How severe is CVE-2026-54739?

CVE-2026-54739 has a CVSS 4.0 base score of 6.9, rated medium severity.

Is CVE-2026-54739 being actively exploited?

It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (36th percentile), an estimate of the probability of exploitation in the next 30 days.

How do I fix CVE-2026-54739?

Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.

When was CVE-2026-54739 published?

CVE-2026-54739 was published on 2026-08-19 and last updated on 2026-08-21.

CVE-2026-54739

CVE-2026-54739 is a medium-severity vulnerability with a CVSS 4.0 base score of 6.9. It is not currently listed as actively exploited by CISA, and its EPSS exploit-prediction score is low. The underlying weakness is classified as CWE-204.

Key facts

Severity: Medium (CVSS 4.0 base score 6.9)
EPSS exploit prediction: 0% (36th percentile)
Actively exploited: Not listed in CISA KEV
Weakness: CWE-204
Published: 2026-08-19
Last modified: 2026-08-21

Description

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.

Frequently asked questions

What is CVE-2026-54739?: 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.
How severe is CVE-2026-54739?: CVE-2026-54739 has a CVSS 4.0 base score of 6.9, rated medium severity.
Is CVE-2026-54739 being actively exploited?: It is not currently listed in CISA's KEV catalog. Its EPSS exploit-prediction score is 0% (36th percentile), an estimate of the probability of exploitation in the next 30 days.
How do I fix CVE-2026-54739?: Review the linked vendor and NVD advisories for patched versions and mitigations, then upgrade or apply the recommended workaround.
When was CVE-2026-54739 published?: CVE-2026-54739 was published on 2026-08-19 and last updated on 2026-08-21.

References

Other CWE-204 vulnerabilities

CVE-2018-25350 — Critical (CVSS 9.8): userSpice 4.3.24 contains a username enumeration vulnerability that allows unauthenticated attackers to discover valid…
CVE-2026-15747 — Critical (CVSS 9.1): Mojolicious versions from 4.59 before 9.48 for Perl expose a stable representation of the session CSRF token to a…
CVE-2026-69519 — High (CVSS 8.6): Observable response discrepancy in Azure Stack HCI allows an unauthorized attacker to disclose information over a…
CVE-2025-5485 — High (CVSS 8.6): User names used to access the web management interface are limited to the device identifier, which is a numerical…
CVE-2026-27462 — High (CVSS 7.5): Combodo iTop is a web based IT service management tool. Prior to 3.2.3, iTop returns different responses for…
CVE-2026-33419 — High (CVSS 7.5): MinIO is a high-performance object storage system. Prior to RELEASE.2026-03-17T21-25-16Z, MinIO AIStor's STS (Security…

Browse all CWE-204 vulnerabilities →