CVE-2026-78551 RansomLook Login Endpoint Allows Timing-Based Username Enumeration and Unthrottled Authentication Attempts
Attackers can enumerate valid usernames via timing differences and cause denial of service through unthrottled login attempts.
- Exploited in the wild
- Not known
- Login needed
- Not needed
- Attackable
- Over the internet
- Victim action needed
- Not needed
- Fix
- None listed
What is CVE-2026-78551?
CVE-2026-78551 is a high-severity vulnerability in ransomlook, published on 24 Aug 2026. An attacker can exploit it over the internet without logging in, with no action from the victim.
Official description
RansomLook contains multiple weaknesses in its authentication endpoint that allow an unauthenticated remote attacker to enumerate valid usernames, perform unrestricted password-guessing attacks, and potentially exhaust application worker resources.
For local authentication, the login implementation previously checked whether a submitted username existed before invoking the password hash verification function. Requests containing a nonexistent username therefore returned significantly faster than requests for valid accounts, for which the computationally expensive password verification routine was executed. A remote attacker could measure these response-time differences to determine which usernames correspond to valid RansomLook accounts.
In addition, the /login endpoint did not restrict the number or frequency of failed authentication attempts. An attacker could consequently perform password brute-force, dictionary, password-spraying, or credential-stuffing attacks against known accounts without server-side throttling. For valid usernames, each authentication attempt also invokes the password key-derivation function, which consumes a significant amount of CPU time. A sufficiently high rate of login attempts could therefore occupy the application's synchronous Gunicorn workers and cause a denial of service affecting the entire application.
The issue has been addressed by always performing password verification using a randomly generated dummy password hash when the supplied username does not exist, eliminating the username-dependent timing discrepancy. Failed authentication attempts are additionally rate-limited per client IP address using Valkey/Redis, with five failed attempts within five minutes resulting in a one-hour block. The reverse-proxy configuration was also updated so that the application derives the client address from a trusted X-Forwarded-For value that cannot be overridden by a client-supplied header.
Which versions are affected by CVE-2026-78551?
How do I fix CVE-2026-78551?
No fix has been published for CVE-2026-78551 yet. Limit exposure of the affected software until one is available.
Is CVE-2026-78551 being exploited?
There is no known exploitation of CVE-2026-78551 in the wild and no public exploit code.
Sources
Also known as GHSA-vrw5-9j4x-chw5.