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Do not treat one Failed password line as proof that a server was breached. Treat it as one event in an authentication transaction. Identify the host, time zone, account, source address, authentication method and sshd process, then follow the surrounding records until you can establish whether authentication succeeded, account policy denied access, or a session actually opened.
On Red Hat-family systems, the traditional file is usually /var/log/secure. A systemd host may send the same information to the journal instead. The examples and commands below are read-only starting points; use the log source configured on your host and preserve the original timestamps before changing or deleting anything.
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There is no universal Linux path for SSH and PAM events. Red Hat-family configurations commonly write authentication-module messages to /var/log/secure. Hosts managed primarily through systemd may retain equivalent records in journald, where journalctl is the appropriate reader. Rotation, forwarding and local policy can also mean that only part of the history is on the machine.
Check a traditional secure log
sudo grep -E 'sshd|sudo|su|login|pam_' /var/log/secureUse the rotated files, such as /var/log/secure-20261014, only when your retention policy and file naming confirm that they contain the period you need. Do not silently combine timestamps from files that use different time zones or rotation conventions.
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Check journald
sudo journalctl -t sshd --since '24 hours ago'
sudo journalctl _COMM=sshd --since '24 hours ago'The first query asks for records tagged sshd; the second filters by the executable name recorded by the journal. If either query is empty, try the other form and inspect the service’s actual unit and command name. A missing result does not prove that no login occurred: the journal may have been vacuumed, forwarded elsewhere, or configured with a different identifier.
Preserve context before investigating
- Record the hostname, collection time and local time zone.
- Copy the relevant file or journal slice without editing the original.
- Keep the
sshdPID, account name, source address and nearby timestamps together. - Note whether the host uses local accounts, SSSD/IdM/AD, another PAM provider, or centralized log collection.
These details prevent a later reviewer from mistaking a time-zone conversion, a reused address or a second authentication provider for a different event.
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Consider this representative entry:
Oct 28 11:11:12 hostname sshd[13412]: Failed password for illegal user admin from 172.16.59.10 port 33762 ssh2Read the fields from left to right, but interpret the result only after finding the rest of the transaction.
| Part | What it tells you | What it does not tell you |
|---|---|---|
Oct 28 11:11:12 |
Timestamp as recorded by the host. | It may not be UTC; confirm the host’s time zone and clock status. |
hostname |
Machine that emitted the record. | It does not identify the operator who initiated the connection. |
sshd[13412] |
SSH daemon and process ID handling this connection. | A PID alone is not a user identity; correlate it with nearby lines. |
Failed password |
This password exchange was rejected at that point. | It does not establish that the entire login transaction failed. |
illegal user admin |
The name was not a valid local account when sshd checked it. |
It does not identify the person behind the source address. |
from 172.16.59.10 |
Network address presented as the remote source. | It may represent a NAT gateway, proxy, scanner, shared host or translated address rather than one person. |
port 33762 |
Source TCP port for this connection. | It is temporary connection metadata, not a destination service port or user identifier. |
ssh2 |
SSH protocol version reported by the client exchange. | It is not a statement that authentication succeeded. |
Compare events on several axes: sequence in time, username, source address and port, authentication method, result, and whether a session opened. Grouping only by source IP can combine unrelated users behind a shared gateway or split one investigation across changing addresses.
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“Illegal user” and “invalid user”
These labels mean that the requested name was not a valid local account at the point sshd checked it. They are common in automated scanning and password guessing, but they do not reveal who sent the request. A valid account name followed by repeated Failed password records is also evidence of attempts, not evidence of compromise.
Decide what a burst of failures means
Repeated attempts from one source can indicate password guessing. Red Hat describes repeated attempts as potentially indicative of an attacker trying to guess a password, especially for a privileged account. That interpretation is a lead for investigation, not a verdict.
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Signals that deserve closer review
- Many attempts in a short interval, especially against several account names.
