CVE-2026-54739 in Lemmy
Summary
by MITRE • 08/20/2026
Lemmy is a link aggregator and forum for the fediverse. Prior to 0.19.19 and 1.0.0-beta.1, Lemmy's login endpoint in crates/api/api/src/local_user/login.rs returns different errors depending on whether the username_or_email value exists. LocalUserView::find_by_email_or_name propagates a NotFound response for an unknown account, while an existing account with a wrong password returns LemmyErrorType::IncorrectLogin. This observable response discrepancy, including HTTP 404 for a nonexistent account and HTTP 400 for an incorrect password on an existing account, allows an unauthenticated attacker to confirm registered usernames or email addresses and use the results for targeted credential attacks or social engineering. This issue is fixed in versions 0.19.19 and 1.0.0-beta.1.
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Analysis
by VulDB Data Team • 08/20/2026
The vulnerability identified in Lemmy, a link aggregator and forum software within the fediverse ecosystem, represents a classic information disclosure flaw stemming from inconsistent error handling during authentication processes. Specifically located in the login endpoint at crates/api/api/src/local_user/login.rs, this issue manifests prior to versions 0.19.19 and 1.0.0-beta.1 through divergent HTTP response codes based on the validity of the provided credentials. When an attacker submits a username or email address that does not correspond to any registered account in the system, the application returns a NotFound response, typically associated with an HTTP 404 status code. Conversely, when the submitted identifier corresponds to an existing user but is paired with an incorrect password, the application triggers a LemmyErrorType::IncorrectLogin error, which results in an HTTP 400 Bad Request status code. This behavioral discrepancy creates a reliable side-channel that allows unauthenticated actors to enumerate valid accounts within the system without needing prior knowledge of specific passwords or internal database structures.
From a technical perspective, this flaw is rooted in how the LocalUserView::find_by_email_or_name function propagates errors up the call stack and into the HTTP response layer. Instead of normalizing error responses to provide uniform feedback for all authentication failures, the application preserves the distinct state of whether the user entity exists or merely has invalid credentials. This lack of abstraction between internal data retrieval states and external API responses violates fundamental security design principles regarding consistent failure modes. The presence of two distinct HTTP status codes serves as a binary oracle, enabling attackers to distinguish between non-existent users and existing ones with high precision. Such enumeration capabilities are particularly dangerous in federated networks where user identities may be linked across multiple instances or used for broader social engineering campaigns against the community infrastructure.
The operational impact of this vulnerability extends beyond simple account verification. By confirming which email addresses or usernames are registered, an attacker can construct targeted phishing attacks that appear legitimate because they reference real accounts and potentially personalized details derived from public profiles associated with those verified identities. Furthermore, this information aids in credential stuffing campaigns by allowing attackers to filter out invalid targets before launching password spraying attempts against known valid users, thereby increasing the efficiency of brute-force or dictionary-based attacks while reducing noise and detection probability. In the context of privacy-focused platforms like Lemmy, exposing which email addresses are associated with active accounts also constitutes a significant privacy breach, as it reveals user contact information to malicious actors who may aggregate this data for spamming or further reconnaissance activities against individual users.
To mitigate this vulnerability, organizations running affected versions must upgrade immediately to version 0.19.19 or later, which includes the necessary patches to normalize authentication responses. For environments where immediate upgrading is not feasible due to operational constraints, implementing a reverse proxy or web application firewall rule can help mask these specific HTTP status code differences by returning generic error messages for all login failures regardless of the underlying cause. Security best practices dictate that authentication endpoints should always return identical response codes and message bodies for both invalid usernames and incorrect passwords, ensuring that attackers cannot gain any information about account existence through timing analysis or response inspection. Additionally, implementing rate limiting on the login endpoint can further reduce the effectiveness of automated enumeration tools by introducing delays or temporary blocks after a certain number of failed attempts, thereby adding another layer of defense against brute-force and user-enumeration attacks aligned with MITRE ATT&CK techniques such as T1078 Valid Accounts and T1110 Brute Force.