Session Fixation
Attack Pattern ID: 61 (Standard Attack Pattern Completeness: Complete)Typical Severity: HighStatus: Draft
+ Description


The attacker induces a client to establish a session with the target software using a session identifier provided by the attacker. Once the user successfully authenticates to the target software, the attacker uses the (now privileged) session identifier in their own transactions. This attack leverages the fact that the target software either relies on client-generated session identifiers or maintains the same session identifiers after privilege elevation.

Attack Execution Flow

  1. Setup the Attack:

    Setup a session: The attacker has to setup a trap session that provides a valid session identifier, or select an arbitrary identifier, depending on the mechanism employed by the application. A trap session is a dummy session established with the application by the attacker and is used solely for the purpose of obtaining valid session identifiers. The attacker may also be required to periodically refresh the trap session in order to obtain valid session identifiers.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    The attacker chooses a predefined identifier that he knows.

    env-Web env-Peer2Peer env-CommProtocol env-ClientServer

    The attacker creates a trap session for the victim.

    env-Web env-Peer2Peer env-CommProtocol env-ClientServer


    IDtypeIndicator DescriptionEnvironments

    The application accepts predefined, or user-provided session IDs

    env-Web env-Peer2Peer env-CommProtocol env-ClientServer

    The application ignores predefined, or user-provided session IDs and provides new session IDs.

    env-Web env-Peer2Peer env-CommProtocol env-ClientServer


    IDtypeOutcome Description
    A trap session or a predefined session ID is established.

    Security Controls

    IDtypeSecurity Control Description
    Detect and alert on users who provide unknown session IDs in their connection establishment. Since this also fits the scenario where a user's session has expired, the heuristic must be a bit smarter, perhaps looking for an unusually high number of such occurrences in a short time frame.
    Detect and alert on multiple origins connecting with the same predefined session ID.
  1. Attract a Victim:

    Fixate the session: The attacker now needs to transfer the session identifier from the trap session to the victim by introducing the session identifier into the victim's browser. This is known as fixating the session. The session identifier can be introduced into the victim's browser by leveraging cross site scripting vulnerability, using META tags or setting HTTP response headers in a variety of ways.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    Attackers can put links on web sites (such as forums, blogs, or comment forms).


    Attackers can establish rogue proxy servers for network protocols that give out the session ID and then redirect the connection to the legitimate service.

    env-Peer2Peer env-ClientServer env-CommProtocol

    Attackers can email attack URLs to potential victims through spam and phishing techniques.



    IDtypeOutcome Description
    A victim makes a connection according to the attacker's design.

    Security Controls

    IDtypeSecurity Control Description
    Record referrers from web clients that connect with predefined session IDs. Alert when referrers do not match known, acceptable sites.
  1. Abuse the Victim's Session:

    Takeover the fixated session: Once the victim has achieved a higher level of privilege, possibly by logging into the application, the attacker can now take over the session using the fixated session identifier.

    Attack Step Techniques

    IDAttack Step Technique DescriptionEnvironments

    The attacker loads the predefined session ID into his browser and browses to protected data or functionality.


    The attacker loads the predefined session ID into his software and utilizes functionality with the rights of the victim.

    env-CommProtocol env-ClientServer env-Peer2Peer


    IDtypeOutcome Description
    The attacker gains access to data or functionality with the rights of the victim.

    Security Controls

    IDtypeSecurity Control Description
    Detect and alert on multiple simultaneous uses of the same session ID from different origins.
    Disconnect all simultaneous users of the same session ID when they arrive from different origins.
+ Attack Prerequisites

Session identifiers that remain unchanged when the privilege levels change.

Permissive session management mechanism that accepts random user-generated session identifiers

Predicatable session identifiers

+ Typical Likelihood of Exploit

Likelihood: Medium

+ Methods of Attack
  • Time and State
  • Injection
+ Examples-Instances


Consider a banking application that issues a session identifier in the URL to a user before login, and uses the same identifier to identify the customer following successful authentication. An attacker can easily leverage session fixation to access a victim's account by having the victim click on a forged link that contains a valid session identifier from a trapped session setup by the attacker. Once the victim is authenticated, the attacker can take over the session and continue with the same levels of privilege as the victim.


An attacker can hijack user sessions, bypass authentication controls and possibly gain administrative privilege by fixating the session of a user authenticating to the Management Console on certain versions of Macromedia JRun 4.0. This can be achieved by setting the session identifier in the user's browser and having the user authenticate to the Management Console. Session fixation is possible since the application server does not regenerate session identifiers when there is a change in the privilege levels.