- Requests for nonexistent names such as
admin,testor other guessed defaults. - Attempts arriving when no approved administrator or automation job should be connecting.
- A sequence that changes from failures to an accepted password or public key.
- Activity spread across many source addresses, which may indicate distributed scanning rather than one host.
Signals that can have an ordinary explanation
- An administrator repeatedly mistyped a password or used an expired credential.
- An automation system retained an old secret and retries on a schedule.
- A NAT gateway makes several legitimate users appear to come from one address.
- An external identity provider rejects a local PAM check before another provider succeeds.
- A monitoring or vulnerability scanner is authorized but noisy.
The source address is an observation about network origin, not proof of a human identity. Confirm expected owners through inventory, VPN, bastion, automation and provider records.
Why one PAM failure line can be misleading
PAM can call more than one authentication provider during a single SSH transaction. A local provider can report a failure even though a later provider authenticates the user successfully. Red Hat documents this pattern for an IdM/AD/SSSD user:
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pam_unix(sshd:auth): authentication failure; logname= uid=0 euid=0 tty=ssh ruser= rhost=client1.example.com user=user22
pam_sss(sshd:auth): authentication success; logname= uid=0 euid=0 tty=ssh ruser= rhost=client1.example.com user=user22In that sequence, the pam_unix message is a false failure report from the local provider; the SSSD provider succeeds. Therefore, do not count every pam_unix(...:auth) message as a separate attack and do not conclude that the final login failed until you inspect the complete window.
Build a transaction window
- Start with the suspicious line’s timestamp and
sshdPID. - Collect lines from the same PID a few seconds before and after it.
- Search the same username and source address, including records emitted by SSSD or another identity service.
- Look for an explicit accepted method, session-open record, or a disconnect without a session.
- Compare the result with login databases and audit records before assigning an outcome.
Do not infer success merely because an authentication failure is followed by silence. Logs may be incomplete, rotated or filtered.
Separate authentication from account and access policy
A correct password can still lead to a denied login. The message family tells you which branch to investigate next.
| Message family | Likely stage | Next check |
|---|---|---|
Failed password, Authentication failure, pam_unix(...:auth) |
Authentication exchange | Check the method, account, source, other PAM providers and eventual SSH result. |
pam_access(sshd:account): access denied |
Account authorization or origin rule | Inspect PAM access rules, groups, permitted hosts and time-based policy. |
PAM: User account has expired |
Account state | Check local or directory-provider expiry data and the organization’s renewal process. |
pam_sss success or failure |
External identity provider participation | Correlate SSSD and directory-service logs with the SSH transaction. |
Labeling all four cases “wrong password” creates the wrong remediation. An access rule, expired account or directory outage requires a different response from credential guessing.
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Confirm whether a login actually succeeded
Use several independent records. Each source answers a different question, and command availability depends on distribution and PAM configuration.
Inspect session history
last -Fai | head -50
lastloglast reads the binary /var/log/wtmp database and shows current and previous login sessions. lastlog reads /var/log/lastlog; its purpose is to show each user’s last login rather than every session. A missing entry can reflect retention, disabled accounting or a session type that does not write the expected record.
Inspect failed-login history
sudo lastb -Fai | head -50lastb reads the bad-login database, commonly /var/log/btmp. Treat it as evidence of recorded bad attempts, not a replacement for the detailed sshd and PAM sequence. The Linux-PAM pam_lastlog facility can also display the number and date of failed attempts recorded in btmp when configured to do so.
Check lockout state
sudo faillock --user USERNAMEpam_faillock stores failed attempts per user and can lock an account after configured failures. Record the configured threshold, interval and lockout duration before interpreting a burst. A lockout proves that the configured control counted failures; it does not by itself identify the source or establish a successful login.
