of the proposed evidence.
VIII. Lawfulness of Obtaining the Messages
The lawfulness of obtaining messages has also been clearly addressed by the case law. STS 90/2021 of 7 October, issued by the Military Chamber of the Supreme Court, in line with constitutional doctrine, states that recording or preserving a conversation by one of the participants does not violate the right to secrecy of communications protected under Article 18.3 of the Spanish Constitution.
The reason is that secrecy protects communications against third parties external to the conversation, not against those who participate in it. There is no secrecy vis-à-vis the recipient of the message. Consequently, introducing a conversation into evidence by one of its participants is, in principle, lawful.
This does not exclude potential issues relating to privacy rights or the right against self-incrimination in cases involving deception, provocation, or institutional superiority. Outside such scenarios, however, the obtaining of the evidence is generally lawful.
IX. Evidentiary Assessment and the Presumption of Innocence
Finally, the case law stresses that instant messaging evidence is subject to the principle of free evaluation of evidence. However, this freedom does not equate to arbitrariness. Where such messages form the core of the accusation, the evidentiary standard must be more demanding.
STS 90/2021 provides a paradigmatic example by denying sufficient incriminating value to uncorroborated screenshots lacking identification of the device or proof of authorship. Under such conditions, the evidence lacks the solidity necessary to rebut the presumption of innocence.
The essential principle reaffirmed is that digital evidence alone may be insufficient unless accompanied by additional guarantees ensuring its authenticity and reliability.
X. Conclusion
The Supreme Court’s doctrine regarding instant messaging evidence reflects a carefully balanced approach. Far from adopting either total distrust or uncritical acceptance, the case law recognizes the usefulness of these means while subjecting their evidentiary value to rigorous scrutiny.
Authenticity of origin, integrity of content, proper identification of the participants, and lawfulness of acquisition stand as the pillars of their validity. On this basis, and through a joint assessment of all the evidence, courts may assign these messages the evidentiary weight they deserve in each individual case.
Ultimately, digital evidence is neither inferior nor superior to other forms of evidence — but it is different. And that difference requires, as the Supreme Court has repeatedly emphasized since STS 300/2015, a degree of caution that has now become a true standard of judicial assessment.