Related Vulnerabilities


+ Attacker Skills or Knowledge Required

Skill or Knowledge Level: Low

Only basic skills are required to determine and fixate session identifiers in a user's browser. Subsequent attacks may require greater skill levels depending on the attacker's motives.

+ Resources Required


+ Probing Techniques

Determining whether the target application server accepts preset session identifiers is relatively easy. The attacker may try setting session identifiers in the URL or hidden form fields or in cookies, depending upon application design. Having access to an account or by utilizing a dummy account, the attacker can determine whether the preset session identifiers are accepted or not.

With code or design in hand, the attacker can readily verify whether preset session identifiers are accepted and whether identifiers are regenerated, and possible destroyed, when privilege levels change.

+ Indicators-Warnings of Attack

There are no indicators for the server since a fixated session identifier is similar to an ordinarily generated one. However, too many invalid sessions due to invalid session identifiers is a potential warning.

A client can be suspicious if a received link contains preset session identifiers. However, this depends on the client's knowledge of such an issue. Also, fixation through Cross Site Scripting or hidden form fields is usually difficult to detect.

+ Solutions and Mitigations

Use a strict session management mechanism that only accepts locally generated session identifiers: This prevents attackers from fixating session identifiers of their own choice.

Regenerate and destroy session identifiers when there is a change in the level of privilege: This ensures that even though a potential victim may have followed a link with a fixated identifier, a new one is issued when the level of privilege changes.

Use session identifiers that are difficult to guess or brute-force: One way for the attackers to obtain valid session identifiers is by brute-forcing or guessing them. By choosing session identifiers that are sufficiently random, brute-forcing or guessing becomes very difficult.

+ Attack Motivation-Consequences
  • Privilege Escalation
+ Injection Vector

GET or POST data, Hidden form fields and session cookies

+ Payload

Preset session identifier

+ Activation Zone

Target application's session management mechanism

+ Payload Activation Impact

The payload activation impact is that a session identifier of the attacker's choice is considered valid and trust decisions by the application will be based on such a fixated identifier.

+ Related Weaknesses
CWE-IDWeakness NameWeakness Relationship Type
384Session FixationTargeted
361Time and StateSecondary
664Improper Control of a Resource Through its LifetimeTargeted
732Incorrect Permission Assignment for Critical ResourceTargeted
+ Related Attack Patterns
NatureTypeIDNameDescriptionView(s) this relationship pertains toView\(s\)
ChildOfAttack PatternAttack Pattern21Exploitation of Session Variables, Resource IDs and other Trusted Credentials 
Mechanism of Attack (primary)1000
ChildOfCategoryCategory370WASC Threat Classification 2.0 - WASC-37 - Session Fixation 
WASC Threat Classification 2.0333
CanFollowAttack PatternAttack Pattern196Session Credential Falsification through Forging

In a Session Fixation attack, the attacker provides a credential and coerces a user into using that credential when authenticating with the server. If the format of credentials is anything but trivial, the attacker would need to forge a valid-looking credential first.

Mechanism of Attack1000
+ Relevant Security Requirements

Regenerate session identifiers upon each new request. This ensures that fixated session identifiers are rendered obsolete.

Regenerate a session identifier everytime a user enters an authenticated session and destroy the identifier when the user logs out of an authenticated session.

Set appropriate expiry times on cookies that contain session identifiers. This helps limit the window of opportunity for an attacker to use the identifier.

Do not use session identifiers as part of URLs or hidden form fields. It becomes easy for an attacker to trick a user into a fixated session when session identifiers are easily accessible.

Authenticate every transaction by requesting credentials. This ensures that only a legitimate user of the application can proceed with the transaction. If an attacker seeks to perform any such authenticated transaction, valid credentials will be required even though session fixation may have been successful earlier.

+ Related Security Principles
  • Complete Mediation

  • Reluctance to Trust

  • Defense in Depth

+ Related Guidelines
  • Never Use Unvalidated Input as Part of a Directive to any Internal Component

+ Purposes
  • Penetration
+ CIA Impact
Confidentiality Impact: HighIntegrity Impact: HighAvailability Impact: Low
+ Technical Context
Architectural Paradigms
+ References
G. Hoglund and G. McGraw. "Exploiting Software: How to Break Code". Addison-Wesley. February 2004.

CWE - Session Fixation

CWE - Time and State

+ Content History
Chiradeep B Chhaya2007-01-29Second Draft
Malik HamroCigital, Inc2007-02-27Reformat to new schema and review
Sean BarnumCigital, Inc2007-03-05Review and revise
Richard StruseVOXEM, Inc2007-03-26Review and feedback leading to changes in Description and Related Attack Patterns
Sean BarnumCigital, Inc2007-04-13Modified pattern content according to review and feedback