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Use journald for the same period
sudo journalctl -t sshd --since '2026-10-28 11:05:00' --until '2026-10-28 11:20:00'
sudo journalctl _COMM=sshd --since '2026-10-28 11:05:00' --until '2026-10-28 11:20:00'Adjust the dates and time zone to the host. Search for accepted password or public-key messages, session-open records and session-close or disconnect records. A successful authentication followed by immediate disconnect is different from an authentication that never succeeded, so capture both parts.
Check audit records for higher-confidence evidence
When Linux auditing is enabled, search the audit stream for USER_LOGIN records. Red Hat identifies fields such as addr, terminal and res=failed as useful failed-login evidence. Match the audit timestamp, account and address to the SSH and PAM window. Audit records are subject to their own configuration and retention, so an absent record is not conclusive.
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A repeatable investigation workflow
- Preserve the evidence. Export the relevant secure-log lines or journal slice, retaining original timestamps, host name and time zone. Save a hash or other chain-of-custody information if your incident process requires it.
- Define the scope. Choose a time window wide enough to include retries, provider messages, session creation and disconnect. Include rotated logs or centralized copies when local retention is shorter.
- Group by transaction. Use the
sshdPID, username, source address, source port and close timestamps. Do not merge events solely because they share an IP. - Classify each line. Mark it as password/authentication, access policy, expiry, external identity, session open, session close or unrelated noise.
- Find the final result. Look for accepted password or public key, PAM success, session-open records, and subsequent close records. If they conflict, preserve the conflict rather than choosing the most alarming line.
- Cross-check databases and controls. Compare
last,lastlog,lastb,faillock, journald and auditUSER_LOGINrecords. - Validate the source. Check whether the address belongs to an administrator, automation host, VPN, bastion, NAT gateway, authorized scanner or unknown network. Treat attribution as unresolved when those records do not identify the user.
- Apply response procedures. If a session is confirmed and unauthorized, follow the host’s incident-response plan, preserve evidence, contain access as authorized, and rotate credentials or keys only in a coordinated way. Do not erase logs while attempting to “clean up” the server.
Common interpretations and their limits
Many failures for a nonexistent account
This pattern is consistent with scanning or guessing of account names. It shows that connection attempts reached SSH and that the requested names were not valid local accounts at that check. It does not show that an attacker reached a shell or that the source address belongs to a particular person.
Failures followed by an accepted key
A client can try several methods. A rejected password followed by Accepted publickey may be a legitimate key-based login, or it may be an unauthorized key. Determine which account and key were accepted, then verify the session and the key’s ownership through your access records.
pam_unix failure followed by pam_sss success
This is the documented multi-provider case. Treat the later provider result and session evidence as part of the same transaction, not as two independent login attempts.
Account-expired or access-denied messages
These are authorization or account-state outcomes. Investigate expiry data, PAM access configuration, group membership and directory policy rather than repeatedly resetting a password.
Quick Recap
Practical pitfalls
- Wrong file: Searching
/var/log/secureon a journal-only host can produce a false “no activity” conclusion. - Wrong time zone: A copied line can appear to precede or follow another event until both are converted consistently.
- Overcounting retries: One SSH connection can produce several PAM messages and method attempts.
- IP attribution: NAT, proxies and shared infrastructure make address-only attribution unreliable.
- Binary history assumptions:
wtmp,btmpandlastlogare binary databases; do not edit them as text. - Lockout misreading:
faillockreflects configured counting and timers, not an attacker identity. - Evidence destruction: Rotating, vacuuming or deleting logs before export can remove the only record that links an attempt to a session.
Quick decision checklist
- Am I reading the host’s configured source: secure file, journald, or a centralized collector?
- Have I recorded the original time zone, host and collection time?
- What account classification, source address, source port and SSH method are shown?
- Is the line an authentication failure, an account-policy denial, an expiry, or an external-provider result?
- Do nearby records show an accepted method, PAM success, session open or session close?
- Do
last,lastlog,lastb,faillockand audit records agree? - Is the source an expected administrator, automation system, shared gateway or unknown network?
- Have I preserved the evidence before making containment or credential changes?
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